[C++] Implement "Deducing this" (P0847R7)

[cor3ntin]

Summary

This patch implements P0847R7 (partially),
CWG2561 and CWG2653.

[aaron.ballman]

clang/lib/Sema/SemaOverload.cpp

15255–15258

15273–15274

15510

15960–15962

[cor3ntin]

Address Aaron's style comments

[cor3ntin]

Fix bogus edits

[cor3ntin]

clang/include/clang/Sema/Sema.h

3775

Maybe we should document the whole function, but for that I'd need to better understand UseMemberUsingDeclRules

The other solution might be to have an IsOverride function such that both IsOverride and IsOverload function would dispatch to the same internal function.

clang/lib/Sema/SemaOverload.cpp

911–915

Yes, although I need to figure out a test

996–999

Yes, although I need to figure out a test

2846–2850

I've elected not to modify any of the Objective C code paths. I have no idea how Objective c++ inherit new features nor how deducing this would impact it.

3170–3172

What would be the type of Reversed ? llvm::reverse(Old) : Old ? there is no common type between the two branches afaict

6250

Here the first line of the function would have to be

Expr* Local = Obj; and then Obj would not be used again. I'm keeping it, lest you insist :)

15960–15962

Oh wow, I didn't spot the gnarly double conversion here. Thanks!

[cor3ntin]

Always consider rewrite candidates that are member
functions

[cor3ntin]

clang/lib/Sema/SemaOverload.cpp

911–915

We need changes.
Which changes is not obvious to me. https://lists.isocpp.org/core/2023/08/14711.php

996–999

UGH, phab ate my comment...
This does not need change.

if the operator has 2 non object parameters, it is implied that it is a non-member binary operator

[aaron.ballman]

Phew, that completes my first pass through the review! I'm also adding @erichkeane as a reviewer now that he's off sabbatical.

clang/include/clang/Basic/DiagnosticSemaKinds.td

9378–9381

These changes seem like they're orthogonal to the patch. Should we split them off into their own NFC commit so we can get them out of here?

clang/test/CXX/drs/dr25xx.cpp

104–106

I'd like to see two other tests:

struct D2 : B {
  void f(this B&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
};

to demonstrate we also catch it when naming the base class instead of the derived class. And:

struct T {};
struct D3 : B {
  void f(this T&); // Okay, not an override
};

void func() {
  T t;
  t.f(); // Verify this calls D3::f() and not B::f(), probably as a codegen test
}

clang/test/CXX/drs/dr26xx.cpp

1

Do we need -Wno-c++2b-extensions? All the changes in the file are protected by c++23 version checks.

125

You should update the other comments as well. :-)

128

176

clang/test/CXX/over/over.load/p2-0x.cpp

10–13

Spurious whitespace changes, this whole file can be reverted I think.

clang/test/CXX/special/class.copy/p20.cpp

14

Spurious whitespace changes, this whole file can be reverted I think.

clang/test/CXX/special/class.copy/p25-0x.cpp

115–118

Might as well skip using EXPLICIT_PARAMETER for these since they're in the guarded block already anyway?

clang/test/CodeGenCXX/cxx2b-deducing-this.cpp

4

I'd like to see more codegen tests in general -- for example, a test that demonstrates we properly handle code like:

struct B {
  virtual void f();
};

struct T {};
struct D3 : B {
  void f(this T&); // Okay, not an override
};

void func() {
  T t;
  t.f(); // Verify this calls D3::f() and not B::f()
}

but also tests that show that we do the correct thing for calling conventions (do explicit object parameter functions act as __fastcall functions?), explicit object parameters in lambdas, call through a pointer to member function, and so on.

Another test that could be interesting is how chained calls look (roughly):

struct S {
  void foo(this const S&);
};

struct T {
  S bar(this const &T);
};

void func() {
  T t;
  t.bar().foo();
}

clang/test/CodeGenCXX/cxx2b-mangle-deducing-this.cpp

1

Is -fno-rtti necessary for some reason?

clang/test/CodeGenCXX/microsoft-abi-explicit-object-parameters.cpp

1

Same question here about -fno-rtti.

clang/test/SemaCXX/cxx2b-deducing-this-coro.cpp

1–4

clang/test/SemaCXX/cxx2b-deducing-this.cpp

19

We've got an inconsistency with our diagnostic wording; this one says const explicitly, but the other ones say have qualifiers. Should these be unified?

28

Should we add a fix-it for this situation or is that overkill?

49

Should this hide the other virtual function? Isn't this morally equivalent to:

struct B {
  virtual void func();
};

struct D : B {
  void func() const;
};

https://godbolt.org/z/ja8Mx9aaE

51

This is not a virtual function, it would hide the virtual function in this case, wouldn't it?

57

I wonder if we want to reword this ever-so-slightly to the explicit object parameter cannot have a default argument to help clarify this situation:

void f(this const auto & = Test{}, int i = 12);

58

62

I'd like an additional test case:

struct B {
  void foo();
  int n;
  static int i;
};

struct D : B {
  void bar(this auto) {
    foo(); // error
    n = 12; // error
    i = 100; // Okay, I presume?
  }
};

94

Other tests I'd like to see are:

struct Frobble;
auto nothingIsOkay = [i = 0](this const Frobble &) {};

struct Frobble {} f; // Should cause a diagnostic on the lambda?
nothingIsOkay(f);

and

auto alsoOk = [](this const Test &) {}; // Fine because there's no capture?
alsoOk(Test{});

173

How about:

[i = 0](this const auto &) mutable { i++; }();

183

192

Errr, this diagnostic seems likely to be hard for users to act on: this non-static member function has an object argument! And it's even the right type!

If the model for the feature is "these are just static functions with funky lookup rules", I kind of wonder if this should be accepted instead of rejected. But if it needs to be rejected, I think we should have a better diagnostic, perhaps along the lines of "a call to an explicit object member function cannot specify the explicit object argument" with a fix-it to remove that argument. WDYT?

200

Another test that might be interesting is:

struct S {
  void f(this int);
  void f(this float);

  operator int() const;
  operator float() const;

  void test(this const S &s) {
    s.f(); // Ambiguous call
  }
};

I'm especially curious how well the notes come out.

Also, what about this?

struct S {
  void s(this short);

  operator int() const;

  void test(this const S &val) {
    val.s();
  }
};

struct T {
  void s(this int);

  operator short() const;

  void test(this const T &val) {
    val.s();
  }
};

to test how implicit conversions factor in.

[cor3ntin]

Address some comments (typos, add tests, reword diags)

clang/test/CXX/drs/dr25xx.cpp

104–106

I might need help with the codegen test setup, it's sill my nemesis. Or we can put it somewhere else

clang/test/SemaCXX/cxx2b-deducing-this.cpp

19

You prefer "const-qualified"?

28

What did the user intend if they put it there? It seems overkill.

49

Same reply as below

51

Per wg21.link/CWG2554

If a virtual member function F is declared in a class B, and, in a class D derived (directly or indirectly) from B, a declaration of a member function G corresponds (6.4.1 [basic.scope.scope]) to a declaration of F as if declared in D (12.2.2.1 [over.match.funcs.general]), ignoring trailing requires-clauses, and, if G is an explicit object member function, ignoring object parameters, and, if G is an implicit object member function, F and G have the same ref-qualifier (or absence thereof), then G overrides [ Footnote: ... ] F .

In particular. if G is an explicit object member function, ignoring object parameters

173

Line 10

[cor3ntin]

Reuse the logic/diag message diagnosing qualifiers
of static member functions, instead of rolling out
an ad-hoc bit of code.

[cor3ntin]

clang/test/SemaCXX/cxx2b-deducing-this.cpp

19

I'm now reusing the same code path we had for static, and same diagnostic message

[cor3ntin]

Add more lambda tests

[cor3ntin]

Adapt DR574 tests

[cor3ntin]

clang/include/clang/Basic/DiagnosticSemaKinds.td

7260–7265

Can you clarify, what do you think is missing test coverage?

clang/lib/Sema/SemaOverload.cpp

1397

Might as well. Not sure it changes anything.

clang/test/SemaCXX/cxx2b-deducing-this.cpp

30–31

Should we simply remove the newly added diagnostic then?

192

UGH, phab is confused, I'm no longer sure which diag you are concerned about...

[cor3ntin]

Rename DiagnoseMissingQualifiersInAddressOfOperand

[cor3ntin]

clang/lib/Sema/SemaOverload.cpp

13034

I don't think FixOverloadedFunctionReference can ever be null, and if it can, maybe it's better to let it assert so that we know we missed something, right?

[steakhal]

I checked out the code to see how does the Static Analyzer work after this.
I'm impressed that it seems to work.
Do you mind adding my test file to this patch?
clang/test/Analysis/cxx2b-deducing-this.cpp:

// RUN: %clang_analyze_cc1 -std=c++2b -verify %s \
// RUN:   -analyzer-checker=core,debug.ExprInspection

template <typename T> void clang_analyzer_dump(T);

struct S {
  int num;
  S *orig;

  void a(this auto Self) {
    clang_analyzer_dump(&Self);     // expected-warning {{&Self}}
    clang_analyzer_dump(Self.orig); // expected-warning {{&s}}
    clang_analyzer_dump(Self.num);       // expected-warning {{5 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{5 S32b}}

    Self.num = 1;
    clang_analyzer_dump(Self.num);       // expected-warning {{1 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{5 S32b}}
  }

  void b(this auto& Self) {
    clang_analyzer_dump(&Self);     // expected-warning {{&s}}
    clang_analyzer_dump(Self.orig); // expected-warning {{&s}}
    clang_analyzer_dump(Self.num);       // expected-warning {{5 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{5 S32b}}

    Self.num = 2;
    clang_analyzer_dump(Self.num);       // expected-warning {{2 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{2 S32b}}
  }

  void c(this S Self) {
    clang_analyzer_dump(&Self);     // expected-warning {{&Self}}
    clang_analyzer_dump(Self.orig); // expected-warning {{&s}}
    clang_analyzer_dump(Self.num);       // expected-warning {{2 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{2 S32b}}

    Self.num = 3;
    clang_analyzer_dump(Self.num);       // expected-warning {{3 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{2 S32b}}
  }

  void c(this S Self, int I) {
    clang_analyzer_dump(I); // expected-warning {{11 S32b}}
    clang_analyzer_dump(&Self);     // expected-warning {{&Self}}
    clang_analyzer_dump(Self.orig); // expected-warning {{&s}}
    clang_analyzer_dump(Self.num);       // expected-warning {{2 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{2 S32b}}

    Self.num = 4;
    clang_analyzer_dump(Self.num);       // expected-warning {{4 S32b}}
    clang_analyzer_dump(Self.orig->num); // expected-warning {{2 S32b}}
  }
};

void top() {
  S s = {/*num=*/5, /*orig=*/&s};
  s.a();
  s.b(); // This call changes 's.num' to 2.
  s.c();
  s.c(11);
}

Thank you for implementing (deducing) this!

[erichkeane]

Mostly just nits (plus my hatred for C style casts :) ), I didn't see any issues with the approach.

clang/include/clang/AST/ASTLambda.h

45

Under what cases does this end up being null? It seems like this condition shouldn't be necessary, and it doesn't seem like we should have a case where we create a method/function without a type set?

clang/lib/AST/Decl.cpp

3624

3629

clang/lib/CodeGen/CGExprCXX.cpp

72

clang/lib/Sema/SemaDeclCXX.cpp

14942

These casts should likely not be C-style/clobber casts.

14965

same here

15050

here and elsewhere.

[cor3ntin]

Address Erich's comments.

[cor3ntin]

Add analyzer tests (I will gladly admit i do not understand the output :p)

[cor3ntin]

clang/include/clang/AST/ASTLambda.h

45

If you have invalid captures for example, you can end up with no type. (but we do need to create a method because you need to have a context to which attach the captures to)

clang/test/CodeGenCXX/cxx2b-deducing-this.cpp

4

That first example is ill-formed (it is an override, not allowed)

I will need help for codegen tests.
For __thiscall, are we even doing the same thing? https://compiler-explorer.com/z/KTea6W36T

[cor3ntin]

Add chained member calls code gen tests

[cor3ntin]

clang/lib/AST/DeclCXX.cpp

841

https://cplusplus.github.io/CWG/issues/2787.html
Do we want to implement that resolution now?

[cor3ntin]

clang/test/CodeGenCXX/cxx2b-deducing-this.cpp

4

NVM, just different optimization levels

[cor3ntin]

Explicit object member functions are not __thiscall by default.

[cor3ntin]

Remove accidentally commited file

[cor3ntin]

Add simple lambda codegen test

[cor3ntin]

Additional code gens tests (lambda with capture, function ptr)

[cor3ntin]

Rebase

[cor3ntin]

Rebase, fix codegen tests

[cor3ntin]

Update issue list

[cor3ntin]

Fix cxx status page

[aaron.ballman]

I think we're getting pretty close! My goal is to get this landed ASAP; I do not think it needs to be hidden behind a feature flag (-std=c++2b is sufficient), and it's good that we're not defining the feature test macro yet. There were a few outstanding questions still that would be good to get answers for. Having some codegen code owner eyes on this would be appreciated, so adding them as reviewers.

clang/include/clang/AST/ASTLambda.h

45

This does feel rather unclean, though it may be fine for now. Instead of a null type, I would expect the declaration to be marked as invalid (and perhaps for callers to avoid calling isLambdaCallWithImplicitObjectParameter() on an invalid declaration so that we can turn the predicate into an assertion).

Let's add a FIXME for now?

clang/include/clang/AST/Decl.h

1722–1724

spurious whitespace

clang/include/clang/AST/DeclCXX.h

2063–2064

Still needs some docs explaining the difference for folks.

clang/include/clang/Basic/DiagnosticSemaKinds.td

7260–7265

explicit object parameters are incompatible with C++ standards before C++2b is missing test coverage.

7262–7265

9378–9381

These changes can still be split off into their own NFC commit.

9443–9444

I don't see any tests modified as a result of this change (and the change seems orthogonal in a similar way as the above changes).

clang/include/clang/Sema/Sema.h

3775

The other solution might be to have an IsOverride function such that both IsOverride and IsOverload function would dispatch to the same internal function.

I think that's a clean solution.

clang/lib/AST/ComputeDependence.cpp

502–504

clang/lib/AST/DeclCXX.cpp

841

https://cplusplus.github.io/CWG/issues/2787.html
Do we want to implement that resolution now?

I think it's fine to do that resolution in a follow-up closely after landing this, unless you want to do it now because it's trivial enough. I don't imagine CWG is going to come back with a drastically different resolution, do you? CC @shafik for opinions on reading CWG tea leaves.

2526

clang/lib/AST/ItaniumMangle.cpp

1729–1731

Based on the discussion in that thread, I think this direction is reasonable. Ping @rjmccall for any last-minute concerns before we make this part of the ABI.

clang/lib/CodeGen/CGExpr.cpp

4274

Still hoping @efriedma or @rjmccall can comment on the codegen changes.

clang/lib/Parse/ParseDecl.cpp

5756

This can still be split off to make the review shorter for folks.

clang/lib/Sema/SemaDeclCXX.cpp

11265

I think this early return should also be pulled.

clang/lib/Sema/SemaExpr.cpp

20551

clang/lib/Sema/SemaOverload.cpp

1299–1300

Any new thoughts on this?

2846–2850

Objective-C++ is a pure extension over the top of C++, so C++ functionality should behave in the expected way in ObjC++. But because this is with ObjC pointers (which are kind of a special thing), it's not clear to me either. I say let's punt on it; but final word comes from @rjmccall

3170–3172

Oh derp. :-)

6250

I don't insist.

6254–6258

Still wondering about test coverage for this bit.

clang/test/CodeGenCXX/cxx2b-deducing-this.cpp

4

That first example is ill-formed (it is an override, not allowed)

Oh you're right, I missed a very important "not" in this chain of logic:

https://eel.is/c++draft/class#virtual-2.sentence-1
https://eel.is/c++draft/basic.scope.scope#4.3
https://eel.is/c++draft/basic.scope.scope#4.4

Coupled with the direction in CWG2553 and CWG2554.

clang/test/SemaCXX/cxx2b-deducing-this.cpp

30–31

I think the new diagnostic is helpful; I'm more wondering if we can emit just the new diagnostic and skip destructor cannot have any parameters if we already said it can't have an explicit object parameter?

192

static void h() {
    f();       // expected-error{{call to non-static member function without an object argument}}
    f(Over_Call_Func_Example{});   // expected-error{{call to non-static member function without an object argument}}
    Over_Call_Func_Example{}.f();   // ok
}

from around line 214 in the latest patch; the second error seems hard to understand because there is an object argument being passed, it's just not the form allowed.

[cor3ntin]

• Nitpicks
• Add more override tests
• Add diagnostic tests for earlier language modes
• Add fixme in areas than need cleaning and unimplemented core issues

[cor3ntin]

Rebase, format, add release notes

[cor3ntin]

clang/include/clang/AST/ASTLambda.h

45

I added a fixme.
I spent some time thinking about alternatives but i can't think of something obvious at the moment.
Making a lambda being parsed invalid does not seem ideal either

clang/include/clang/Basic/DiagnosticSemaKinds.td

9378–9381

Discussed offline, we can keep those

clang/include/clang/Sema/Sema.h

3775

I added IsOverride

clang/lib/Parse/ParseDecl.cpp

5756

Discussed offline, we can keep these here

clang/lib/Sema/SemaOverload.cpp

911–915

Added link to https://cplusplus.github.io/CWG/issues/2797.html

1299–1300

Nope, I will add a fixme

6254–6258

What specific test would you like to see?

clang/test/CXX/drs/dr25xx.cpp

104–106

Added the first test. the second test is still an override. I guess i can still add it but it is IF

clang/test/SemaCXX/cxx2b-deducing-this.cpp

30–31

FYI i remember looking at that and i think it was non trivial, I'd rather punt it

192

You have a rewording suggestion?

[aaron.ballman]

clang/include/clang/Sema/Sema.h

3775

Oh, I was thinking of something different than what you did. Can we do:

bool IsOverload(FunctionDecl *New, FunctionDecl *Old,
                  bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs = true);
bool IsOverride(FunctionDecl *MD, FunctionDecl *BaseMD);

in the header file, and then in the implementation we dispatch to the same static helper function?

(What I was hoping to get rid of was UseOverrideRules from the public signature entirely so callers don't have to wonder what that means.)

clang/lib/Sema/SemaOverload.cpp

6254–6258

Whatever one exercises this code path. :-D I *think* that's going to be:

struct S {
  void foo(this S&&);
};

void func() {
  S{}.foo();
}

but am not 100% sure.

clang/test/CXX/drs/dr25xx.cpp

104–106

No need to add it in that case.

clang/test/SemaCXX/cxx2b-deducing-this.cpp

30–31

Okay, when we land this, can you file an issue so we don't lose track of the improvement?

192

How about we split it into two diagnostics:

static void h() {
  f(); // expected-error{{call to non-static member function without an object argument}}
  f(Over_Call_Func_Example{});   // expected-error{{cannot pass an explicit object as an argument to the call; did you mean to use '.'?}}
  Over_Call_Func_Example{}.f(); // ok
}

the idea being: if the call is to an explicit object function and the user provided an extra argument to the call whose type and position matches the explicit object parameter, we tell the users "that's now how you use this" and best-case give them a fix-it to move the argument to before the call and add a .?

[aaron.ballman]

clang/test/SemaCXX/cxx2b-deducing-this.cpp

192

@cor3ntin and I discussed this suggestion off-list and came to the conclusion that it would be ideal to improve the error here (and perhaps have a note that points to the this in the declaration of f() + a fixit) but it would be difficult to come up with a heuristic for in all cases. Consider a declaration like void f(this auto, Over_Call_Func_Example); which is called with f(Over_Call_Func_Example{}); from within a static function as a corollary test case to the above.

Let's punt on this for now.

[cor3ntin]

• Add code gen test for materialized temporary
• Split IsOverload in 2 functions
• Fix Previous rebase

[cor3ntin]

clang/include/clang/Sema/Sema.h

3775

Done!

clang/test/SemaCXX/cxx2b-deducing-this.cpp

30–31

Sure!

[aaron.ballman]

Aside from some minor nits in the release notes, I think this LGTM. I'm going to hold off on accepting officially for a day or two just so other code reviews have a chance to weigh in (the codegen code owners were only added to the review today).

clang/docs/ReleaseNotes.rst

94–98 ↗

(On Diff #557447)

[cor3ntin]

Fix Release notes.

[aaron.ballman]

Accepting as-is; we'll keep our eyes peeled for any post-commit feedback.

Thank you for all the hard work on this one, it was a big, involved patch!

[Endill]

clang/docs/ReleaseNotes.rst

94–98 ↗

(On Diff #557447)

Closing words was not set in this version. have been lost.

[zequanwu]

This comment was removed by zequanwu.

[nikic]

FYI this causes some compile-time regression: http://llvm-compile-time-tracker.com/compare.php?from=bc7d88faf1a595ab59952a2054418cdd0d9eeee8&to=af4751738db89a142a8880c782d12d4201b222a8&stat=instructions:u About 0.7% for C++ code at O0. Sample for a file with >2% would be btSoftSoftCollisionAlgorithm.cpp.

Not sure to what degree this is expected or not.

[aaron.ballman]

In D140828#4653389, @nikic wrote:

FYI this causes some compile-time regression: http://llvm-compile-time-tracker.com/compare.php?from=bc7d88faf1a595ab59952a2054418cdd0d9eeee8&to=af4751738db89a142a8880c782d12d4201b222a8&stat=instructions:u About 0.7% for C++ code at O0. Sample for a file with >2% would be btSoftSoftCollisionAlgorithm.cpp.

Not sure to what degree this is expected or not.

I think it's at least somewhat expected, but unfortunate. We basically have to check "is this an explicit object argument function or not" in a bunch of places in the compiler and I think that perhaps the extra branching is what's causing the perf situation. I'm not certain what can be done beyond "hook up a profiler and start poking around". Thoughts, @cor3ntin?

clang/include/clang/AST/ASTLambda.h

inline bool isLambdaCallOperator(const CXXMethodDecl *MD) {
   return MD->getOverloadedOperator() == OO_Call;
 }
 
 inline bool isLambdaCallOperator(const DeclContext *DC) {
   if (!DC || !isa<CXXMethodDecl>(DC)) return false;
   return isLambdaCallOperator(cast<CXXMethodDecl>(DC));
 }
 
+inline bool isLambdaCallWithExplicitObjectParameter(const DeclContext *DC) {
+  return isLambdaCallOperator(DC) &&
+         cast<CXXMethodDecl>(DC)->isExplicitObjectMemberFunction();
+}
+
+inline bool isLambdaCallWithImplicitObjectParameter(const DeclContext *DC) {
+  return isLambdaCallOperator(DC) &&
+         !cast<CXXMethodDecl>(DC)->getType().isNull() &&
+         !cast<CXXMethodDecl>(DC)->isExplicitObjectMemberFunction();
+}
+
 inline bool isGenericLambdaCallOperatorSpecialization(const CXXMethodDecl *MD) {
   if (!MD) return false;
   const CXXRecordDecl *LambdaClass = MD->getParent();
   if (LambdaClass && LambdaClass->isGenericLambda())
     return isLambdaCallOperator(MD) &&
                     MD->isFunctionTemplateSpecialization();
   return false;
 }

clang/include/clang/AST/Decl.h

class ParmVarDeclBitfields {
     unsigned DefaultArgKind : 2;
 
     /// Whether this parameter undergoes K&R argument promotion.
     unsigned IsKNRPromoted : 1;
 
     /// Whether this parameter is an ObjC method parameter or not.
     unsigned IsObjCMethodParam : 1;
 
     /// If IsObjCMethodParam, a Decl::ObjCDeclQualifier.
     /// Otherwise, the number of function parameter scopes enclosing
     /// the function parameter scope in which this parameter was
     /// declared.
     unsigned ScopeDepthOrObjCQuals : NumScopeDepthOrObjCQualsBits;
 
     /// The number of parameters preceding this parameter in the
     /// function parameter scope in which it was declared.
     unsigned ParameterIndex : NumParameterIndexBits;
   };
public:
   }
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == ImplicitParam; }
 };
 
 /// Represents a parameter to a function.
 class ParmVarDecl : public VarDecl {
+
 public:
   enum { MaxFunctionScopeDepth = 255 };
   enum { MaxFunctionScopeIndex = 255 };
 
 protected:
   ParmVarDecl(Kind DK, ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
               SourceLocation IdLoc, IdentifierInfo *Id, QualType T,
               TypeSourceInfo *TInfo, StorageClass S, Expr *DefArg)
       : VarDecl(DK, C, DC, StartLoc, IdLoc, Id, T, TInfo, S) {
public:
   ///   are promoted to double.
   bool isKNRPromoted() const {
     return ParmVarDeclBits.IsKNRPromoted;
   }
   void setKNRPromoted(bool promoted) {
     ParmVarDeclBits.IsKNRPromoted = promoted;
   }
 
+  bool isExplicitObjectParameter() const {
+    return ExplicitObjectParameterIntroducerLoc.isValid();
+  }
+
+  void setExplicitObjectParameterLoc(SourceLocation Loc) {
+    ExplicitObjectParameterIntroducerLoc = Loc;
+  }
+
+  SourceLocation getExplicitObjectParamThisLoc() const {
+    return ExplicitObjectParameterIntroducerLoc;
+  }
+
   Expr *getDefaultArg();
   const Expr *getDefaultArg() const {
     return const_cast<ParmVarDecl *>(this)->getDefaultArg();
   }
 
   void setDefaultArg(Expr *defarg);
 
   /// Retrieve the source range that covers the entire default
public:
   /// the DeclContext appropriately.
   void setOwningFunction(DeclContext *FD) { setDeclContext(FD); }
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == ParmVar; }
 
 private:
+  friend class ASTDeclReader;
+
   enum { ParameterIndexSentinel = (1 << NumParameterIndexBits) - 1 };
+  SourceLocation ExplicitObjectParameterIntroducerLoc;
 
   void setParameterIndex(unsigned parameterIndex) {
     if (parameterIndex >= ParameterIndexSentinel) {
       setParameterIndexLarge(parameterIndex);
       return;
     }
 
     ParmVarDeclBits.ParameterIndex = parameterIndex;
void setParams(ArrayRef<ParmVarDecl *> NewParamInfo) {
     setParams(getASTContext(), NewParamInfo);
   }
 
   /// Returns the minimum number of arguments needed to call this function. This
   /// may be fewer than the number of function parameters, if some of the
   /// parameters have default arguments (in C++).
   unsigned getMinRequiredArguments() const;
 
+  /// Returns the minimum number of non-object arguments needed to call this
+  /// function. This produces the same value as getMinRequiredArguments except
+  /// it does not count the explicit object argument, if any.
+  unsigned getMinRequiredExplicitArguments() const;
+
+  bool hasCXXExplicitFunctionObjectParameter() const;
+
+  unsigned getNumNonObjectParams() const;
+
+  const ParmVarDecl *getNonObjectParameter(unsigned I) const {
+    return getParamDecl(hasCXXExplicitFunctionObjectParameter() ? I + 1 : I);
+  }
+
+  ParmVarDecl *getNonObjectParameter(unsigned I) {
+    return getParamDecl(hasCXXExplicitFunctionObjectParameter() ? I + 1 : I);
+  }
+
   /// Determine whether this function has a single parameter, or multiple
   /// parameters where all but the first have default arguments.
   ///
   /// This notion is used in the definition of copy/move constructors and
   /// initializer list constructors. Note that, unlike getMinRequiredArguments,
   /// parameter packs are not treated specially here.
   bool hasOneParamOrDefaultArgs() const;
 

clang/include/clang/AST/DeclCXX.h

Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
          StorageClass SC, bool UsesFPIntrin, bool isInline,
          ConstexprSpecKind ConstexprKind, SourceLocation EndLocation,
          Expr *TrailingRequiresClause = nullptr);
 
   static CXXMethodDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
   bool isStatic() const;
   bool isInstance() const { return !isStatic(); }
-
+  bool isExplicitObjectMemberFunction() const;
+  bool isImplicitObjectMemberFunction() const;
   /// Returns true if the given operator is implicitly static in a record
   /// context.
   static bool isStaticOverloadedOperator(OverloadedOperatorKind OOK) {
     // [class.free]p1:
     // Any allocation function for a class T is a static member
     // (even if not explicitly declared static).
     // [class.free]p6 Any deallocation function for a class X is a static member
     // (even if not explicitly declared static).
public:
   /// that for the call operator of a lambda closure type, this returns the
   /// desugared 'this' type (a pointer to the closure type), not the captured
   /// 'this' type.
   QualType getThisType() const;
 
   /// Return the type of the object pointed by \c this.
   ///
   /// See getThisType() for usage restriction.
-  QualType getThisObjectType() const;
 
-  static QualType getThisType(const FunctionProtoType *FPT,
-                              const CXXRecordDecl *Decl);
+  QualType getFunctionObjectParameterReferenceType() const;
+  QualType getFunctionObjectParameterType() const {
+    return getFunctionObjectParameterReferenceType().getNonReferenceType();
+  }
 
-  static QualType getThisObjectType(const FunctionProtoType *FPT,
-                                    const CXXRecordDecl *Decl);
+  unsigned getNumExplicitParams() const {
+    return getNumParams() - (isExplicitObjectMemberFunction() ? 1 : 0);
+  }
+
+  static QualType getThisType(const FunctionProtoType *FPT,
 
   Qualifiers getMethodQualifiers() const {
     return getType()->castAs<FunctionProtoType>()->getMethodQuals();
   }
 
   /// Retrieve the ref-qualifier associated with this method.
   ///
   /// In the following example, \c f() has an lvalue ref-qualifier, \c g()

clang/include/clang/AST/Expr.h

DeclRefExpr(const ASTContext &Ctx, NestedNameSpecifierLoc QualifierLoc,
               SourceLocation TemplateKWLoc, ValueDecl *D,
               bool RefersToEnlosingVariableOrCapture,
               const DeclarationNameInfo &NameInfo, NamedDecl *FoundD,
               const TemplateArgumentListInfo *TemplateArgs, QualType T,
               ExprValueKind VK, NonOdrUseReason NOUR);
 
   /// Construct an empty declaration reference expression.
   explicit DeclRefExpr(EmptyShell Empty) : Expr(DeclRefExprClass, Empty) {}
 
 public:
   DeclRefExpr(const ASTContext &Ctx, ValueDecl *D,
               bool RefersToEnclosingVariableOrCapture, QualType T,
               ExprValueKind VK, SourceLocation L,
               const DeclarationNameLoc &LocInfo = DeclarationNameLoc(),
               NonOdrUseReason NOUR = NOUR_None);
 
   static DeclRefExpr *
public:
   }
 
   bool isImmediateEscalating() const {
     return DeclRefExprBits.IsImmediateEscalating;
   }
 
   void setIsImmediateEscalating(bool Set) {
     DeclRefExprBits.IsImmediateEscalating = Set;
   }
 
+  bool isCapturedByCopyInLambdaWithExplicitObjectParameter() const {
+    return DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter;
+  }
+
+  void setCapturedByCopyInLambdaWithExplicitObjectParameter(
+      bool Set, const ASTContext &Context) {
+    DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = Set;
+    setDependence(computeDependence(this, Context));
+  }
+
   static bool classof(const Stmt *T) {
     return T->getStmtClass() == DeclRefExprClass;
   }
 
   // Iterators
   child_range children() {
     return child_range(child_iterator(), child_iterator());
   }

clang/include/clang/AST/Stmt.h

class DeclRefExprBitfields {
 
     unsigned : NumExprBits;
 
     unsigned HasQualifier : 1;
     unsigned HasTemplateKWAndArgsInfo : 1;
     unsigned HasFoundDecl : 1;
     unsigned HadMultipleCandidates : 1;
     unsigned RefersToEnclosingVariableOrCapture : 1;
+    unsigned CapturedByCopyInLambdaWithExplicitObjectParameter : 1;
     unsigned NonOdrUseReason : 2;
     unsigned IsImmediateEscalating : 1;
 
     /// The location of the declaration name itself.
     SourceLocation Loc;
   };
 
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

 // different types with the same name.  Remove this note when the above note
 // can handle that case properly.
 def note_ovl_candidate_non_deduced_mismatch_qualified : Note<
     "candidate template ignored: could not match %q0 against %q1">;
 
 // Note that we don't treat templates differently for this diagnostic.
 def note_ovl_candidate_arity : Note<"candidate "
     "%sub{select_ovl_candidate_kind}0,1,2 not viable: "
-    "requires%select{ at least| at most|}3 %4 argument%s4, but %5 "
+    "requires%select{ at least| at most|}3 %4 "
+    "%select{|non-object }6argument%s4, but %5 "
     "%plural{1:was|:were}5 provided">;
 
 def note_ovl_candidate_arity_one : Note<"candidate "
     "%sub{select_ovl_candidate_kind}0,1,2 not viable: "
     "%select{requires at least|allows at most single|requires single}3 "
+    "%select{|non-object }6"
     "argument %4, but %plural{0:no|:%5}5 arguments were provided">;
 
 def note_ovl_candidate_deleted : Note<
     "candidate %sub{select_ovl_candidate_kind}0,1,2 has been "
     "%select{explicitly made unavailable|explicitly deleted|"
     "implicitly deleted}3">;
 
 // Giving the index of the bad argument really clutters this message, and

 def err_ovl_ambiguous_object_call : Error<
   "call to object of type %0 is ambiguous">;
 def err_ovl_ambiguous_subscript_call : Error<
   "call to subscript operator of type %0 is ambiguous">;
 def err_ovl_deleted_object_call : Error<
   "call to deleted function call operator in type %0">;
 def note_ovl_surrogate_cand : Note<"conversion candidate of type %0">;
 def err_member_call_without_object : Error<
-  "call to non-static member function without an object argument">;
+  "call to %select{non-static|explicit}0 member function without an object argument">;
 
 // C++ Address of Overloaded Function
 def err_addr_ovl_no_viable : Error<
   "address of overloaded function %0 does not match required type %1">;
 def err_addr_ovl_ambiguous : Error<
   "address of overloaded function %0 is ambiguous">;
 def err_addr_ovl_not_func_ptrref : Error<
   "address of overloaded function %0 cannot be converted to type %1">;
def warn_logical_not_on_lhs_of_check : Warning<
   InGroup<LogicalNotParentheses>;
 def note_logical_not_fix : Note<
   "add parentheses after the '!' to evaluate the "
   "%select{comparison|bitwise operator}0 first">;
 def note_logical_not_silence_with_parens : Note<
   "add parentheses around left hand side expression to silence this warning">;
 
 def err_invalid_this_use : Error<
-  "invalid use of 'this' outside of a non-static member function">;
+  "invalid use of 'this' %select{outside of a non-static member function"
+  "|in a function with an explicit object parameter}0">;
 def err_this_static_member_func : Error<
   "'this' cannot be%select{| implicitly}0 used in a static member function "
   "declaration">;
-def err_invalid_member_use_in_static_method : Error<
-  "invalid use of member %0 in static member function">;
+def err_invalid_member_use_in_method : Error<
+  "invalid use of member %0 in %select{static|explicit object}1 member function">;
+
 def err_invalid_qualified_function_type : Error<
   "%select{non-member function|static member function|deduction guide}0 "
   "%select{of type %2 |}1cannot have '%3' qualifier">;
 def err_compound_qualified_function_type : Error<
   "%select{block pointer|pointer|reference}0 to function type %select{%2 |}1"
   "cannot have '%3' qualifier">;
 def err_qualified_function_typeid : Error<
   "type operand %0 of 'typeid' cannot have '%1' qualifier">;
 
+def err_cxx20_deducing_this  : Error<
+  "explicit object parameters are incompatible with C++ standards before C++2b">;
+def err_explicit_object_default_arg: Error<
+  "an explicit object parameter cannot have a default argument">;
+def err_explicit_object_parameter_pack: Error<
+  "an explicit object parameter cannot be a function parameter pack">;
+def err_explicit_object_parameter_must_be_first: Error<
+  "an explicit object parameter can only appear as the first parameter "
+  "of the %select{function|lambda}0">;
+def err_explicit_object_parameter_nonmember: Error<
+  "an explicit object parameter cannot appear in a "
+  "%select{static|virtual|non-member}0 %select{function|lambda}1">;
+def err_explicit_object_parameter_constructor: Error<
+  "an explicit object parameter cannot appear in a %select{constructor|destructor}0">;
+def err_explicit_object_parameter_qualifiers: Error<
+  "a %select{function|lambda}0 with an explicit object parameter cannot "
+  "%select{be const|be mutable|be reference-qualified|have qualifiers}1">;
+def err_invalid_explicit_object_type_in_lambda: Error<
+  "invalid explicit object parameter type %0 in lambda with capture; "
+  "the type must be the same as, or derived from, the lambda">;
+
 def err_ref_qualifier_overload : Error<
   "cannot overload a member function %select{without a ref-qualifier|with "
   "ref-qualifier '&'|with ref-qualifier '&&'}0 with a member function %select{"
   "without a ref-qualifier|with ref-qualifier '&'|with ref-qualifier '&&'}1">;
 
 def err_invalid_non_static_member_use : Error<
   "invalid use of non-static data member %0">;
 def err_nested_non_static_member_use : Error<
   "%select{call to non-static member function|use of non-static data member}0 "
   "%2 of %1 from nested type %3">;
 def warn_cxx98_compat_non_static_member_use : Warning<
def err_typecheck_call_not_function : Error<
   "called object type %0 is not a function or function pointer">;
 def err_call_incomplete_return : Error<
   "calling function with incomplete return type %0">;
 def err_call_function_incomplete_return : Error<
   "calling %0 with incomplete return type %1">;
 def err_call_incomplete_argument : Error<
   "argument type %0 is incomplete">;
 def err_typecheck_call_too_few_args : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected %1, have %2">;
 def err_typecheck_call_too_few_args_one : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }2arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "single argument %1 was not specified">;
 def err_typecheck_call_too_few_args_at_least : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected at least %1, have %2">;
 def err_typecheck_call_too_few_args_at_least_one : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }2arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "at least argument %1 must be specified">;
 def err_typecheck_call_too_few_args_suggest : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected %1, have %2; did you mean %3?">;
+  "expected %1, have %2; did you mean %4?">;
 def err_typecheck_call_too_few_args_at_least_suggest : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected at least %1, have %2; did you mean %3?">;
+  "expected at least %1, have %2; did you mean %4?">;
 def err_typecheck_call_too_many_args : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected %1, have %2">;
 def err_typecheck_call_too_many_args_one : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected single argument %1, have %2 arguments">;
 def err_typecheck_call_too_many_args_at_most : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected at most %1, have %2">;
 def err_typecheck_call_too_many_args_at_most_one : Error<
   "too many %select{|||execution configuration }0arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected at most single argument %1, have %2 arguments">;
+  "expected at most single %select{|non-object }3argument %1, "
+  "have %2%select{|non-object}3 arguments">;
 def err_typecheck_call_too_many_args_suggest : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected %1, have %2; did you mean %3?">;
+  "expected %1, have %2; did you mean %4?">;
 def err_typecheck_call_too_many_args_at_most_suggest : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected at most %1, have %2; did you mean %3?">;
+  "expected at most %1, have %2; did you mean %4?">;
 
 def err_arc_typecheck_convert_incompatible_pointer : Error<
   "incompatible pointer types passing retainable parameter of type %0"
   "to a CF function expecting %1 type">;
 
 def err_builtin_fn_use : Error<"builtin functions must be directly called">;
 
 def warn_call_wrong_number_of_arguments : Warning<

 def ext_explicit_conversion_functions : ExtWarn<
   "explicit conversion functions are a C++11 extension">, InGroup<CXX11>;
 def warn_cxx98_compat_explicit_conversion_functions : Warning<
   "explicit conversion functions are incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 
 // C++11 defaulted functions
 def err_defaulted_special_member_params : Error<
-  "an explicitly-defaulted %select{|copy |move }0constructor cannot "
+  "an explicitly-defaulted %sub{select_special_member_kind}0 cannot "
   "have default arguments">;
 def err_defaulted_special_member_variadic : Error<
-  "an explicitly-defaulted %select{|copy |move }0constructor cannot "
+  "an explicitly-defaulted %sub{select_special_member_kind}0 cannot "
   "be variadic">;
 def err_defaulted_special_member_return_type : Error<
   "explicitly-defaulted %select{copy|move}0 assignment operator must "
   "return %1">;
 def err_defaulted_special_member_quals : Error<
   "an explicitly-defaulted %select{copy|move}0 assignment operator may not "
   "have 'const'%select{, 'constexpr'|}1 or 'volatile' qualifiers">;
 def err_defaulted_special_member_volatile_param : Error<

 def ext_defaulted_comparison : ExtWarn<
   "defaulted comparison operators are a C++20 extension">, InGroup<CXX20>;
 def warn_cxx17_compat_defaulted_comparison : Warning<
   "defaulted comparison operators are incompatible with C++ standards "
   "before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
 def err_defaulted_comparison_template : Error<
   "comparison operator template cannot be defaulted">;
 def err_defaulted_comparison_num_args : Error<
   "%select{non-member|member}0 %sub{select_defaulted_comparison_kind}1"
-  " comparison operator must have %select{2|1}0 parameters">;
+  " must have %select{2|1}0 parameters">;
 def err_defaulted_comparison_param : Error<
   "invalid parameter type for defaulted %sub{select_defaulted_comparison_kind}0"
   "; found %1, expected %2%select{| or %4}3">;
 def err_defaulted_comparison_param_unknown : Error<
   "invalid parameter type for non-member defaulted"
   " %sub{select_defaulted_comparison_kind}0"
   "; found %1, expected class or reference to a constant class">;
 def err_defaulted_comparison_param_mismatch : Error<

clang/include/clang/Sema/ScopeInfo.h

public:
   /// The lambda's compiler-generated \c operator().
   CXXMethodDecl *CallOperator = nullptr;
 
   /// Indicate that we parsed the parameter list
   /// at which point the mutability of the lambda
   /// is known.
   bool AfterParameterList = true;
 
+  ParmVarDecl *ExplicitObjectParameter = nullptr;
+
   /// Source range covering the lambda introducer [...].
   SourceRange IntroducerRange;
 
   /// Source location of the '&' or '=' specifying the default capture
   /// type, if any.
   SourceLocation CaptureDefaultLoc;
 
   /// The number of captures in the \c Captures list that are
public:
 
   bool hasPotentialCaptures() const {
     return getNumPotentialVariableCaptures() ||
                                   PotentialThisCaptureLocation.isValid();
   }
 
   void visitPotentialCaptures(
       llvm::function_ref<void(ValueDecl *, Expr *)> Callback) const;
+
+  bool lambdaCaptureShouldBeConst() const;
 };
 
 FunctionScopeInfo::WeakObjectProfileTy::WeakObjectProfileTy()
     : Base(nullptr, false) {}
 
 FunctionScopeInfo::WeakObjectProfileTy
 FunctionScopeInfo::WeakObjectProfileTy::getSentinel() {
   FunctionScopeInfo::WeakObjectProfileTy Result;

clang/include/clang/Sema/Sema.h

public:
   bool CheckEquivalentExceptionSpec(
       const FunctionProtoType *Old, SourceLocation OldLoc,
       const FunctionProtoType *New, SourceLocation NewLoc);
   bool CheckEquivalentExceptionSpec(
       const PartialDiagnostic &DiagID, const PartialDiagnostic & NoteID,
       const FunctionProtoType *Old, SourceLocation OldLoc,
       const FunctionProtoType *New, SourceLocation NewLoc);
   bool handlerCanCatch(QualType HandlerType, QualType ExceptionType);
-  bool CheckExceptionSpecSubset(const PartialDiagnostic &DiagID,
-                                const PartialDiagnostic &NestedDiagID,
-                                const PartialDiagnostic &NoteID,
-                                const PartialDiagnostic &NoThrowDiagID,
-                                const FunctionProtoType *Superset,
-                                SourceLocation SuperLoc,
-                                const FunctionProtoType *Subset,
-                                SourceLocation SubLoc);
-  bool CheckParamExceptionSpec(const PartialDiagnostic &NestedDiagID,
-                               const PartialDiagnostic &NoteID,
-                               const FunctionProtoType *Target,
-                               SourceLocation TargetLoc,
-                               const FunctionProtoType *Source,
-                               SourceLocation SourceLoc);
+  bool CheckExceptionSpecSubset(
+      const PartialDiagnostic &DiagID, const PartialDiagnostic &NestedDiagID,
+      const PartialDiagnostic &NoteID, const PartialDiagnostic &NoThrowDiagID,
+      const FunctionProtoType *Superset, bool SkipSupersetFirstParameter,
+      SourceLocation SuperLoc, const FunctionProtoType *Subset,
+      bool SkipSubsetFirstParameter, SourceLocation SubLoc);
+  bool CheckParamExceptionSpec(
+      const PartialDiagnostic &NestedDiagID, const PartialDiagnostic &NoteID,
+      const FunctionProtoType *Target, bool SkipTargetFirstParameter,
+      SourceLocation TargetLoc, const FunctionProtoType *Source,
+      bool SkipSourceFirstParameter, SourceLocation SourceLoc);
 
   TypeResult ActOnTypeName(Scope *S, Declarator &D);
 
   /// The parser has parsed the context-sensitive type 'instancetype'
   /// in an Objective-C message declaration. Return the appropriate type.
   ParsedType ActOnObjCInstanceType(SourceLocation Loc);
 
   /// Abstract class used to diagnose incomplete types.
public:
   bool canFullyTypeCheckRedeclaration(ValueDecl *NewD, ValueDecl *OldD,
                                       QualType NewT, QualType OldT);
   void CheckMain(FunctionDecl *FD, const DeclSpec &D);
   void CheckMSVCRTEntryPoint(FunctionDecl *FD);
   void CheckHLSLEntryPoint(FunctionDecl *FD);
   Attr *getImplicitCodeSegOrSectionAttrForFunction(const FunctionDecl *FD,
                                                    bool IsDefinition);
   void CheckFunctionOrTemplateParamDeclarator(Scope *S, Declarator &D);
-  Decl *ActOnParamDeclarator(Scope *S, Declarator &D);
+  Decl *ActOnParamDeclarator(Scope *S, Declarator &D,
+                             SourceLocation ExplicitThisLoc = {});
   ParmVarDecl *BuildParmVarDeclForTypedef(DeclContext *DC,
                                           SourceLocation Loc,
                                           QualType T);
   ParmVarDecl *CheckParameter(DeclContext *DC, SourceLocation StartLoc,
                               SourceLocation NameLoc, IdentifierInfo *Name,
                               QualType T, TypeSourceInfo *TSInfo,
                               StorageClass SC);
   void ActOnParamDefaultArgument(Decl *param,
public:
   };
   OverloadKind CheckOverload(Scope *S,
                              FunctionDecl *New,
                              const LookupResult &OldDecls,
                              NamedDecl *&OldDecl,
                              bool UseMemberUsingDeclRules);
   bool IsOverload(FunctionDecl *New, FunctionDecl *Old,
                   bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs = true,
-                  bool ConsiderRequiresClauses = true);
+                  bool UseOverrideRules = false);
 
   // Calculates whether the expression Constraint depends on an enclosing
   // template, for the purposes of [temp.friend] p9.
   // TemplateDepth is the 'depth' of the friend function, which is used to
   // compare whether a declaration reference is referring to a containing
   // template, or just the current friend function. A 'lower' TemplateDepth in
   // the AST refers to a 'containing' template. As the constraint is
   // uninstantiated, this is relative to the 'top' of the TU.
bool IsPointerConversion(Expr *From, QualType FromType, QualType ToType,
                            bool InOverloadResolution,
                            QualType& ConvertedType, bool &IncompatibleObjC);
   bool isObjCPointerConversion(QualType FromType, QualType ToType,
                                QualType& ConvertedType, bool &IncompatibleObjC);
   bool isObjCWritebackConversion(QualType FromType, QualType ToType,
                                  QualType &ConvertedType);
   bool IsBlockPointerConversion(QualType FromType, QualType ToType,
                                 QualType& ConvertedType);
+
+  bool FunctionParamTypesAreEqual(ArrayRef<QualType> Old,
+                                  ArrayRef<QualType> New,
+                                  unsigned *ArgPos = nullptr,
+                                  bool Reversed = false);
+
   bool FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
                                   const FunctionProtoType *NewType,
                                   unsigned *ArgPos = nullptr,
                                   bool Reversed = false);
   void HandleFunctionTypeMismatch(PartialDiagnostic &PDiag,
                                   QualType FromType, QualType ToType);
 
   void maybeExtendBlockObject(ExprResult &E);
public:
 
   bool CanPerformCopyInitialization(const InitializedEntity &Entity,
                                     ExprResult Init);
   ExprResult PerformCopyInitialization(const InitializedEntity &Entity,
                                        SourceLocation EqualLoc,
                                        ExprResult Init,
                                        bool TopLevelOfInitList = false,
                                        bool AllowExplicit = false);
-  ExprResult PerformObjectArgumentInitialization(Expr *From,
-                                                 NestedNameSpecifier *Qualifier,
-                                                 NamedDecl *FoundDecl,
-                                                 CXXMethodDecl *Method);
+  ExprResult InitializeExplicitObjectArgument(Sema &S, Expr *Obj,
+                                              FunctionDecl *Fun);
+  ExprResult PerformImplicitObjectArgumentInitialization(
+      Expr *From, NestedNameSpecifier *Qualifier, NamedDecl *FoundDecl,
 
   /// Check that the lifetime of the initializer (and its subobjects) is
   /// sufficient for initializing the entity, and perform lifetime extension
   /// (when permitted) if not.
   void checkInitializerLifetime(const InitializedEntity &Entity, Expr *Init);
 
   ExprResult PerformContextuallyConvertToBool(Expr *From);
   ExprResult PerformContextuallyConvertToObjCPointer(Expr *From);
FunctionDecl *ResolveSingleFunctionTemplateSpecialization(
       TemplateSpecCandidateSet *FailedTSC = nullptr);
 
   bool ResolveAndFixSingleFunctionTemplateSpecialization(
       ExprResult &SrcExpr, bool DoFunctionPointerConversion = false,
       bool Complain = false, SourceRange OpRangeForComplaining = SourceRange(),
       QualType DestTypeForComplaining = QualType(),
       unsigned DiagIDForComplaining = 0);
 
-  Expr *FixOverloadedFunctionReference(Expr *E,
-                                       DeclAccessPair FoundDecl,
+  ExprResult FixOverloadedFunctionReference(Expr *E, DeclAccessPair FoundDecl,
                                             FunctionDecl *Fn);
   ExprResult FixOverloadedFunctionReference(ExprResult,
                                             DeclAccessPair FoundDecl,
                                             FunctionDecl *Fn);
 
   void AddOverloadedCallCandidates(UnresolvedLookupExpr *ULE,
                                    ArrayRef<Expr *> Args,
                                    OverloadCandidateSet &CandidateSet,
                                    bool PartialOverloading = false);
public:
   ExprResult CreateBuiltinUnaryOp(SourceLocation OpLoc, UnaryOperatorKind Opc,
                                   Expr *InputExpr, bool IsAfterAmp = false);
   ExprResult BuildUnaryOp(Scope *S, SourceLocation OpLoc, UnaryOperatorKind Opc,
                           Expr *Input, bool IsAfterAmp = false);
   ExprResult ActOnUnaryOp(Scope *S, SourceLocation OpLoc, tok::TokenKind Op,
                           Expr *Input, bool IsAfterAmp = false);
 
   bool isQualifiedMemberAccess(Expr *E);
+  bool DiagnoseMissingQualifiersInAddressOfOperand(SourceLocation OpLoc,
+                                                   const Expr *Op,
+                                                   const CXXMethodDecl *MD);
+
   QualType CheckAddressOfOperand(ExprResult &Operand, SourceLocation OpLoc);
 
   bool CheckTypeTraitArity(unsigned Arity, SourceLocation Loc, size_t N);
 
   bool ActOnAlignasTypeArgument(StringRef KWName, ParsedType Ty,
                                 SourceLocation OpLoc, SourceRange R);
   bool CheckAlignasTypeArgument(StringRef KWName, TypeSourceInfo *TInfo,
                                 SourceLocation OpLoc, SourceRange R);
CXXMethodDecl *CreateLambdaCallOperator(SourceRange IntroducerRange,
                                           CXXRecordDecl *Class);
   void CompleteLambdaCallOperator(
       CXXMethodDecl *Method, SourceLocation LambdaLoc,
       SourceLocation CallOperatorLoc, Expr *TrailingRequiresClause,
       TypeSourceInfo *MethodTyInfo, ConstexprSpecKind ConstexprKind,
       StorageClass SC, ArrayRef<ParmVarDecl *> Params,
       bool HasExplicitResultType);
 
+  void DiagnoseInvalidExplicitObjectParameterInLambda(CXXMethodDecl *Method);
+
   /// Perform initialization analysis of the init-capture and perform
   /// any implicit conversions such as an lvalue-to-rvalue conversion if
   /// not being used to initialize a reference.
   ParsedType actOnLambdaInitCaptureInitialization(
       SourceLocation Loc, bool ByRef, SourceLocation EllipsisLoc,
       IdentifierInfo *Id, LambdaCaptureInitKind InitKind, Expr *&Init) {
     return ParsedType::make(buildLambdaInitCaptureInitialization(
         Loc, ByRef, EllipsisLoc, std::nullopt, Id,
public:
   NamedDecl *ActOnFriendFunctionDecl(Scope *S, Declarator &D,
                                      MultiTemplateParamsArg TemplateParams);
 
   QualType CheckConstructorDeclarator(Declarator &D, QualType R,
                                       StorageClass& SC);
   void CheckConstructor(CXXConstructorDecl *Constructor);
   QualType CheckDestructorDeclarator(Declarator &D, QualType R,
                                      StorageClass& SC);
   bool CheckDestructor(CXXDestructorDecl *Destructor);
   void CheckConversionDeclarator(Declarator &D, QualType &R,
                                  StorageClass& SC);
   Decl *ActOnConversionDeclarator(CXXConversionDecl *Conversion);
   bool CheckDeductionGuideDeclarator(Declarator &D, QualType &R,
                                      StorageClass &SC);
   void CheckDeductionGuideTemplate(FunctionTemplateDecl *TD);
 
   void CheckExplicitlyDefaultedFunction(Scope *S, FunctionDecl *MD);
 
   bool CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
                                              CXXSpecialMember CSM,
                                              SourceLocation DefaultLoc);
   void CheckDelayedMemberExceptionSpecs();
 
   bool CheckExplicitlyDefaultedComparison(Scope *S, FunctionDecl *MD,
                                           DefaultedComparisonKind DCK);
   void DeclareImplicitEqualityComparison(CXXRecordDecl *RD,
                                          FunctionDecl *Spaceship);
   void DefineDefaultedComparison(SourceLocation Loc, FunctionDecl *FD,
                                  DefaultedComparisonKind DCK);
 
+  void CheckExplicitObjectMemberFunction(Declarator &D, DeclarationName Name,
+                                         QualType R, bool IsLambda,
+                                         DeclContext *DC = nullptr);
+  void CheckExplicitObjectMemberFunction(DeclContext *DC, Declarator &D,
+                                         DeclarationName Name, QualType R);
+  void CheckExplicitObjectLambda(Declarator &D);
+
   //===--------------------------------------------------------------------===//
   // C++ Derived Classes
   //
 
   /// ActOnBaseSpecifier - Parsed a base specifier
   CXXBaseSpecifier *CheckBaseSpecifier(CXXRecordDecl *Class,
                                        SourceRange SpecifierRange,
                                        bool Virtual, AccessSpecifier Access,
public:
   bool CheckOverridingFunctionAttributes(const CXXMethodDecl *New,
                                          const CXXMethodDecl *Old);
 
   /// CheckOverridingFunctionReturnType - Checks whether the return types are
   /// covariant, according to C++ [class.virtual]p5.
   bool CheckOverridingFunctionReturnType(const CXXMethodDecl *New,
                                          const CXXMethodDecl *Old);
 
+  // Check that the overriding method has no explicit object parameter.
+  bool CheckExplicitObjectOverride(CXXMethodDecl *New,
+                                   const CXXMethodDecl *Old);
+
   /// CheckOverridingFunctionExceptionSpec - Checks whether the exception
   /// spec is a subset of base spec.
   bool CheckOverridingFunctionExceptionSpec(const CXXMethodDecl *New,
                                             const CXXMethodDecl *Old);
 
   bool CheckPureMethod(CXXMethodDecl *Method, SourceRange InitRange);
 
   /// CheckOverrideControl - Check C++11 override control semantics.
TemplateDeductionResult FinishTemplateArgumentDeduction(
       bool PartialOverloading = false,
       llvm::function_ref<bool()> CheckNonDependent = []{ return false; });
 
   TemplateDeductionResult DeduceTemplateArguments(
       FunctionTemplateDecl *FunctionTemplate,
       TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
       FunctionDecl *&Specialization, sema::TemplateDeductionInfo &Info,
       bool PartialOverloading, bool AggregateDeductionCandidate,
+      QualType ObjectType, Expr::Classification ObjectClassification,
       llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent);
 
   TemplateDeductionResult
   DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
                           TemplateArgumentListInfo *ExplicitTemplateArgs,
                           QualType ArgFunctionType,
                           FunctionDecl *&Specialization,
                           sema::TemplateDeductionInfo &Info,
                           bool IsAddressOfFunction = false);
 
-  TemplateDeductionResult
-  DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
-                          QualType ToType,
-                          CXXConversionDecl *&Specialization,
-                          sema::TemplateDeductionInfo &Info);
+  TemplateDeductionResult DeduceTemplateArguments(
+      FunctionTemplateDecl *FunctionTemplate, QualType ObjectType,
+      Expr::Classification ObjectClassification, QualType ToType,
+      CXXConversionDecl *&Specialization, sema::TemplateDeductionInfo &Info);
 
   TemplateDeductionResult
   DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
                           TemplateArgumentListInfo *ExplicitTemplateArgs,
                           FunctionDecl *&Specialization,
                           sema::TemplateDeductionInfo &Info,
                           bool IsAddressOfFunction = false);
 

clang/lib/AST/ASTImporter.cpp

if (GetImportedOrCreateDecl(ToParm, D, Importer.getToContext(), DC,
                               ToType, D->getParameterKind()))
     return ToParm;
   return ToParm;
 }
 
 Error ASTNodeImporter::ImportDefaultArgOfParmVarDecl(
     const ParmVarDecl *FromParam, ParmVarDecl *ToParam) {
   ToParam->setHasInheritedDefaultArg(FromParam->hasInheritedDefaultArg());
+  ToParam->setExplicitObjectParameterLoc(
+      FromParam->getExplicitObjectParamThisLoc());
   ToParam->setKNRPromoted(FromParam->isKNRPromoted());
 
   if (FromParam->hasUninstantiatedDefaultArg()) {
     if (auto ToDefArgOrErr = import(FromParam->getUninstantiatedDefaultArg()))
       ToParam->setUninstantiatedDefaultArg(*ToDefArgOrErr);
     else
       return ToDefArgOrErr.takeError();
   } else if (FromParam->hasUnparsedDefaultArg()) {

clang/lib/AST/ASTStructuralEquivalence.cpp

static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
                                      CXXMethodDecl *Method1,
                                      CXXMethodDecl *Method2) {
   bool PropertiesEqual =
       Method1->getDeclKind() == Method2->getDeclKind() &&
       Method1->getRefQualifier() == Method2->getRefQualifier() &&
       Method1->getAccess() == Method2->getAccess() &&
       Method1->getOverloadedOperator() == Method2->getOverloadedOperator() &&
       Method1->isStatic() == Method2->isStatic() &&
+      Method1->isImplicitObjectMemberFunction() ==
+          Method2->isImplicitObjectMemberFunction() &&
       Method1->isConst() == Method2->isConst() &&
       Method1->isVolatile() == Method2->isVolatile() &&
       Method1->isVirtual() == Method2->isVirtual() &&
       Method1->isPure() == Method2->isPure() &&
       Method1->isDefaulted() == Method2->isDefaulted() &&
       Method1->isDeleted() == Method2->isDeleted();
   if (!PropertiesEqual)
     return false;

clang/lib/AST/ComputeDependence.cpp

ExprDependence clang::computeDependence(DeclRefExpr *E, const ASTContext &Ctx) {
   Deps |= toExprDependenceForImpliedType(Type->getDependence()) &
           ExprDependence::Error;
 
   // C++ [temp.dep.expr]p3:
   //   An id-expression is type-dependent if it contains:
 
   //    - an identifier associated by name lookup with one or more declarations
   //      declared with a dependent type
+  //    - an identifier associated by name lookup with an entity captured by
+  //    copy ([expr.prim.lambda.capture])
+  //      in a lambda-expression that has an explicit object parameter whose
+  //      type is dependent ([dcl.fct]),
   //
   // [The "or more" case is not modeled as a DeclRefExpr. There are a bunch
   // more bullets here that we handle by treating the declaration as having a
   // dependent type if they involve a placeholder type that can't be deduced.]
   if (Type->isDependentType())
     Deps |= ExprDependence::TypeValueInstantiation;
   else if (Type->isInstantiationDependentType())
     Deps |= ExprDependence::Instantiation;
 
+  //    - an identifier associated by name lookup with an entity captured by
+  //    copy ([expr.prim.lambda.capture])
+  if (E->isCapturedByCopyInLambdaWithExplicitObjectParameter()) {
+    Deps |= ExprDependence::Type;
+  }
+
   //    - a conversion-function-id that specifies a dependent type
   if (Decl->getDeclName().getNameKind() ==
       DeclarationName::CXXConversionFunctionName) {
     QualType T = Decl->getDeclName().getCXXNameType();
     if (T->isDependentType())
       return Deps | ExprDependence::TypeValueInstantiation;
 
     if (T->isInstantiationDependentType())

clang/lib/AST/Decl.cpp

if (!Param->isParameterPack()) {
       ++MinParamsSoFar;
       if (!Param->hasDefaultArg())
         NumRequiredArgs = MinParamsSoFar;
     }
   }
   return NumRequiredArgs;
 }
 
+bool FunctionDecl::hasCXXExplicitFunctionObjectParameter() const {
+  return getNumParams() != 0 && getParamDecl(0)->isExplicitObjectParameter();
+}
+
+unsigned FunctionDecl::getNumNonObjectParams() const {
+  return getNumParams() - (unsigned)hasCXXExplicitFunctionObjectParameter();
+}
+
+unsigned FunctionDecl::getMinRequiredExplicitArguments() const {
+  return getMinRequiredArguments() -
+         (hasCXXExplicitFunctionObjectParameter() ? 1 : 0);
+}
+
 bool FunctionDecl::hasOneParamOrDefaultArgs() const {
   return getNumParams() == 1 ||
          (getNumParams() > 1 &&
           llvm::all_of(llvm::drop_begin(parameters()),
                        [](ParmVarDecl *P) { return P->hasDefaultArg(); }));
 }
 
 /// The combination of the extern and inline keywords under MSVC forces

clang/lib/AST/DeclCXX.cpp

void CXXRecordDecl::addedMember(Decl *D) {
   if (const auto *Method = dyn_cast<CXXMethodDecl>(D)) {
     if (isa<CXXDestructorDecl>(D))
       SMKind |= SMF_Destructor;
 
     if (Method->isCopyAssignmentOperator()) {
       SMKind |= SMF_CopyAssignment;
 
       const auto *ParamTy =
-          Method->getParamDecl(0)->getType()->getAs<ReferenceType>();
+          Method->getNonObjectParameter(0)->getType()->getAs<ReferenceType>();
       if (!ParamTy || ParamTy->getPointeeType().isConstQualified())
         data().HasDeclaredCopyAssignmentWithConstParam = true;
     }
 
     if (Method->isMoveAssignmentOperator())
       SMKind |= SMF_MoveAssignment;
 
     // Keep the list of conversion functions up-to-date.
if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
         PreventedBy.push_back(FD);
         Result = false;
       }
     }
   }
   return Result;
 }
 
+bool CXXMethodDecl::isExplicitObjectMemberFunction() const {
+  // C++2b [dcl.fct]p6:
+  // An explicit object member function is a non-static member
+  // function with an explicit object parameter
+  return !isStatic() && hasCXXExplicitFunctionObjectParameter();
+}
+
+bool CXXMethodDecl::isImplicitObjectMemberFunction() const {
+  return !isStatic() && !hasCXXExplicitFunctionObjectParameter();
+}
+
 bool CXXMethodDecl::isCopyAssignmentOperator() const {
   // C++0x [class.copy]p17:
   //  A user-declared copy assignment operator X::operator= is a non-static
   //  non-template member function of class X with exactly one parameter of
   //  type X, X&, const X&, volatile X& or const volatile X&.
   if (/*operator=*/getOverloadedOperator() != OO_Equal ||
       /*non-static*/ isStatic() ||
-      /*non-template*/getPrimaryTemplate() || getDescribedFunctionTemplate() ||
-      getNumParams() != 1)
+
+      /*non-template*/ getPrimaryTemplate() || getDescribedFunctionTemplate() ||
+      getNumExplicitParams() != 1)
     return false;
 
-  QualType ParamType = getParamDecl(0)->getType();
+  QualType ParamType = getNonObjectParameter(0)->getType();
   if (const auto *Ref = ParamType->getAs<LValueReferenceType>())
     ParamType = Ref->getPointeeType();
 
   ASTContext &Context = getASTContext();
   QualType ClassType
     = Context.getCanonicalType(Context.getTypeDeclType(getParent()));
   return Context.hasSameUnqualifiedType(ClassType, ParamType);
 }
 
 bool CXXMethodDecl::isMoveAssignmentOperator() const {
   // C++0x [class.copy]p19:
   //  A user-declared move assignment operator X::operator= is a non-static
   //  non-template member function of class X with exactly one parameter of type
   //  X&&, const X&&, volatile X&&, or const volatile X&&.
   if (getOverloadedOperator() != OO_Equal || isStatic() ||
       getPrimaryTemplate() || getDescribedFunctionTemplate() ||
-      getNumParams() != 1)
+      getNumExplicitParams() != 1)
     return false;
 
-  QualType ParamType = getParamDecl(0)->getType();
+  QualType ParamType = getNonObjectParameter(0)->getType();
   if (!ParamType->isRValueReferenceType())
     return false;
   ParamType = ParamType->getPointeeType();
 
   ASTContext &Context = getASTContext();
   QualType ClassType
     = Context.getCanonicalType(Context.getTypeDeclType(getParent()));
   return Context.hasSameUnqualifiedType(ClassType, ParamType);
CXXMethodDecl::overridden_methods() const {
   if (isa<CXXConstructorDecl>(this))
     return overridden_method_range(nullptr, nullptr);
   return getASTContext().overridden_methods(this);
 }
 
 static QualType getThisObjectType(ASTContext &C, const FunctionProtoType *FPT,
                                   const CXXRecordDecl *Decl) {
   QualType ClassTy = C.getTypeDeclType(Decl);
   return C.getQualifiedType(ClassTy, FPT->getMethodQuals());
 }
 
 QualType CXXMethodDecl::getThisType(const FunctionProtoType *FPT,
                                     const CXXRecordDecl *Decl) {
   ASTContext &C = Decl->getASTContext();
   QualType ObjectTy = ::getThisObjectType(C, FPT, Decl);
   return C.getPointerType(ObjectTy);
 }
 
-QualType CXXMethodDecl::getThisObjectType(const FunctionProtoType *FPT,
-                                          const CXXRecordDecl *Decl) {
-  ASTContext &C = Decl->getASTContext();
-  return ::getThisObjectType(C, FPT, Decl);
-}
-
 QualType CXXMethodDecl::getThisType() const {
   // C++ 9.3.2p1: The type of this in a member function of a class X is X*.
   // If the member function is declared const, the type of this is const X*,
   // if the member function is declared volatile, the type of this is
   // volatile X*, and if the member function is declared const volatile,
   // the type of this is const volatile X*.
   assert(isInstance() && "No 'this' for static methods!");
   return CXXMethodDecl::getThisType(getType()->castAs<FunctionProtoType>(),
                                     getParent());
 }
 
-QualType CXXMethodDecl::getThisObjectType() const {
-  // Ditto getThisType.
-  assert(isInstance() && "No 'this' for static methods!");
-  return CXXMethodDecl::getThisObjectType(
-      getType()->castAs<FunctionProtoType>(), getParent());
+QualType CXXMethodDecl::getFunctionObjectParameterReferenceType() const {
+  if (isExplicitObjectMemberFunction())
+    return parameters()[0]->getType();
+
+  ASTContext &C = getParentASTContext();
+  const FunctionProtoType *FPT = getType()->castAs<FunctionProtoType>();
+  QualType Type = ::getThisObjectType(C, FPT, getParent());
+  RefQualifierKind RK = FPT->getRefQualifier();
+  if (RK == RefQualifierKind::RQ_RValue)
+    return C.getRValueReferenceType(Type);
+  return C.getLValueReferenceType(Type);
 }
 
 bool CXXMethodDecl::hasInlineBody() const {
   // If this function is a template instantiation, look at the template from
   // which it was instantiated.
   const FunctionDecl *CheckFn = getTemplateInstantiationPattern();
   if (!CheckFn)
     CheckFn = this;

clang/lib/AST/DeclPrinter.cpp

 
 void DeclPrinter::VisitLabelDecl(LabelDecl *D) {
   Out << *D << ":";
 }
 
 void DeclPrinter::VisitVarDecl(VarDecl *D) {
   prettyPrintPragmas(D);
 
+  if (const auto *Param = dyn_cast<ParmVarDecl>(D);
+      Param && Param->isExplicitObjectParameter())
+    Out << "this ";
+
   QualType T = D->getTypeSourceInfo()
     ? D->getTypeSourceInfo()->getType()
     : D->getASTContext().getUnqualifiedObjCPointerType(D->getType());
 
   if (!Policy.SuppressSpecifiers) {
     StorageClass SC = D->getStorageClass();
     if (SC != SC_None)
       Out << VarDecl::getStorageClassSpecifierString(SC) << " ";
 

clang/lib/AST/Expr.cpp

DeclRefExpr::DeclRefExpr(const ASTContext &Ctx, ValueDecl *D,
                          NonOdrUseReason NOUR)
     : Expr(DeclRefExprClass, T, VK, OK_Ordinary), D(D), DNLoc(LocInfo) {
   DeclRefExprBits.HasQualifier = false;
   DeclRefExprBits.HasTemplateKWAndArgsInfo = false;
   DeclRefExprBits.HasFoundDecl = false;
   DeclRefExprBits.HadMultipleCandidates = false;
   DeclRefExprBits.RefersToEnclosingVariableOrCapture =
       RefersToEnclosingVariableOrCapture;
+  DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = false;
   DeclRefExprBits.NonOdrUseReason = NOUR;
   DeclRefExprBits.IsImmediateEscalating = false;
   DeclRefExprBits.Loc = L;
   setDependence(computeDependence(this, Ctx));
 }
 
 DeclRefExpr::DeclRefExpr(const ASTContext &Ctx,
                          NestedNameSpecifierLoc QualifierLoc,
new (getTrailingObjects<NestedNameSpecifierLoc>())
         NestedNameSpecifierLoc(QualifierLoc);
   DeclRefExprBits.HasFoundDecl = FoundD ? 1 : 0;
   if (FoundD)
     *getTrailingObjects<NamedDecl *>() = FoundD;
   DeclRefExprBits.HasTemplateKWAndArgsInfo
     = (TemplateArgs || TemplateKWLoc.isValid()) ? 1 : 0;
   DeclRefExprBits.RefersToEnclosingVariableOrCapture =
       RefersToEnclosingVariableOrCapture;
+  DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = false;
   DeclRefExprBits.NonOdrUseReason = NOUR;
   if (TemplateArgs) {
     auto Deps = TemplateArgumentDependence::None;
     getTrailingObjects<ASTTemplateKWAndArgsInfo>()->initializeFrom(
         TemplateKWLoc, *TemplateArgs, getTrailingObjects<TemplateArgumentLoc>(),
         Deps);
     assert(!(Deps & TemplateArgumentDependence::Dependent) &&
            "built a DeclRefExpr with dependent template args");

clang/lib/AST/ExprClassification.cpp

 static Cl::Kinds ClassifyDecl(ASTContext &Ctx, const Decl *D) {
   // C++ [expr.prim.general]p6: The result is an lvalue if the entity is a
   //   function, variable, or data member and a prvalue otherwise.
   // In C, functions are not lvalues.
   // In addition, NonTypeTemplateParmDecl derives from VarDecl but isn't an
   // lvalue unless it's a reference type (C++ [temp.param]p6), so we need to
   // special-case this.
 
-  if (isa<CXXMethodDecl>(D) && cast<CXXMethodDecl>(D)->isInstance())
+  if (const auto *M = dyn_cast<CXXMethodDecl>(D)) {
+    if (M->isImplicitObjectMemberFunction())
       return Cl::CL_MemberFunction;
+    if (M->isStatic())
+      return Cl::CL_LValue;
+    return Cl::CL_PRValue;
+  }
 
   bool islvalue;
   if (const auto *NTTParm = dyn_cast<NonTypeTemplateParmDecl>(D))
     islvalue = NTTParm->getType()->isReferenceType() ||
                NTTParm->getType()->isRecordType();
   else
     islvalue =
         isa<VarDecl, FieldDecl, IndirectFieldDecl, BindingDecl, MSGuidDecl,
if (isa<ObjCPropertyRefExpr>(Base))
       return Cl::CL_SubObjCPropertySetting;
     return ClassifyInternal(Ctx, E->getBase());
   }
 
   //   -- If E2 is a [...] member function, [...]
   //      -- If it refers to a static member function [...], then E1.E2 is an
   //         lvalue; [...]
   //      -- Otherwise [...] E1.E2 is a prvalue.
-  if (const auto *Method = dyn_cast<CXXMethodDecl>(Member))
-    return Method->isStatic() ? Cl::CL_LValue : Cl::CL_MemberFunction;
+  if (const auto *Method = dyn_cast<CXXMethodDecl>(Member)) {
+    if (Method->isStatic())
+      return Cl::CL_LValue;
+    if (Method->isImplicitObjectMemberFunction())
+      return Cl::CL_MemberFunction;
+    return Cl::CL_PRValue;
+  }
 
   //   -- If E2 is a member enumerator [...], the expression E1.E2 is a prvalue.
   // So is everything else we haven't handled yet.
   return Cl::CL_PRValue;
 }
 
 static Cl::Kinds ClassifyBinaryOp(ASTContext &Ctx, const BinaryOperator *E) {
   assert(Ctx.getLangOpts().CPlusPlus &&
          "This is only relevant for C++.");

clang/lib/AST/ExprConstant.cpp

APValue *EvalInfo::createHeapAlloc(const Expr *E, QualType T, LValue &LV) {
   assert(Result.second && "reused a heap alloc index?");
   Result.first->second.AllocExpr = E;
   return &Result.first->second.Value;
 }
 
 /// Produce a string describing the given constexpr call.
 void CallStackFrame::describe(raw_ostream &Out) const {
   unsigned ArgIndex = 0;
-  bool IsMemberCall = isa<CXXMethodDecl>(Callee) &&
-                      !isa<CXXConstructorDecl>(Callee) &&
-                      cast<CXXMethodDecl>(Callee)->isInstance();
+  bool IsMemberCall =
+      isa<CXXMethodDecl>(Callee) && !isa<CXXConstructorDecl>(Callee) &&
+      cast<CXXMethodDecl>(Callee)->isImplicitObjectMemberFunction();
 
   if (!IsMemberCall)
     Callee->getNameForDiagnostic(Out, Info.Ctx.getPrintingPolicy(),
                                  /*Qualified=*/false);
 
   if (This && IsMemberCall) {
     if (const auto *MCE = dyn_cast_if_present<CXXMemberCallExpr>(CallExpr)) {
       const Expr *Object = MCE->getImplicitObjectArgument();
if (Object->getType()->isPointerType() && Object->isPRValue())
     return EvaluatePointer(Object, This, Info);
 
   if (Object->isGLValue())
     return EvaluateLValue(Object, This, Info);
 
   if (Object->getType()->isLiteralType(Info.Ctx))
     return EvaluateTemporary(Object, This, Info);
 
+  if (Object->getType()->isRecordType() && Object->isPRValue())
+    return EvaluateTemporary(Object, This, Info);
+
   Info.FFDiag(Object, diag::note_constexpr_nonliteral) << Object->getType();
   return false;
 }
 
 /// HandleMemberPointerAccess - Evaluate a member access operation and build an
 /// lvalue referring to the result.
 ///
 /// \param Info - Information about the ongoing evaluation.
if (CalleeType->isSpecificBuiltinType(BuiltinType::BoundMember)) {
       }
 
       // For an (overloaded) assignment expression, evaluate the RHS before the
       // LHS.
       auto *OCE = dyn_cast<CXXOperatorCallExpr>(E);
       if (OCE && OCE->isAssignmentOp()) {
         assert(Args.size() == 2 && "wrong number of arguments in assignment");
         Call = Info.CurrentCall->createCall(FD);
-        if (!EvaluateArgs(isa<CXXMethodDecl>(FD) ? Args.slice(1) : Args, Call,
-                          Info, FD, /*RightToLeft=*/true))
+        bool HasThis = false;
+        if (const auto *MD = dyn_cast<CXXMethodDecl>(FD))
+          HasThis = MD->isImplicitObjectMemberFunction();
+        if (!EvaluateArgs(HasThis ? Args.slice(1) : Args, Call, Info, FD,
+                          /*RightToLeft=*/true))
           return false;
       }
 
       // Overloaded operator calls to member functions are represented as normal
       // calls with '*this' as the first argument.
       const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD);
-      if (MD && !MD->isStatic()) {
+      if (MD && MD->isImplicitObjectMemberFunction()) {
         // FIXME: When selecting an implicit conversion for an overloaded
         // operator delete, we sometimes try to evaluate calls to conversion
         // operators without a 'this' parameter!
         if (Args.empty())
           return Error(E);
 
         if (!EvaluateObjectArgument(Info, Args[0], ThisVal))
           return false;
bool handleCallExpr(const CallExpr *E, APValue &Result,
     if (This) {
       auto *NamedMember = dyn_cast<CXXMethodDecl>(FD);
       if (NamedMember && NamedMember->isVirtual() && !HasQualifier) {
         // Perform virtual dispatch, if necessary.
         FD = HandleVirtualDispatch(Info, E, *This, NamedMember,
                                    CovariantAdjustmentPath);
         if (!FD)
           return false;
-      } else {
+      } else if (NamedMember && NamedMember->isImplicitObjectMemberFunction()) {
         // Check that the 'this' pointer points to an object of the right type.
         // FIXME: If this is an assignment operator call, we may need to change
         // the active union member before we check this.
         if (!checkNonVirtualMemberCallThisPointer(Info, E, *This, NamedMember))
           return false;
       }
     }
 
bool Expr::EvaluateWithSubstitution(APValue &Value, ASTContext &Ctx,
   Info.InConstantContext = true;
 
   LValue ThisVal;
   const LValue *ThisPtr = nullptr;
   if (This) {
 #ifndef NDEBUG
     auto *MD = dyn_cast<CXXMethodDecl>(Callee);
     assert(MD && "Don't provide `this` for non-methods.");
-    assert(!MD->isStatic() && "Don't provide `this` for static methods.");
+    assert(MD->isImplicitObjectMemberFunction() &&
+           "Don't provide `this` for methods without an implicit object.");
 #endif
     if (!This->isValueDependent() &&
         EvaluateObjectArgument(Info, This, ThisVal) &&
         !Info.EvalStatus.HasSideEffects)
       ThisPtr = &ThisVal;
 
     // Ignore any side-effects from a failed evaluation. This is safe because
     // they can't interfere with any other argument evaluation.
bool Expr::isPotentialConstantExpr(const FunctionDecl *FD,
   APValue Scratch;
   if (const CXXConstructorDecl *CD = dyn_cast<CXXConstructorDecl>(FD)) {
     // Evaluate the call as a constant initializer, to allow the construction
     // of objects of non-literal types.
     Info.setEvaluatingDecl(This.getLValueBase(), Scratch);
     HandleConstructorCall(&VIE, This, Args, CD, Info, Scratch);
   } else {
     SourceLocation Loc = FD->getLocation();
-    HandleFunctionCall(Loc, FD, (MD && MD->isInstance()) ? &This : nullptr,
-                       &VIE, Args, CallRef(), FD->getBody(), Info, Scratch,
-                       /*ResultSlot=*/nullptr);
+    HandleFunctionCall(
+        Loc, FD, (MD && MD->isImplicitObjectMemberFunction()) ? &This : nullptr,
   }
 
   return Diags.empty();
 }
 
 bool Expr::isPotentialConstantExprUnevaluated(Expr *E,
                                               const FunctionDecl *FD,
                                               SmallVectorImpl<

clang/lib/AST/Interp/ByteCodeEmitter.cpp

if (!Ty->isVoidType() && !Ctx.classify(Ty)) {
     ParamOffset += align(primSize(PT_Ptr));
   }
 
   // If the function decl is a member decl, the next parameter is
   // the 'this' pointer. This parameter is pop()ed from the
   // InterpStack when calling the function.
   bool HasThisPointer = false;
   if (const auto *MD = dyn_cast<CXXMethodDecl>(FuncDecl)) {
-    if (MD->isInstance()) {
+    if (MD->isImplicitObjectMemberFunction()) {
       HasThisPointer = true;
       ParamTypes.push_back(PT_Ptr);
       ParamOffsets.push_back(ParamOffset);
       ParamOffset += align(primSize(PT_Ptr));
     }
 
     // Set up lambda capture to closure record field mapping.
     if (isLambdaCallOperator(MD)) {

clang/lib/AST/ItaniumMangle.cpp

case DeclarationName::CXXDestructorName:
     break;
 
   case DeclarationName::CXXOperatorName:
     if (ND && Arity == UnknownArity) {
       Arity = cast<FunctionDecl>(ND)->getNumParams();
 
       // If we have a member function, we need to include the 'this' pointer.
       if (const auto *MD = dyn_cast<CXXMethodDecl>(ND))
-        if (!MD->isStatic())
+        if (MD->isImplicitObjectMemberFunction())
           Arity++;
     }
     [[fallthrough]];
   case DeclarationName::CXXConversionFunctionName:
   case DeclarationName::CXXLiteralOperatorName:
     mangleOperatorName(Name, Arity);
     writeAbiTags(ND, AdditionalAbiTags);
     break;
void CXXNameMangler::mangleNestedName(GlobalDecl GD,
   //   ::= N [<CV-qualifiers>] [<ref-qualifier>] <template-prefix>
   //       <template-args> E
 
   Out << 'N';
   if (const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(ND)) {
     Qualifiers MethodQuals = Method->getMethodQualifiers();
     // We do not consider restrict a distinguishing attribute for overloading
     // purposes so we must not mangle it.
+    if (Method->isExplicitObjectMemberFunction())
+      Out << 'H';
     MethodQuals.removeRestrict();
     mangleQualifiers(MethodQuals);
     mangleRefQualifier(Method->getRefQualifier());
   }
 
   // Check if we have a template.
   const TemplateArgumentList *TemplateArgs = nullptr;
   if (GlobalDecl TD = isTemplate(GD, TemplateArgs)) {
     mangleTemplatePrefix(TD, NoFunction);

clang/lib/AST/JSONNodeDumper.cpp

 
 void JSONNodeDumper::VisitUsingShadowDecl(const UsingShadowDecl *USD) {
   JOS.attribute("target", createBareDeclRef(USD->getTargetDecl()));
 }
 
 void JSONNodeDumper::VisitVarDecl(const VarDecl *VD) {
   VisitNamedDecl(VD);
   JOS.attribute("type", createQualType(VD->getType()));
+  if (const auto *P = dyn_cast<ParmVarDecl>(VD))
+    attributeOnlyIfTrue("explicitObjectParameter",
+                        P->isExplicitObjectParameter());
 
   StorageClass SC = VD->getStorageClass();
   if (SC != SC_None)
     JOS.attribute("storageClass", VarDecl::getStorageClassSpecifierString(SC));
   switch (VD->getTLSKind()) {
   case VarDecl::TLS_Dynamic: JOS.attribute("tls", "dynamic"); break;
   case VarDecl::TLS_Static: JOS.attribute("tls", "static"); break;
   case VarDecl::TLS_None: break;

clang/lib/AST/Mangle.cpp

#endif
   Out << '@';
   if (!Proto) {
     Out << '0';
     return;
   }
   assert(!Proto->isVariadic());
   unsigned ArgWords = 0;
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD))
-    if (!MD->isStatic())
+    if (MD->isImplicitObjectMemberFunction())
       ++ArgWords;
   uint64_t DefaultPtrWidth = TI.getPointerWidth(LangAS::Default);
   for (const auto &AT : Proto->param_types()) {
     // If an argument type is incomplete there is no way to get its size to
     // correctly encode into the mangling scheme.
     // Follow GCCs behaviour by simply breaking out of the loop.
     if (AT->isIncompleteType())
       break;

clang/lib/AST/MicrosoftMangle.cpp

void MicrosoftCXXNameMangler::mangleFunctionType(const FunctionType *T,
   if (D) Range = D->getSourceRange();
 
   bool IsInLambda = false;
   bool IsStructor = false, HasThisQuals = ForceThisQuals, IsCtorClosure = false;
   CallingConv CC = T->getCallConv();
   if (const CXXMethodDecl *MD = dyn_cast_or_null<CXXMethodDecl>(D)) {
     if (MD->getParent()->isLambda())
       IsInLambda = true;
-    if (MD->isInstance())
+    if (MD->isImplicitObjectMemberFunction())
       HasThisQuals = true;
     if (isa<CXXDestructorDecl>(MD)) {
       IsStructor = true;
     } else if (isa<CXXConstructorDecl>(MD)) {
       IsStructor = true;
       IsCtorClosure = (StructorType == Ctor_CopyingClosure ||
                        StructorType == Ctor_DefaultClosure) &&
                       isStructorDecl(MD);
if (!Proto) {
     // within an overloadable function in C. We mangle these as the absence of
     // any parameter types (not even an empty parameter list).
     Out << '@';
   } else if (Proto->getNumParams() == 0 && !Proto->isVariadic()) {
     Out << 'X';
   } else {
     // Happens for function pointer type arguments for example.
     for (unsigned I = 0, E = Proto->getNumParams(); I != E; ++I) {
+      // Explicit object parameters are prefixed by "_V".
+      if (I == 0 && D && D->getParamDecl(I)->isExplicitObjectParameter())
+        Out << "_V";
+
       mangleFunctionArgumentType(Proto->getParamType(I), Range);
       // Mangle each pass_object_size parameter as if it's a parameter of enum
       // type passed directly after the parameter with the pass_object_size
       // attribute. The aforementioned enum's name is __pass_object_size, and we
       // pretend it resides in a top-level namespace called __clang.
       //
       // FIXME: Is there a defined extension notation for the MS ABI, or is it
       // necessary to just cross our fingers and hope this type+namespace
if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD)) {
     if (isa<CXXDestructorDecl>(MD) && isStructorDecl(MD) &&
         StructorType == Dtor_Complete) {
       IsVirtual = false;
     }
     switch (MD->getAccess()) {
       case AS_none:
         llvm_unreachable("Unsupported access specifier");
       case AS_private:
-        if (MD->isStatic())
+        if (!MD->isImplicitObjectMemberFunction())
           Out << 'C';
         else if (IsVirtual)
           Out << 'E';
         else
           Out << 'A';
         break;
       case AS_protected:
-        if (MD->isStatic())
+        if (!MD->isImplicitObjectMemberFunction())
           Out << 'K';
         else if (IsVirtual)
           Out << 'M';
         else
           Out << 'I';
         break;
       case AS_public:
-        if (MD->isStatic())
+        if (!MD->isImplicitObjectMemberFunction())
           Out << 'S';
         else if (IsVirtual)
           Out << 'U';
         else
           Out << 'Q';
     }
   } else {
     Out << 'Y';

clang/lib/AST/TextNodeDumper.cpp

void TextNodeDumper::VisitFieldDecl(const FieldDecl *D) {
   if (D->isMutable())
     OS << " mutable";
   if (D->isModulePrivate())
     OS << " __module_private__";
 }
 
 void TextNodeDumper::VisitVarDecl(const VarDecl *D) {
   dumpName(D);
+  if (const auto *P = dyn_cast<ParmVarDecl>(D);
+      P && P->isExplicitObjectParameter())
+    OS << " this";
+
   dumpType(D->getType());
   StorageClass SC = D->getStorageClass();
   if (SC != SC_None)
     OS << ' ' << VarDecl::getStorageClassSpecifierString(SC);
   switch (D->getTLSKind()) {
   case VarDecl::TLS_None:
     break;
   case VarDecl::TLS_Static:

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

Environment::Environment(DataflowAnalysisContext &DACtx,
 
   if (const auto *MethodDecl = dyn_cast<CXXMethodDecl>(&DeclCtx)) {
     auto *Parent = MethodDecl->getParent();
     assert(Parent != nullptr);
     if (Parent->isLambda())
       MethodDecl = dyn_cast<CXXMethodDecl>(Parent->getDeclContext());
 
     // FIXME: Initialize the ThisPointeeLoc of lambdas too.
-    if (MethodDecl && !MethodDecl->isStatic()) {
-      QualType ThisPointeeType = MethodDecl->getThisObjectType();
+    if (MethodDecl && MethodDecl->isImplicitObjectMemberFunction()) {
+      QualType ThisPointeeType = MethodDecl->getFunctionObjectParameterType();
       ThisPointeeLoc =
           &cast<RecordValue>(createValue(ThisPointeeType))->getLoc();
     }
   }
 }
 
 bool Environment::canDescend(unsigned MaxDepth,
                              const DeclContext *Callee) const {

clang/lib/Analysis/ThreadSafetyCommon.cpp

CapabilityExpr SExprBuilder::translateAttrExpr(const Expr *AttrExp,
 
   if (Self) {
     assert(!Ctx.SelfArg && "Ambiguous self argument");
     Ctx.SelfArg = Self;
 
     // If the attribute has no arguments, then assume the argument is "this".
     if (!AttrExp)
       return CapabilityExpr(
-          Self, ClassifyDiagnostic(cast<CXXMethodDecl>(D)->getThisObjectType()),
+          Self,
+          ClassifyDiagnostic(
+              cast<CXXMethodDecl>(D)->getFunctionObjectParameterType()),
           false);
     else  // For most attributes.
       return translateAttrExpr(AttrExp, &Ctx);
   }
 
   // If the attribute has no arguments, then assume the argument is "this".
   if (!AttrExp)
     return translateAttrExpr(cast<const Expr *>(Ctx.SelfArg), nullptr);

clang/lib/CodeGen/CGCall.cpp

 CodeGenTypes::arrangeCXXMethodDeclaration(const CXXMethodDecl *MD) {
   assert(!isa<CXXConstructorDecl>(MD) && "wrong method for constructors!");
   assert(!isa<CXXDestructorDecl>(MD) && "wrong method for destructors!");
 
   CanQualType FT = GetFormalType(MD).getAs<Type>();
   setCUDAKernelCallingConvention(FT, CGM, MD);
   auto prototype = FT.getAs<FunctionProtoType>();
 
-  if (MD->isInstance()) {
+  if (MD->isImplicitObjectMemberFunction()) {
     // The abstract case is perfectly fine.
     const CXXRecordDecl *ThisType = TheCXXABI.getThisArgumentTypeForMethod(MD);
     return arrangeCXXMethodType(ThisType, prototype.getTypePtr(), MD);
   }
 
   return arrangeFreeFunctionType(prototype);
 }
 
return arrangeLLVMFunctionInfo(ResultType, FnInfoOpts::IsInstanceMethod,
                                  ArgTypes, Info, ParamInfos, Required);
 }
 
 /// Arrange the argument and result information for the declaration or
 /// definition of the given function.
 const CGFunctionInfo &
 CodeGenTypes::arrangeFunctionDeclaration(const FunctionDecl *FD) {
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD))
-    if (MD->isInstance())
+    if (MD->isImplicitObjectMemberFunction())
       return arrangeCXXMethodDeclaration(MD);
 
   CanQualType FTy = FD->getType()->getCanonicalTypeUnqualified();
 
   assert(isa<FunctionType>(FTy));
   setCUDAKernelCallingConvention(FTy, CGM, FD);
 
   // When declaring a function without a prototype, always use a

clang/lib/CodeGen/CGClass.cpp

struct CallBaseDtor final : EHScopeStack::Cleanup {
 
     void Emit(CodeGenFunction &CGF, Flags flags) override {
       const CXXRecordDecl *DerivedClass =
         cast<CXXMethodDecl>(CGF.CurCodeDecl)->getParent();
 
       const CXXDestructorDecl *D = BaseClass->getDestructor();
       // We are already inside a destructor, so presumably the object being
       // destroyed should have the expected type.
-      QualType ThisTy = D->getThisObjectType();
+      QualType ThisTy = D->getFunctionObjectParameterType();
       Address Addr =
         CGF.GetAddressOfDirectBaseInCompleteClass(CGF.LoadCXXThisAddress(),
                                                   DerivedClass, BaseClass,
                                                   BaseIsVirtual);
       CGF.EmitCXXDestructorCall(D, Dtor_Base, BaseIsVirtual,
                                 /*Delegating=*/false, Addr, ThisTy);
     }
   };
void CodeGenFunction::EmitDestructorBody(FunctionArgList &Args) {
   // The call to operator delete in a deleting destructor happens
   // outside of the function-try-block, which means it's always
   // possible to delegate the destructor body to the complete
   // destructor.  Do so.
   if (DtorType == Dtor_Deleting) {
     RunCleanupsScope DtorEpilogue(*this);
     EnterDtorCleanups(Dtor, Dtor_Deleting);
     if (HaveInsertPoint()) {
-      QualType ThisTy = Dtor->getThisObjectType();
+      QualType ThisTy = Dtor->getFunctionObjectParameterType();
       EmitCXXDestructorCall(Dtor, Dtor_Complete, /*ForVirtualBase=*/false,
                             /*Delegating=*/false, LoadCXXThisAddress(), ThisTy);
     }
     return;
   }
 
   // If the body is a function-try-block, enter the try before
   // anything else.
void CodeGenFunction::EmitDestructorBody(FunctionArgList &Args) {
   case Dtor_Complete:
     assert((Body || getTarget().getCXXABI().isMicrosoft()) &&
            "can't emit a dtor without a body for non-Microsoft ABIs");
 
     // Enter the cleanup scopes for virtual bases.
     EnterDtorCleanups(Dtor, Dtor_Complete);
 
     if (!isTryBody) {
-      QualType ThisTy = Dtor->getThisObjectType();
+      QualType ThisTy = Dtor->getFunctionObjectParameterType();
       EmitCXXDestructorCall(Dtor, Dtor_Base, /*ForVirtualBase=*/false,
                             /*Delegating=*/false, LoadCXXThisAddress(), ThisTy);
       break;
     }
 
     // Fallthrough: act like we're in the base variant.
     [[fallthrough]];
 
struct CallDelegatingCtorDtor final : EHScopeStack::Cleanup {
 
     CallDelegatingCtorDtor(const CXXDestructorDecl *D, Address Addr,
                            CXXDtorType Type)
       : Dtor(D), Addr(Addr), Type(Type) {}
 
     void Emit(CodeGenFunction &CGF, Flags flags) override {
       // We are calling the destructor from within the constructor.
       // Therefore, "this" should have the expected type.
-      QualType ThisTy = Dtor->getThisObjectType();
+      QualType ThisTy = Dtor->getFunctionObjectParameterType();
       CGF.EmitCXXDestructorCall(Dtor, Type, /*ForVirtualBase=*/false,
                                 /*Delegating=*/true, Addr, ThisTy);
     }
   };
 } // end anonymous namespace
 
 void
 CodeGenFunction::EmitDelegatingCXXConstructorCall(const CXXConstructorDecl *Ctor,

clang/lib/CodeGen/CGDebugInfo.cpp

for (unsigned i = 0, e = Args.Args.size(); i != e; ++i) {
     case TemplateArgument::Declaration: {
       const ValueDecl *D = TA.getAsDecl();
       QualType T = TA.getParamTypeForDecl().getDesugaredType(CGM.getContext());
       llvm::DIType *TTy = getOrCreateType(T, Unit);
       llvm::Constant *V = nullptr;
       // Skip retrieve the value if that template parameter has cuda device
       // attribute, i.e. that value is not available at the host side.
       if (!CGM.getLangOpts().CUDA || CGM.getLangOpts().CUDAIsDevice ||
           !D->hasAttr<CUDADeviceAttr>()) {
-        const CXXMethodDecl *MD;
         // Variable pointer template parameters have a value that is the address
         // of the variable.
         if (const auto *VD = dyn_cast<VarDecl>(D))
           V = CGM.GetAddrOfGlobalVar(VD);
         // Member function pointers have special support for building them,
         // though this is currently unsupported in LLVM CodeGen.
-        else if ((MD = dyn_cast<CXXMethodDecl>(D)) && MD->isInstance())
+        else if (const auto *MD = dyn_cast<CXXMethodDecl>(D);
+                 MD && MD->isImplicitObjectMemberFunction())
           V = CGM.getCXXABI().EmitMemberFunctionPointer(MD);
         else if (const auto *FD = dyn_cast<FunctionDecl>(D))
           V = CGM.GetAddrOfFunction(FD);
         // Member data pointers have special handling too to compute the fixed
         // offset within the object.
         else if (const auto *MPT =
                      dyn_cast<MemberPointerType>(T.getTypePtr())) {
           // These five lines (& possibly the above member function pointer
           // handling) might be able to be refactored to use similar code in
           // CodeGenModule::getMemberPointerConstant

clang/lib/CodeGen/CGExpr.cpp

static LValue EmitFunctionDeclLValue(CodeGenFunction &CGF, const Expr *E,
   llvm::Value *V = EmitFunctionDeclPointer(CGF.CGM, GD);
   CharUnits Alignment = CGF.getContext().getDeclAlign(FD);
   return CGF.MakeAddrLValue(V, E->getType(), Alignment,
                             AlignmentSource::Decl);
 }
 
 static LValue EmitCapturedFieldLValue(CodeGenFunction &CGF, const FieldDecl *FD,
                                       llvm::Value *ThisValue) {
-  QualType TagType = CGF.getContext().getTagDeclType(FD->getParent());
-  LValue LV = CGF.MakeNaturalAlignAddrLValue(ThisValue, TagType);
-  return CGF.EmitLValueForField(LV, FD);
+
+  return CGF.EmitLValueForLambdaField(FD, ThisValue);
 }
 
 /// Named Registers are named metadata pointing to the register name
 /// which will be read from/written to as an argument to the intrinsic
 /// @llvm.read/write_register.
 /// So far, only the name is being passed down, but other options such as
 /// register type, allocation type or even optimization options could be
 /// passed down via the metadata node.
 static LValue EmitGlobalNamedRegister(const VarDecl *VD, CodeGenModule &CGM) {
LValue CodeGenFunction::EmitMemberExpr(const MemberExpr *E) {
   if (const auto *FD = dyn_cast<FunctionDecl>(ND))
     return EmitFunctionDeclLValue(*this, E, FD);
 
   llvm_unreachable("Unhandled member declaration!");
 }
 
 /// Given that we are currently emitting a lambda, emit an l-value for
 /// one of its members.
-LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field) {
-  if (CurCodeDecl) {
-    assert(cast<CXXMethodDecl>(CurCodeDecl)->getParent()->isLambda());
-    assert(cast<CXXMethodDecl>(CurCodeDecl)->getParent() == Field->getParent());
-  }
-  QualType LambdaTagType =
-    getContext().getTagDeclType(Field->getParent());
-  LValue LambdaLV = MakeNaturalAlignAddrLValue(CXXABIThisValue, LambdaTagType);
+///
+LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field,
+                                                 llvm::Value *ThisValue) {
+  bool HasExplicitObjectParameter = false;
+  if (const auto *MD = dyn_cast_if_present<CXXMethodDecl>(CurCodeDecl)) {
+    HasExplicitObjectParameter = MD->isExplicitObjectMemberFunction();
+    assert(MD->getParent()->isLambda());
+    assert(MD->getParent() == Field->getParent());
+  }
+  LValue LambdaLV;
+  if (HasExplicitObjectParameter) {
+    const VarDecl *D = cast<CXXMethodDecl>(CurCodeDecl)->getParamDecl(0);
+    auto It = LocalDeclMap.find(D);
+    assert(It != LocalDeclMap.end() && "explicit parameter not loaded?");
+    Address AddrOfExplicitObject = It->getSecond();
+    if (D->getType()->isReferenceType())
+      LambdaLV = EmitLoadOfReferenceLValue(AddrOfExplicitObject, D->getType(),
+                                           AlignmentSource::Decl);
+    else
+      LambdaLV = MakeNaturalAlignAddrLValue(AddrOfExplicitObject.getPointer(),
+                                            D->getType().getNonReferenceType());
+  } else {
+    QualType LambdaTagType = getContext().getTagDeclType(Field->getParent());
+    LambdaLV = MakeNaturalAlignAddrLValue(ThisValue, LambdaTagType);
+  }
   return EmitLValueForField(LambdaLV, Field);
 }
 
+LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field) {
+  return EmitLValueForLambdaField(Field, CXXABIThisValue);
+}
+
 /// Get the field index in the debug info. The debug info structure/union
 /// will ignore the unnamed bitfields.
 unsigned CodeGenFunction::getDebugInfoFIndex(const RecordDecl *Rec,
                                              unsigned FieldIndex) {
   unsigned I = 0, Skipped = 0;
 
   for (auto *F : Rec->getDefinition()->fields()) {
     if (I == FieldIndex)
if (E->getCallee()->getType()->isBlockPointerType())
     return EmitBlockCallExpr(E, ReturnValue);
 
   if (const auto *CE = dyn_cast<CXXMemberCallExpr>(E))
     return EmitCXXMemberCallExpr(CE, ReturnValue);
 
   if (const auto *CE = dyn_cast<CUDAKernelCallExpr>(E))
     return EmitCUDAKernelCallExpr(CE, ReturnValue);
 
+  // A CXXOperatorCallExpr is created even for explicit object methods, but
+  // these should be treated like static function call.
   if (const auto *CE = dyn_cast<CXXOperatorCallExpr>(E))
-    if (const CXXMethodDecl *MD =
-          dyn_cast_or_null<CXXMethodDecl>(CE->getCalleeDecl()))
+    if (const auto *MD =
+            dyn_cast_if_present<CXXMethodDecl>(CE->getCalleeDecl());
+        MD && MD->isImplicitObjectMemberFunction())
       return EmitCXXOperatorMemberCallExpr(CE, MD, ReturnValue);
 
   CGCallee callee = EmitCallee(E->getCallee());
 
   if (callee.isBuiltin()) {
     return EmitBuiltinExpr(callee.getBuiltinDecl(), callee.getBuiltinID(),
                            E, ReturnValue);
   }

clang/lib/CodeGen/CGExprCXX.cpp

 commonEmitCXXMemberOrOperatorCall(CodeGenFunction &CGF, GlobalDecl GD,
                                   llvm::Value *This, llvm::Value *ImplicitParam,
                                   QualType ImplicitParamTy, const CallExpr *CE,
                                   CallArgList &Args, CallArgList *RtlArgs) {
   auto *MD = cast<CXXMethodDecl>(GD.getDecl());
 
   assert(CE == nullptr || isa<CXXMemberCallExpr>(CE) ||
          isa<CXXOperatorCallExpr>(CE));
-  assert(MD->isInstance() &&
+  assert(MD->isImplicitObjectMemberFunction() &&
          "Trying to emit a member or operator call expr on a static method!");
 
   // Push the this ptr.
   const CXXRecordDecl *RD =
       CGF.CGM.getCXXABI().getThisArgumentTypeForMethod(GD);
   Args.add(RValue::get(This), CGF.getTypes().DeriveThisType(RD, MD));
 
   // If there is an implicit parameter (e.g. VTT), emit it.
   if (ImplicitParam) {
     Args.add(RValue::get(ImplicitParam), ImplicitParamTy);
   }
 
   const FunctionProtoType *FPT = MD->getType()->castAs<FunctionProtoType>();
   RequiredArgs required = RequiredArgs::forPrototypePlus(FPT, Args.size());
   unsigned PrefixSize = Args.size() - 1;
 
   // And the rest of the call args.
   if (RtlArgs) {
     // Special case: if the caller emitted the arguments right-to-left already
     // (prior to emitting the *this argument), we're done. This happens for
     // assignment operators.
     Args.addFrom(*RtlArgs);
   } else if (CE) {
     // Special case: skip first argument of CXXOperatorCall (it is "this").
-    unsigned ArgsToSkip = isa<CXXOperatorCallExpr>(CE) ? 1 : 0;
+    unsigned ArgsToSkip = 0;
+    if (const auto *Op = dyn_cast<CXXOperatorCallExpr>(CE)) {
+      if (const auto *M = dyn_cast<CXXMethodDecl>(Op->getCalleeDecl()))
+        ArgsToSkip = (unsigned)!M->isExplicitObjectMemberFunction();
+    }
     CGF.EmitCallArgs(Args, FPT, drop_begin(CE->arguments(), ArgsToSkip),
                      CE->getDirectCallee());
   } else {
     assert(
         FPT->getNumParams() == 0 &&
         "No CallExpr specified for function with non-zero number of arguments");
   }
   return {required, PrefixSize};
 }
return EmitCall(CGM.getTypes().arrangeCXXMethodCall(Args, FPT, required,
                   Callee, ReturnValue, Args, nullptr, E == MustTailCall,
                   E->getExprLoc());
 }
 
 RValue
 CodeGenFunction::EmitCXXOperatorMemberCallExpr(const CXXOperatorCallExpr *E,
                                                const CXXMethodDecl *MD,
                                                ReturnValueSlot ReturnValue) {
-  assert(MD->isInstance() &&
+  assert(MD->isImplicitObjectMemberFunction() &&
          "Trying to emit a member call expr on a static method!");
   return EmitCXXMemberOrOperatorMemberCallExpr(
       E, MD, ReturnValue, /*HasQualifier=*/false, /*Qualifier=*/nullptr,
       /*IsArrow=*/false, E->getArg(0));
 }
 
 RValue CodeGenFunction::EmitCUDAKernelCallExpr(const CUDAKernelCallExpr *E,
                                                ReturnValueSlot ReturnValue) {

clang/lib/CodeGen/CodeGenFunction.h

public:
   LValue EmitPseudoObjectLValue(const PseudoObjectExpr *e);
 
   llvm::Value *EmitIvarOffset(const ObjCInterfaceDecl *Interface,
                               const ObjCIvarDecl *Ivar);
   llvm::Value *EmitIvarOffsetAsPointerDiff(const ObjCInterfaceDecl *Interface,
                                            const ObjCIvarDecl *Ivar);
   LValue EmitLValueForField(LValue Base, const FieldDecl* Field);
   LValue EmitLValueForLambdaField(const FieldDecl *Field);
+  LValue EmitLValueForLambdaField(const FieldDecl *Field,
+                                  llvm::Value *ThisValue);
 
   /// EmitLValueForFieldInitialization - Like EmitLValueForField, except that
   /// if the Field is a reference, this will return the address of the reference
   /// and not the address of the value stored in the reference.
   LValue EmitLValueForFieldInitialization(LValue Base,
                                           const FieldDecl* Field);
 
   LValue EmitLValueForIvar(QualType ObjectTy,

clang/lib/CodeGen/CodeGenFunction.cpp

if (getLangOpts().OpenMP && CurCodeDecl)
     CGM.getOpenMPRuntime().emitFunctionProlog(*this, CurCodeDecl);
 
   // Handle emitting HLSL entry functions.
   if (D && D->hasAttr<HLSLShaderAttr>())
     CGM.getHLSLRuntime().emitEntryFunction(FD, Fn);
 
   EmitFunctionProlog(*CurFnInfo, CurFn, Args);
 
-  if (isa_and_nonnull<CXXMethodDecl>(D) &&
-      cast<CXXMethodDecl>(D)->isInstance()) {
+  if (const CXXMethodDecl *MD = dyn_cast_if_present<CXXMethodDecl>(D);
+      MD && !MD->isStatic()) {
+    bool IsInLambda =
+        MD->getParent()->isLambda() && MD->getOverloadedOperator() == OO_Call;
+    if (MD->isImplicitObjectMemberFunction())
       CGM.getCXXABI().EmitInstanceFunctionProlog(*this);
-    const CXXMethodDecl *MD = cast<CXXMethodDecl>(D);
-    if (MD->getParent()->isLambda() &&
-        MD->getOverloadedOperator() == OO_Call) {
+    if (IsInLambda) {
       // We're in a lambda; figure out the captures.
       MD->getParent()->getCaptureFields(LambdaCaptureFields,
                                         LambdaThisCaptureField);
       if (LambdaThisCaptureField) {
         // If the lambda captures the object referred to by '*this' - either by
         // value or by reference, make sure CXXThisValue points to the correct
         // object.
 
if (IsInLambda) {
       for (auto *FD : MD->getParent()->fields()) {
         if (FD->hasCapturedVLAType()) {
           auto *ExprArg = EmitLoadOfLValue(EmitLValueForLambdaField(FD),
                                            SourceLocation()).getScalarVal();
           auto VAT = FD->getCapturedVLAType();
           VLASizeMap[VAT->getSizeExpr()] = ExprArg;
         }
       }
-    } else {
+    } else if (MD->isImplicitObjectMemberFunction()) {
       // Not in a lambda; just use 'this' from the method.
       // FIXME: Should we generate a new load for each use of 'this'?  The
       // fast register allocator would be happier...
       CXXThisValue = CXXABIThisValue;
     }
 
     // Check the 'this' pointer once per function, if it's available.
     if (CXXABIThisValue) {

 }
 
 QualType CodeGenFunction::BuildFunctionArgList(GlobalDecl GD,
                                                FunctionArgList &Args) {
   const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
   QualType ResTy = FD->getReturnType();
 
   const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD);
-  if (MD && MD->isInstance()) {
+  if (MD && MD->isImplicitObjectMemberFunction()) {
     if (CGM.getCXXABI().HasThisReturn(GD))
       ResTy = MD->getThisType();
     else if (CGM.getCXXABI().hasMostDerivedReturn(GD))
       ResTy = CGM.getContext().VoidPtrTy;
     CGM.getCXXABI().buildThisParam(*this, Args);
   }
 
   // The base version of an inheriting constructor whose constructed base is a

clang/lib/CodeGen/CodeGenModule.cpp

static bool requiresMemberFunctionPointerTypeMetadata(CodeGenModule &CGM,
                                                       const CXXMethodDecl *MD) {
   // Check that the type metadata can ever actually be used by a call.
   if (!CGM.getCodeGenOpts().LTOUnit ||
       !CGM.HasHiddenLTOVisibility(MD->getParent()))
     return false;
 
   // Only functions whose address can be taken with a member function pointer
   // need this sort of type metadata.
-  return !MD->isStatic() && !MD->isVirtual() && !isa<CXXConstructorDecl>(MD) &&
-         !isa<CXXDestructorDecl>(MD);
+  return MD->isImplicitObjectMemberFunction() && !MD->isVirtual() &&
+         !isa<CXXConstructorDecl, CXXDestructorDecl>(MD);
 }
 
 SmallVector<const CXXRecordDecl *, 0>
 CodeGenModule::getMostBaseClasses(const CXXRecordDecl *RD) {
   llvm::SetVector<const CXXRecordDecl *> MostBases;
 
   std::function<void (const CXXRecordDecl *)> CollectMostBases;
   CollectMostBases = [&](const CXXRecordDecl *RD) {
     if (RD->getNumBases() == 0)

clang/lib/Parse/ParseDecl.cpp

#include "clang/Basic/OpenCLImageTypes.def"
   case tok::kw_private:
     return getLangOpts().OpenCL;
   }
 }
 
 bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
                                      DeclSpec::FriendSpecified IsFriend,
                                      const ParsedTemplateInfo *TemplateInfo) {
-  TentativeParsingAction TPA(*this);
-
+  RevertingTentativeParsingAction TPA(*this);
   // Parse the C++ scope specifier.
   CXXScopeSpec SS;
   if (TemplateInfo && TemplateInfo->TemplateParams)
     SS.setTemplateParamLists(*TemplateInfo->TemplateParams);
 
   if (ParseOptionalCXXScopeSpecifier(SS, /*ObjectType=*/nullptr,
                                      /*ObjectHasErrors=*/false,
                                      /*EnteringContext=*/true)) {
-    TPA.Revert();
     return false;
   }
 
   // Parse the constructor name.
   if (Tok.is(tok::identifier)) {
     // We already know that we have a constructor name; just consume
     // the token.
     ConsumeToken();
   } else if (Tok.is(tok::annot_template_id)) {
     ConsumeAnnotationToken();
   } else {
-    TPA.Revert();
     return false;
   }
 
   // There may be attributes here, appertaining to the constructor name or type
   // we just stepped past.
   SkipCXX11Attributes();
 
   // Current class name must be followed by a left parenthesis.
   if (Tok.isNot(tok::l_paren)) {
-    TPA.Revert();
     return false;
   }
   ConsumeParen();
 
   // A right parenthesis, or ellipsis followed by a right parenthesis signals
   // that we have a constructor.
   if (Tok.is(tok::r_paren) ||
       (Tok.is(tok::ellipsis) && NextToken().is(tok::r_paren))) {
-    TPA.Revert();
     return true;
   }
 
   // A C++11 attribute here signals that we have a constructor, and is an
   // attribute on the first constructor parameter.
   if (getLangOpts().CPlusPlus11 &&
       isCXX11AttributeSpecifier(/*Disambiguate*/ false,
                                 /*OuterMightBeMessageSend*/ true)) {
-    TPA.Revert();
     return true;
   }
 
   // If we need to, enter the specified scope.
   DeclaratorScopeObj DeclScopeObj(*this, SS);
   if (SS.isSet() && Actions.ShouldEnterDeclaratorScope(getCurScope(), SS))
     DeclScopeObj.EnterDeclaratorScope();
 
   // Optionally skip Microsoft attributes.
   ParsedAttributes Attrs(AttrFactory);
   MaybeParseMicrosoftAttributes(Attrs);
 
   // Check whether the next token(s) are part of a declaration
   // specifier, in which case we have the start of a parameter and,
   // therefore, we know that this is a constructor.
   // Due to an ambiguity with implicit typename, the above is not enough.
   // Additionally, check to see if we are a friend.
   // If we parsed a scope specifier as well as friend,
   // we might be parsing a friend constructor.
   bool IsConstructor = false;
-  if (isDeclarationSpecifier(IsFriend && !SS.isSet()
+  ImplicitTypenameContext ITC = IsFriend && !SS.isSet()
                                     ? ImplicitTypenameContext::No
-                                 : ImplicitTypenameContext::Yes))
+                                    : ImplicitTypenameContext::Yes;
+  // Constructors cannot have this parameters, but we support that scenario here
+  // to improve diagnostic.
+  if (Tok.is(tok::kw_this)) {
+    ConsumeToken();
+    return isDeclarationSpecifier(ITC);
+  }
+
+  if (isDeclarationSpecifier(ITC))
     IsConstructor = true;
   else if (Tok.is(tok::identifier) ||
            (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier))) {
     // We've seen "C ( X" or "C ( X::Y", but "X" / "X::Y" is not a type.
     // This might be a parenthesized member name, but is more likely to
     // be a constructor declaration with an invalid argument type. Keep
     // looking.
     if (Tok.is(tok::annot_cxxscope))
case tok::r_paren:
       }
       break;
 
     default:
       IsConstructor = true;
       break;
     }
   }
-
-  TPA.Revert();
   return IsConstructor;
 }
 
 /// ParseTypeQualifierListOpt
 ///          type-qualifier-list: [C99 6.7.5]
 ///            type-qualifier
 /// [vendor]   attributes
 ///              [ only if AttrReqs & AR_VendorAttributesParsed ]

 /// [C++]   declaration-specifiers declarator '=' assignment-expression
 /// [C++11]                                       initializer-clause
 /// [GNU]   declaration-specifiers declarator attributes
 ///         declaration-specifiers abstract-declarator[opt]
 /// [C++]   declaration-specifiers abstract-declarator[opt]
 ///           '=' assignment-expression
 /// [GNU]   declaration-specifiers abstract-declarator[opt] attributes
 /// [C++11] attribute-specifier-seq parameter-declaration
+/// [C++2b] attribute-specifier-seq 'this' parameter-declaration
 ///
 void Parser::ParseParameterDeclarationClause(
     DeclaratorContext DeclaratorCtx, ParsedAttributes &FirstArgAttrs,
     SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
     SourceLocation &EllipsisLoc, bool IsACXXFunctionDeclaration) {
 
   // Avoid exceeding the maximum function scope depth.
   // See https://bugs.llvm.org/show_bug.cgi?id=19607
if (FirstArgAttrs.Range.isValid()) {
       MaybeParseCXX11Attributes(ArgDeclAttrs);
 
       // Skip any Microsoft attributes before a param.
       MaybeParseMicrosoftAttributes(ArgDeclSpecAttrs);
     }
 
     SourceLocation DSStart = Tok.getLocation();
 
+    // Parse a C++23 Explicit Object Parameter
+    // We do that in all language modes to produce a better diagnostic.
+    SourceLocation ThisLoc;
+    if (getLangOpts().CPlusPlus && Tok.is(tok::kw_this))
+      ThisLoc = ConsumeToken();
+
     ParseDeclarationSpecifiers(DS, /*TemplateInfo=*/ParsedTemplateInfo(),
                                AS_none, DeclSpecContext::DSC_normal,
                                /*LateAttrs=*/nullptr, AllowImplicitTypename);
+
     DS.takeAttributesFrom(ArgDeclSpecAttrs);
 
     // Parse the declarator.  This is "PrototypeContext" or
     // "LambdaExprParameterContext", because we must accept either
     // 'declarator' or 'abstract-declarator' here.
     Declarator ParmDeclarator(DS, ArgDeclAttrs,
                               DeclaratorCtx == DeclaratorContext::RequiresExpr
                                   ? DeclaratorContext::RequiresExpr
                               : DeclaratorCtx == DeclaratorContext::LambdaExpr
                                   ? DeclaratorContext::LambdaExprParameter
                                   : DeclaratorContext::Prototype);
     ParseDeclarator(ParmDeclarator);
 
+    if (ThisLoc.isValid())
+      ParmDeclarator.SetRangeBegin(ThisLoc);
+
     // Parse GNU attributes, if present.
     MaybeParseGNUAttributes(ParmDeclarator);
     if (getLangOpts().HLSL)
       MaybeParseHLSLSemantics(DS.getAttributes());
 
     if (Tok.is(tok::kw_requires)) {
       // User tried to define a requires clause in a parameter declaration,
       // which is surely not a function declaration.
if (DS.isEmpty() && ParmDeclarator.getIdentifier() == nullptr &&
           Tok.getIdentifierInfo() &&
           Tok.getIdentifierInfo()->isKeyword(getLangOpts())) {
         Diag(Tok, diag::err_keyword_as_parameter) << PP.getSpelling(Tok);
         // Consume the keyword.
         ConsumeToken();
       }
       // Inform the actions module about the parameter declarator, so it gets
       // added to the current scope.
-      Decl *Param = Actions.ActOnParamDeclarator(getCurScope(), ParmDeclarator);
+      Decl *Param =
+          Actions.ActOnParamDeclarator(getCurScope(), ParmDeclarator, ThisLoc);
       // Parse the default argument, if any. We parse the default
       // arguments in all dialects; the semantic analysis in
       // ActOnParamDefaultArgument will reject the default argument in
       // C.
       if (Tok.is(tok::equal)) {
         SourceLocation EqualLoc = Tok.getLocation();
 
         // Parse the default argument

clang/lib/Parse/ParseTentative.cpp

case tok::kw___funcref:
     // Borland
   case tok::kw___pascal:
     return TPResult::True;
 
     // AltiVec
   case tok::kw___vector:
     return TPResult::True;
 
+  case tok::kw_this: {
+    // Try to parse a C++23 Explicit Object Parameter
+    // We do that in all language modes to produce a better diagnostic.
+    if (getLangOpts().CPlusPlus) {
+      RevertingTentativeParsingAction PA(*this);
+      ConsumeToken();
+      return isCXXDeclarationSpecifier(AllowImplicitTypename, BracedCastResult,
+                                       InvalidAsDeclSpec);
+    }
+    return TPResult::False;
+  }
   case tok::annot_template_id: {
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     // If lookup for the template-name found nothing, don't assume we have a
     // definitive disambiguation result yet.
     if ((TemplateId->hasInvalidName() ||
          TemplateId->Kind == TNK_Undeclared_template) &&
         InvalidAsDeclSpec) {
       // 'template-id(' can be a valid expression but not a valid decl spec if

clang/lib/Sema/ScopeInfo.cpp

if (auto *DRE = dyn_cast<DeclRefExpr>(E)) {
       for (ValueDecl *VD : *FP)
         Callback(VD, E);
     } else {
       llvm_unreachable("unexpected expression in potential captures list");
     }
   }
 }
 
+bool LambdaScopeInfo::lambdaCaptureShouldBeConst() const {
+  if (ExplicitObjectParameter)
+    return ExplicitObjectParameter->getType()
+        .getNonReferenceType()
+        .isConstQualified();
+  return !Mutable;
+}
+
 FunctionScopeInfo::~FunctionScopeInfo() { }
 BlockScopeInfo::~BlockScopeInfo() { }
 CapturedRegionScopeInfo::~CapturedRegionScopeInfo() { }

clang/lib/Sema/SemaChecking.cpp

 static bool checkArgCountAtLeast(Sema &S, CallExpr *Call,
                                  unsigned MinArgCount) {
   unsigned ArgCount = Call->getNumArgs();
   if (ArgCount >= MinArgCount)
     return false;
 
   return S.Diag(Call->getEndLoc(), diag::err_typecheck_call_too_few_args)
          << 0 /*function call*/ << MinArgCount << ArgCount
-         << Call->getSourceRange();
+         << /*is non object*/ 0 << Call->getSourceRange();
 }
 
 /// Checks that a call expression's argument count is at most the desired
 /// number. This is useful when doing custom type-checking on a variadic
 /// function. Returns true on error.
 static bool checkArgCountAtMost(Sema &S, CallExpr *Call, unsigned MaxArgCount) {
   unsigned ArgCount = Call->getNumArgs();
   if (ArgCount <= MaxArgCount)
     return false;
   return S.Diag(Call->getEndLoc(),
                 diag::err_typecheck_call_too_many_args_at_most)
          << 0 /*function call*/ << MaxArgCount << ArgCount
-         << Call->getSourceRange();
+         << /*is non object*/ 0 << Call->getSourceRange();
 }
 
 /// Checks that a call expression's argument count is in the desired range. This
 /// is useful when doing custom type-checking on a variadic function. Returns
 /// true on error.
 static bool checkArgCountRange(Sema &S, CallExpr *Call, unsigned MinArgCount,
                                unsigned MaxArgCount) {
   return checkArgCountAtLeast(S, Call, MinArgCount) ||
static bool checkArgCount(Sema &S, CallExpr *Call, unsigned DesiredArgCount) {
   assert(ArgCount > DesiredArgCount && "should have diagnosed this");
 
   // Highlight all the excess arguments.
   SourceRange Range(Call->getArg(DesiredArgCount)->getBeginLoc(),
                     Call->getArg(ArgCount - 1)->getEndLoc());
 
   return S.Diag(Range.getBegin(), diag::err_typecheck_call_too_many_args)
          << 0 /*function call*/ << DesiredArgCount << ArgCount
-         << Call->getArg(1)->getSourceRange();
+         << /*is non object*/ 0 << Call->getArg(1)->getSourceRange();
 }
 
 static bool convertArgumentToType(Sema &S, Expr *&Value, QualType Ty) {
   if (Value->isTypeDependent())
     return false;
 
   InitializedEntity Entity =
       InitializedEntity::InitializeParameter(S.Context, Ty, false);
static bool SemaBuiltinAnnotation(Sema &S, CallExpr *TheCall) {
   TheCall->setType(Ty);
   return false;
 }
 
 static bool SemaBuiltinMSVCAnnotation(Sema &S, CallExpr *TheCall) {
   // We need at least one argument.
   if (TheCall->getNumArgs() < 1) {
     S.Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 1 << TheCall->getNumArgs()
+        << 0 << 1 << TheCall->getNumArgs() << /*is non object*/ 0
         << TheCall->getCallee()->getSourceRange();
     return true;
   }
 
   // All arguments should be wide string literals.
   for (Expr *Arg : TheCall->arguments()) {
     auto *Literal = dyn_cast<StringLiteral>(Arg->IgnoreParenCasts());
     if (!Literal || !Literal->isWide()) {

 ///                    void (^block)(local void*, ...),
 ///                    uint size0, ...)
 static bool SemaOpenCLBuiltinEnqueueKernel(Sema &S, CallExpr *TheCall) {
   unsigned NumArgs = TheCall->getNumArgs();
 
   if (NumArgs < 4) {
     S.Diag(TheCall->getBeginLoc(),
            diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 4 << NumArgs;
+        << 0 << 4 << NumArgs << /*is non object*/ 0;
     return true;
   }
 
   Expr *Arg0 = TheCall->getArg(0);
   Expr *Arg1 = TheCall->getArg(1);
   Expr *Arg2 = TheCall->getArg(2);
   Expr *Arg3 = TheCall->getArg(3);
 

 void Sema::CheckConstructorCall(FunctionDecl *FDecl, QualType ThisType,
                                 ArrayRef<const Expr *> Args,
                                 const FunctionProtoType *Proto,
                                 SourceLocation Loc) {
   VariadicCallType CallType =
       Proto->isVariadic() ? VariadicConstructor : VariadicDoesNotApply;
 
   auto *Ctor = cast<CXXConstructorDecl>(FDecl);
-  CheckArgAlignment(Loc, FDecl, "'this'", Context.getPointerType(ThisType),
-                    Context.getPointerType(Ctor->getThisObjectType()));
+  CheckArgAlignment(
+      Loc, FDecl, "'this'", Context.getPointerType(ThisType),
+      Context.getPointerType(Ctor->getFunctionObjectParameterType()));
 
   checkCall(FDecl, Proto, /*ThisArg=*/nullptr, Args, /*IsMemberFunction=*/true,
             Loc, SourceRange(), CallType);
 }
 
 /// CheckFunctionCall - Check a direct function call for various correctness
 /// and safety properties not strictly enforced by the C type system.
 bool Sema::CheckFunctionCall(FunctionDecl *FDecl, CallExpr *TheCall,
                              const FunctionProtoType *Proto) {
   bool IsMemberOperatorCall = isa<CXXOperatorCallExpr>(TheCall) &&
                               isa<CXXMethodDecl>(FDecl);
   bool IsMemberFunction = isa<CXXMemberCallExpr>(TheCall) ||
                           IsMemberOperatorCall;
   VariadicCallType CallType = getVariadicCallType(FDecl, Proto,
                                                   TheCall->getCallee());
   Expr** Args = TheCall->getArgs();
   unsigned NumArgs = TheCall->getNumArgs();
 
   Expr *ImplicitThis = nullptr;
-  if (IsMemberOperatorCall && !FDecl->isStatic()) {
+  if (IsMemberOperatorCall && !FDecl->isStatic() &&
+      !FDecl->hasCXXExplicitFunctionObjectParameter()) {
     // If this is a call to a non-static member operator, hide the first
     // argument from checkCall.
     // FIXME: Our choice of AST representation here is less than ideal.
     ImplicitThis = Args[0];
     ++Args;
     --NumArgs;
-  } else if (IsMemberFunction && !FDecl->isStatic())
+  } else if (IsMemberFunction && !FDecl->isStatic() &&
+             !FDecl->hasCXXExplicitFunctionObjectParameter())
     ImplicitThis =
         cast<CXXMemberCallExpr>(TheCall)->getImplicitObjectArgument();
 
   if (ImplicitThis) {
     // ImplicitThis may or may not be a pointer, depending on whether . or -> is
     // used.
     QualType ThisType = ImplicitThis->getType();
     if (!ThisType->isPointerType()) {
       assert(!ThisType->isReferenceType());
       ThisType = Context.getPointerType(ThisType);
     }
 
-    QualType ThisTypeFromDecl =
-        Context.getPointerType(cast<CXXMethodDecl>(FDecl)->getThisObjectType());
+    QualType ThisTypeFromDecl = Context.getPointerType(
+        cast<CXXMethodDecl>(FDecl)->getFunctionObjectParameterType());
 
     CheckArgAlignment(TheCall->getRParenLoc(), FDecl, "'this'", ThisType,
                       ThisTypeFromDecl);
   }
 
   checkCall(FDecl, Proto, ImplicitThis, llvm::ArrayRef(Args, NumArgs),
             IsMemberFunction, TheCall->getRParenLoc(),
             TheCall->getCallee()->getSourceRange(), CallType);
ExprResult Sema::BuildAtomicExpr(SourceRange CallRange, SourceRange ExprRange,
 
   unsigned AdjustedNumArgs = NumArgs[Form];
   if ((IsOpenCL || IsHIP) && Op != AtomicExpr::AO__opencl_atomic_init)
     ++AdjustedNumArgs;
   // Check we have the right number of arguments.
   if (Args.size() < AdjustedNumArgs) {
     Diag(CallRange.getEnd(), diag::err_typecheck_call_too_few_args)
         << 0 << AdjustedNumArgs << static_cast<unsigned>(Args.size())
-        << ExprRange;
+        << /*is non object*/ 0 << ExprRange;
     return ExprError();
   } else if (Args.size() > AdjustedNumArgs) {
     Diag(Args[AdjustedNumArgs]->getBeginLoc(),
          diag::err_typecheck_call_too_many_args)
         << 0 << AdjustedNumArgs << static_cast<unsigned>(Args.size())
-        << ExprRange;
+        << /*is non object*/ 0 << ExprRange;
     return ExprError();
   }
 
   // Inspect the first argument of the atomic operation.
   Expr *Ptr = Args[0];
   ExprResult ConvertedPtr = DefaultFunctionArrayLvalueConversion(Ptr);
   if (ConvertedPtr.isInvalid())
     return ExprError();
bool Sema::BuiltinWasmRefNullExtern(CallExpr *TheCall) {
   TheCall->setType(Context.getWebAssemblyExternrefType());
 
   return false;
 }
 
 bool Sema::BuiltinWasmRefNullFunc(CallExpr *TheCall) {
   if (TheCall->getNumArgs() != 0) {
     Diag(TheCall->getBeginLoc(), diag::err_typecheck_call_too_many_args)
-         << 0 /*function call*/ << 0 << TheCall->getNumArgs();
+        << 0 /*function call*/ << /*expected*/ 0 << TheCall->getNumArgs()
+        << /*is non object*/ 0;
     return true;
   }
 
   // This custom type checking code ensures that the nodes are as expected
   // in order to later on generate the necessary builtin.
   QualType Pointee = Context.getFunctionType(Context.VoidTy, {}, {});
   QualType Type = Context.getPointerType(Pointee);
   Pointee = Context.getAddrSpaceQualType(Pointee, LangAS::wasm_funcref);
Sema::SemaBuiltinAtomicOverloaded(ExprResult TheCallResult) {
   CallExpr *TheCall = static_cast<CallExpr *>(TheCallResult.get());
   Expr *Callee = TheCall->getCallee();
   DeclRefExpr *DRE = cast<DeclRefExpr>(Callee->IgnoreParenCasts());
   FunctionDecl *FDecl = cast<FunctionDecl>(DRE->getDecl());
 
   // Ensure that we have at least one argument to do type inference from.
   if (TheCall->getNumArgs() < 1) {
     Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 1 << TheCall->getNumArgs() << Callee->getSourceRange();
+        << 0 << 1 << TheCall->getNumArgs() << /*is non object*/ 0
+        << Callee->getSourceRange();
     return ExprError();
   }
 
   // Inspect the first argument of the atomic builtin.  This should always be
   // a pointer type, whose element is an integral scalar or pointer type.
   // Because it is a pointer type, we don't have to worry about any implicit
   // casts here.
   // FIXME: We don't allow floating point scalars as input.
case Builtin::BI__sync_swap_16:
     BuiltinIndex = 16;
     break;
   }
 
   // Now that we know how many fixed arguments we expect, first check that we
   // have at least that many.
   if (TheCall->getNumArgs() < 1+NumFixed) {
     Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 1 + NumFixed << TheCall->getNumArgs()
+        << 0 << 1 + NumFixed << TheCall->getNumArgs() << /*is non object*/ 0
         << Callee->getSourceRange();
     return ExprError();
   }
 
   Diag(TheCall->getEndLoc(), diag::warn_atomic_implicit_seq_cst)
       << Callee->getSourceRange();
 
   if (WarnAboutSemanticsChange) {
bool Sema::SemaBuiltinVAStartARMMicrosoft(CallExpr *Call) {
   // void __va_start(va_list *ap, const char *named_addr, size_t slot_size,
   //                 const char *named_addr);
 
   Expr *Func = Call->getCallee();
 
   if (Call->getNumArgs() < 3)
     return Diag(Call->getEndLoc(),
                 diag::err_typecheck_call_too_few_args_at_least)
-           << 0 /*function call*/ << 3 << Call->getNumArgs();
+           << 0 /*function call*/ << 3 << Call->getNumArgs()
+           << /*is non object*/ 0;
 
   // Type-check the first argument normally.
   if (checkBuiltinArgument(*this, Call, 0))
     return true;
 
   // Check that the current function is variadic.
   if (checkVAStartIsInVariadicFunction(*this, Func))
     return true;

 
 /// SemaBuiltinShuffleVector - Handle __builtin_shufflevector.
 // This is declared to take (...), so we have to check everything.
 ExprResult Sema::SemaBuiltinShuffleVector(CallExpr *TheCall) {
   if (TheCall->getNumArgs() < 2)
     return ExprError(Diag(TheCall->getEndLoc(),
                           diag::err_typecheck_call_too_few_args_at_least)
                      << 0 /*function call*/ << 2 << TheCall->getNumArgs()
-                     << TheCall->getSourceRange());
+                     << /*is non object*/ 0 << TheCall->getSourceRange());
 
   // Determine which of the following types of shufflevector we're checking:
   // 1) unary, vector mask: (lhs, mask)
   // 2) binary, scalar mask: (lhs, rhs, index, ..., index)
   QualType resType = TheCall->getArg(0)->getType();
   unsigned numElements = 0;
 
   if (!TheCall->getArg(0)->isTypeDependent() &&

 // This is declared to take (const void*, ...) and can take two
 // optional constant int args.
 bool Sema::SemaBuiltinPrefetch(CallExpr *TheCall) {
   unsigned NumArgs = TheCall->getNumArgs();
 
   if (NumArgs > 3)
     return Diag(TheCall->getEndLoc(),
                 diag::err_typecheck_call_too_many_args_at_most)
-           << 0 /*function call*/ << 3 << NumArgs << TheCall->getSourceRange();
+           << 0 /*function call*/ << 3 << NumArgs << /*is non object*/ 0
+           << TheCall->getSourceRange();
 
   // Argument 0 is checked for us and the remaining arguments must be
   // constant integers.
   for (unsigned i = 1; i != NumArgs; ++i)
     if (SemaBuiltinConstantArgRange(TheCall, i, 0, i == 1 ? 1 : 3))
       return true;
 
   return false;
unsigned BuiltinID =
       cast<FunctionDecl>(TheCall->getCalleeDecl())->getBuiltinID();
   bool IsSizeCall = BuiltinID == Builtin::BI__builtin_os_log_format_buffer_size;
 
   unsigned NumArgs = TheCall->getNumArgs();
   unsigned NumRequiredArgs = IsSizeCall ? 1 : 2;
   if (NumArgs < NumRequiredArgs) {
     return Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args)
            << 0 /* function call */ << NumRequiredArgs << NumArgs
-           << TheCall->getSourceRange();
+           << /*is non object*/ 0 << TheCall->getSourceRange();
   }
   if (NumArgs >= NumRequiredArgs + 0x100) {
     return Diag(TheCall->getEndLoc(),
                 diag::err_typecheck_call_too_many_args_at_most)
            << 0 /* function call */ << (NumRequiredArgs + 0xff) << NumArgs
-           << TheCall->getSourceRange();
+           << /*is non object*/ 0 << TheCall->getSourceRange();
   }
   unsigned i = 0;
 
   // For formatting call, check buffer arg.
   if (!IsSizeCall) {
     ExprResult Arg(TheCall->getArg(i));
     InitializedEntity Entity = InitializedEntity::InitializeParameter(
         Context, Context.VoidPtrTy, false);

clang/lib/Sema/SemaCoroutine.cpp

static QualType lookupPromiseType(Sema &S, const FunctionDecl *FD,
   auto AddArg = [&](QualType T) {
     Args.addArgument(TemplateArgumentLoc(
         TemplateArgument(T), S.Context.getTrivialTypeSourceInfo(T, KwLoc)));
   };
   AddArg(FnType->getReturnType());
   // If the function is a non-static member function, add the type
   // of the implicit object parameter before the formal parameters.
   if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
-    if (MD->isInstance()) {
+    if (MD->isImplicitObjectMemberFunction()) {
       // [over.match.funcs]4
       // For non-static member functions, the type of the implicit object
       // parameter is
       //  -- "lvalue reference to cv X" for functions declared without a
       //      ref-qualifier or with the & ref-qualifier
       //  -- "rvalue reference to cv X" for functions declared with the &&
       //      ref-qualifier
       QualType T = MD->getThisType()->castAs<PointerType>()->getPointeeType();
if (PromiseRef.isInvalid())
     return ExprError();
 
   return buildMemberCall(S, PromiseRef.get(), Loc, Name, Args);
 }
 
 VarDecl *Sema::buildCoroutinePromise(SourceLocation Loc) {
   assert(isa<FunctionDecl>(CurContext) && "not in a function scope");
   auto *FD = cast<FunctionDecl>(CurContext);
   bool IsThisDependentType = [&] {
-    if (auto *MD = dyn_cast_or_null<CXXMethodDecl>(FD))
-      return MD->isInstance() && MD->getThisType()->isDependentType();
-    else
-      return false;
+    if (const auto *MD = dyn_cast_if_present<CXXMethodDecl>(FD))
+      return MD->isImplicitObjectMemberFunction() &&
+             MD->getThisType()->isDependentType();
   }();
 
   QualType T = FD->getType()->isDependentType() || IsThisDependentType
                    ? Context.DependentTy
                    : lookupPromiseType(*this, FD, Loc);
   if (T.isNull())
     return nullptr;
 
   auto *VD = VarDecl::Create(Context, FD, FD->getLocation(), FD->getLocation(),
                              &PP.getIdentifierTable().get("__promise"), T,
                              Context.getTrivialTypeSourceInfo(T, Loc), SC_None);
   VD->setImplicit();
   CheckVariableDeclarationType(VD);
   if (VD->isInvalidDecl())
     return nullptr;
 
   auto *ScopeInfo = getCurFunction();
 
   // Build a list of arguments, based on the coroutine function's arguments,
   // that if present will be passed to the promise type's constructor.
   llvm::SmallVector<Expr *, 4> CtorArgExprs;
 
   // Add implicit object parameter.
   if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
-    if (MD->isInstance() && !isLambdaCallOperator(MD)) {
+    if (MD->isImplicitObjectMemberFunction() && !isLambdaCallOperator(MD)) {
       ExprResult ThisExpr = ActOnCXXThis(Loc);
       if (ThisExpr.isInvalid())
         return nullptr;
       ThisExpr = CreateBuiltinUnaryOp(Loc, UO_Deref, ThisExpr.get());
       if (ThisExpr.isInvalid())
         return nullptr;
       CtorArgExprs.push_back(ThisExpr.get());
     }

 }
 
 // Collect placement arguments for allocation function of coroutine FD.
 // Return true if we collect placement arguments succesfully. Return false,
 // otherwise.
 static bool collectPlacementArgs(Sema &S, FunctionDecl &FD, SourceLocation Loc,
                                  SmallVectorImpl<Expr *> &PlacementArgs) {
   if (auto *MD = dyn_cast<CXXMethodDecl>(&FD)) {
-    if (MD->isInstance() && !isLambdaCallOperator(MD)) {
+    if (MD->isImplicitObjectMemberFunction() && !isLambdaCallOperator(MD)) {
       ExprResult ThisExpr = S.ActOnCXXThis(Loc);
       if (ThisExpr.isInvalid())
         return false;
       ThisExpr = S.CreateBuiltinUnaryOp(Loc, UO_Deref, ThisExpr.get());
       if (ThisExpr.isInvalid())
         return false;
       PlacementArgs.push_back(ThisExpr.get());
     }

clang/lib/Sema/SemaDecl.cpp

auto VisitBase = [&] (const CXXBaseSpecifier *Specifier, CXXBasePath &Path) {
     for (NamedDecl *BaseND : BaseRecord->lookup(Name)) {
       CXXMethodDecl *BaseMD =
           dyn_cast<CXXMethodDecl>(BaseND->getCanonicalDecl());
       if (!BaseMD || !BaseMD->isVirtual() ||
           IsOverload(MD, BaseMD, /*UseMemberUsingDeclRules=*/false,
                      /*ConsiderCudaAttrs=*/true,
                      // C++2a [class.virtual]p2 does not consider requires
                      // clauses when overriding.
-                     /*ConsiderRequiresClauses=*/false))
+                     /*UseOverrideRules=*/true))
+        continue;
+      if (!CheckExplicitObjectOverride(MD, BaseMD))
         continue;
-
       if (Overridden.insert(BaseMD).second) {
         MD->addOverriddenMethod(BaseMD);
         CheckOverridingFunctionReturnType(MD, BaseMD);
         CheckOverridingFunctionAttributes(MD, BaseMD);
         CheckOverridingFunctionExceptionSpec(MD, BaseMD);
         CheckIfOverriddenFunctionIsMarkedFinal(MD, BaseMD);
       }
 
if (ConstexprKind == ConstexprSpecKind::Constinit) {
     SemaRef.Diag(D.getDeclSpec().getConstexprSpecLoc(),
                  diag::err_constexpr_wrong_decl_kind)
         << static_cast<int>(ConstexprKind);
     ConstexprKind = ConstexprSpecKind::Unspecified;
     D.getMutableDeclSpec().ClearConstexprSpec();
   }
   Expr *TrailingRequiresClause = D.getTrailingRequiresClause();
 
+  SemaRef.CheckExplicitObjectMemberFunction(DC, D, Name, R);
+
   if (Name.getNameKind() == DeclarationName::CXXConstructorName) {
     // This is a C++ constructor declaration.
     assert(DC->isRecord() &&
            "Constructors can only be declared in a member context");
 
     R = SemaRef.CheckConstructorDeclarator(D, R, SC);
     return CXXConstructorDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
void Sema::CheckFunctionOrTemplateParamDeclarator(Scope *S, Declarator &D) {
   case UnqualifiedIdKind::IK_TemplateId:
   case UnqualifiedIdKind::IK_ConstructorTemplateId:
     // GetNameForDeclarator would not produce a useful name in this case.
     Diag(D.getIdentifierLoc(), diag::err_bad_parameter_name_template_id);
     break;
   }
 }
 
+static void CheckExplicitObjectParameter(Sema &S, ParmVarDecl *P,
+                                         SourceLocation ExplicitThisLoc) {
+  if (!ExplicitThisLoc.isValid())
+    return;
+  assert(S.getLangOpts().CPlusPlus &&
+         "explicit parameter in non-cplusplus mode");
+  if (!S.getLangOpts().CPlusPlus23)
+    S.Diag(ExplicitThisLoc, diag::err_cxx20_deducing_this)
+        << P->getSourceRange();
+
+  // C++2b [dcl.fct/7] An explicit object parameter shall not be a function
+  // parameter pack.
+  if (P->isParameterPack()) {
+    S.Diag(P->getBeginLoc(), diag::err_explicit_object_parameter_pack)
+        << P->getSourceRange();
+    return;
+  }
+  P->setExplicitObjectParameterLoc(ExplicitThisLoc);
+  if (LambdaScopeInfo *LSI = S.getCurLambda())
+    LSI->ExplicitObjectParameter = P;
+}
+
 /// ActOnParamDeclarator - Called from Parser::ParseFunctionDeclarator()
 /// to introduce parameters into function prototype scope.
-Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D) {
+Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D,
+                                 SourceLocation ExplicitThisLoc) {
   const DeclSpec &DS = D.getDeclSpec();
 
   // Verify C99 6.7.5.3p2: The only SCS allowed is 'register'.
 
   // C++03 [dcl.stc]p2 also permits 'auto'.
   StorageClass SC = SC_None;
   if (DS.getStorageClassSpec() == DeclSpec::SCS_register) {
     SC = SC_Register;
Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D,
   // looking like class members in C++.
   ParmVarDecl *New =
       CheckParameter(Context.getTranslationUnitDecl(), D.getBeginLoc(),
                      D.getIdentifierLoc(), II, parmDeclType, TInfo, SC);
 
   if (D.isInvalidType())
     New->setInvalidDecl();
 
+  CheckExplicitObjectParameter(*this, New, ExplicitThisLoc);
+
   assert(S->isFunctionPrototypeScope());
   assert(S->getFunctionPrototypeDepth() >= 1);
   New->setScopeInfo(S->getFunctionPrototypeDepth() - 1,
                     S->getNextFunctionPrototypeIndex());
 
   // Add the parameter declaration into this scope.
   S->AddDecl(New);
   if (II)
static void RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator,
   else if (LCD == LCD_ByCopy)
     LSI->ImpCaptureStyle = CapturingScopeInfo::ImpCap_LambdaByval;
   else if (LCD == LCD_ByRef)
     LSI->ImpCaptureStyle = CapturingScopeInfo::ImpCap_LambdaByref;
   DeclarationNameInfo DNI = CallOperator->getNameInfo();
 
   LSI->IntroducerRange = DNI.getCXXOperatorNameRange();
   LSI->Mutable = !CallOperator->isConst();
+  if (CallOperator->isExplicitObjectMemberFunction())
+    LSI->ExplicitObjectParameter = CallOperator->getParamDecl(0);
 
   // Add the captures to the LSI so they can be noted as already
   // captured within tryCaptureVar.
   auto I = LambdaClass->field_begin();
   for (const auto &C : LambdaClass->captures()) {
     if (C.capturesVariable()) {
       ValueDecl *VD = C.getCapturedVar();
       if (VD->isInitCapture())
static bool AreSpecialMemberFunctionsSameKind(ASTContext &Context,
                                               CXXMethodDecl *M1,
                                               CXXMethodDecl *M2,
                                               Sema::CXXSpecialMember CSM) {
   // We don't want to compare templates to non-templates: See
   // https://github.com/llvm/llvm-project/issues/59206
   if (CSM == Sema::CXXDefaultConstructor)
     return bool(M1->getDescribedFunctionTemplate()) ==
            bool(M2->getDescribedFunctionTemplate());
-  if (!Context.hasSameType(M1->getParamDecl(0)->getType(),
-                           M2->getParamDecl(0)->getType()))
+  // https://github.com/cplusplus/CWG/issues/390
+  if (!Context.hasSameType(M1->getNonObjectParameter(0)->getType(),
+                           M2->getNonObjectParameter(0)->getType()))
     return false;
-  if (!Context.hasSameType(M1->getThisType(), M2->getThisType()))
+  if (!Context.hasSameType(M1->getFunctionObjectParameterReferenceType(),
+                           M2->getFunctionObjectParameterReferenceType()))
     return false;
 
   return true;
 }
 
 /// [class.mem.special]p6:
 /// An eligible special member function is a special member function for which:
 /// - the function is not deleted,

clang/lib/Sema/SemaDeclCXX.cpp

bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
   // C++2a changes the second bullet to instead delete the function if it's
   // defaulted on its first declaration, unless it's "an assignment operator,
   // and its return type differs or its parameter type is not a reference".
   bool DeleteOnTypeMismatch = getLangOpts().CPlusPlus20 && First;
   bool ShouldDeleteForTypeMismatch = false;
   unsigned ExpectedParams = 1;
   if (CSM == CXXDefaultConstructor || CSM == CXXDestructor)
     ExpectedParams = 0;
-  if (MD->getNumParams() != ExpectedParams) {
+  if (MD->getNumExplicitParams() != ExpectedParams) {
     // This checks for default arguments: a copy or move constructor with a
     // default argument is classified as a default constructor, and assignment
     // operations and destructors can't have default arguments.
     Diag(MD->getLocation(), diag::err_defaulted_special_member_params)
       << CSM << MD->getSourceRange();
     HadError = true;
   } else if (MD->isVariadic()) {
     if (DeleteOnTypeMismatch)
if (CSM == CXXCopyConstructor)
     CanHaveConstParam = RD->implicitCopyConstructorHasConstParam();
   else if (CSM == CXXCopyAssignment)
     CanHaveConstParam = RD->implicitCopyAssignmentHasConstParam();
 
   QualType ReturnType = Context.VoidTy;
   if (CSM == CXXCopyAssignment || CSM == CXXMoveAssignment) {
     // Check for return type matching.
     ReturnType = Type->getReturnType();
+    QualType ThisType = MD->getFunctionObjectParameterType();
 
     QualType DeclType = Context.getTypeDeclType(RD);
     DeclType = Context.getElaboratedType(ETK_None, nullptr, DeclType, nullptr);
-    DeclType = Context.getAddrSpaceQualType(DeclType, MD->getMethodQualifiers().getAddressSpace());
+    DeclType = Context.getAddrSpaceQualType(
+        DeclType, ThisType.getQualifiers().getAddressSpace());
     QualType ExpectedReturnType = Context.getLValueReferenceType(DeclType);
 
     if (!Context.hasSameType(ReturnType, ExpectedReturnType)) {
       Diag(MD->getLocation(), diag::err_defaulted_special_member_return_type)
         << (CSM == CXXMoveAssignment) << ExpectedReturnType;
       HadError = true;
     }
 
     // A defaulted special member cannot have cv-qualifiers.
-    if (Type->getMethodQuals().hasConst() || Type->getMethodQuals().hasVolatile()) {
+    if (ThisType.isConstQualified() || ThisType.isVolatileQualified()) {
       if (DeleteOnTypeMismatch)
         ShouldDeleteForTypeMismatch = true;
       else {
         Diag(MD->getLocation(), diag::err_defaulted_special_member_quals)
           << (CSM == CXXMoveAssignment) << getLangOpts().CPlusPlus14;
         HadError = true;
       }
     }
   }
 
   // Check for parameter type matching.
-  QualType ArgType = ExpectedParams ? Type->getParamType(0) : QualType();
+  QualType ArgType =
+      ExpectedParams
+          ? Type->getParamType(MD->isExplicitObjectMemberFunction() ? 1 : 0)
+          : QualType();
   bool HasConstParam = false;
   if (ExpectedParams && ArgType->isReferenceType()) {
     // Argument must be reference to possibly-const T.
     QualType ReferentType = ArgType->getPointeeType();
     HasConstParam = ReferentType.isConstQualified();
 
     if (ReferentType.isVolatileQualified()) {
       if (DeleteOnTypeMismatch)
if (First) {
     if (!Type->hasExceptionSpec()) {
       // C++2a [except.spec]p3:
       //   If a declaration of a function does not have a noexcept-specifier
       //   [and] is defaulted on its first declaration, [...] the exception
       //   specification is as specified below
       FunctionProtoType::ExtProtoInfo EPI = Type->getExtProtoInfo();
       EPI.ExceptionSpec.Type = EST_Unevaluated;
       EPI.ExceptionSpec.SourceDecl = MD;
-      MD->setType(Context.getFunctionType(
-          ReturnType, llvm::ArrayRef(&ArgType, ExpectedParams), EPI));
+      MD->setType(
+          Context.getFunctionType(ReturnType, Type->getParamTypes(), EPI));
     }
   }
 
   if (ShouldDeleteForTypeMismatch || ShouldDeleteSpecialMember(MD, CSM)) {
     if (First) {
       SetDeclDeleted(MD, MD->getLocation());
       if (!inTemplateInstantiation() && !HadError) {
         Diag(MD->getLocation(), diag::warn_defaulted_method_deleted) << CSM;
private:
   ExprResult getParam(unsigned I) {
     ParmVarDecl *PD = FD->getParamDecl(I);
     return getDecl(PD);
   }
 
   ExprPair getCompleteObject() {
     unsigned Param = 0;
     ExprResult LHS;
-    if (isa<CXXMethodDecl>(FD)) {
+    if (const auto *MD = dyn_cast<CXXMethodDecl>(FD);
+        MD && MD->isImplicitObjectMemberFunction()) {
       // LHS is '*this'.
       LHS = S.ActOnCXXThis(Loc);
       if (!LHS.isInvalid())
         LHS = S.CreateBuiltinUnaryOp(Loc, UO_Deref, LHS.get());
     } else {
       LHS = getParam(Param++);
     }
     ExprResult RHS = getParam(Param++);
     assert(Param == FD->getNumParams());
if (MD->getRefQualifier() == RQ_RValue) {
       EPI.RefQualifier = RQ_None;
       MD->setType(Context.getFunctionType(FPT->getReturnType(),
                                           FPT->getParamTypes(), EPI));
     }
 
     // If we're out-of-class, this is the class we're comparing.
     if (!RD)
       RD = MD->getParent();
-
-    if (!MD->isConst()) {
-      SourceLocation InsertLoc;
+    QualType T = MD->getFunctionObjectParameterType();
+    if (!T.isConstQualified()) {
+      SourceLocation Loc, InsertLoc;
+      if (MD->isExplicitObjectMemberFunction()) {
+        Loc = MD->getParamDecl(0)->getBeginLoc();
+        InsertLoc = getLocForEndOfToken(
+            MD->getParamDecl(0)->getExplicitObjectParamThisLoc());
+      } else {
+        Loc = MD->getLocation();
         if (FunctionTypeLoc Loc = MD->getFunctionTypeLoc())
-        InsertLoc = getLocForEndOfToken(Loc.getRParenLoc());
+          InsertLoc = Loc.getRParenLoc();
+      }
       // Don't diagnose an implicit 'operator=='; we will have diagnosed the
       // corresponding defaulted 'operator<=>' already.
       if (!MD->isImplicit()) {
-        Diag(MD->getLocation(), diag::err_defaulted_comparison_non_const)
+        Diag(Loc, diag::err_defaulted_comparison_non_const)
             << (int)DCK << FixItHint::CreateInsertion(InsertLoc, " const");
       }
 
       // Add the 'const' to the type to recover.
       const auto *FPT = MD->getType()->castAs<FunctionProtoType>();
       FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
       EPI.TypeQuals.addConst();
       MD->setType(Context.getFunctionType(FPT->getReturnType(),
                                           FPT->getParamTypes(), EPI));
     }
 
     if (MD->isVolatile()) {
       Diag(MD->getLocation(), diag::err_volatile_comparison_operator);
 
       // Remove the 'volatile' from the type to recover.
       const auto *FPT = MD->getType()->castAs<FunctionProtoType>();
       FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
       EPI.TypeQuals.removeVolatile();
       MD->setType(Context.getFunctionType(FPT->getReturnType(),
                                           FPT->getParamTypes(), EPI));
     }
   }
 
-  if (FD->getNumParams() != (IsMethod ? 1 : 2)) {
+  if ((FD->getNumParams() -
+       (unsigned)FD->hasCXXExplicitFunctionObjectParameter()) !=
+      (IsMethod ? 1 : 2)) {
     // Let's not worry about using a variadic template pack here -- who would do
     // such a thing?
     Diag(FD->getLocation(), diag::err_defaulted_comparison_num_args)
         << int(IsMethod) << int(DCK);
     return true;
   }
 
   const ParmVarDecl *KnownParm = nullptr;
   for (const ParmVarDecl *Param : FD->parameters()) {
+    if (Param->isExplicitObjectParameter())
+      continue;
     QualType ParmTy = Param->getType();
 
     if (!KnownParm) {
       auto CTy = ParmTy;
       // Is it `T const &`?
       bool Ok = !IsMethod;
       QualType ExpectedTy;
       if (RD)
case Sema::CXXMoveAssignment:
       IsAssignment = true;
       break;
     case Sema::CXXDestructor:
       break;
     case Sema::CXXInvalid:
       llvm_unreachable("invalid special member kind");
     }
 
-    if (MD->getNumParams()) {
+    if (MD->getNumExplicitParams()) {
       if (const ReferenceType *RT =
-              MD->getParamDecl(0)->getType()->getAs<ReferenceType>())
+              MD->getNonObjectParameter(0)->getType()->getAs<ReferenceType>())
         ConstArg = RT->getPointeeType().isConstQualified();
     }
   }
 
   Derived &getDerived() { return static_cast<Derived&>(*this); }
 
   /// Is this a "move" special member?
   bool isMove() const {
bool Sema::SpecialMemberIsTrivial(CXXMethodDecl *MD, CXXSpecialMember CSM,
   switch (CSM) {
   case CXXDefaultConstructor:
   case CXXDestructor:
     // Trivial default constructors and destructors cannot have parameters.
     break;
 
   case CXXCopyConstructor:
   case CXXCopyAssignment: {
-    const ParmVarDecl *Param0 = MD->getParamDecl(0);
+    const ParmVarDecl *Param0 = MD->getNonObjectParameter(0);
     const ReferenceType *RT = Param0->getType()->getAs<ReferenceType>();
 
     // When ClangABICompat14 is true, CXX copy constructors will only be trivial
     // if they are not user-provided and their parameter-type-list is equivalent
     // to the parameter-type-list of an implicit declaration. This maintains the
     // behavior before dr2171 was implemented.
     //
     // Otherwise, if ClangABICompat14 is false, All copy constructors can be
case CXXCopyAssignment: {
 
     ConstArg = RT->getPointeeType().isConstQualified();
     break;
   }
 
   case CXXMoveConstructor:
   case CXXMoveAssignment: {
     // Trivial move operations always have non-cv-qualified parameters.
-    const ParmVarDecl *Param0 = MD->getParamDecl(0);
+    const ParmVarDecl *Param0 = MD->getNonObjectParameter(0);
     const RValueReferenceType *RT =
       Param0->getType()->getAs<RValueReferenceType>();
     if (!RT || RT->getPointeeType().getCVRQualifiers()) {
       if (Diagnose)
         Diag(Param0->getLocation(), diag::note_nontrivial_param_type)
           << Param0->getSourceRange() << Param0->getType()
           << Context.getRValueReferenceType(Context.getRecordType(RD));
       return false;
if (DS.hasTypeSpecifier() && !D.isInvalidType()) {
     // place, such as:
     //   struct S { const operator int(); };
     // FIXME: we could provide a fixit to move the qualifiers onto the
     // conversion type.
     Diag(D.getIdentifierLoc(), diag::err_conv_function_with_complex_decl)
         << SourceRange(D.getIdentifierLoc()) << 0;
     D.setInvalidType();
   }
-
   const auto *Proto = R->castAs<FunctionProtoType>();
-
   // Make sure we don't have any parameters.
-  if (Proto->getNumParams() > 0) {
-    Diag(D.getIdentifierLoc(), diag::err_conv_function_with_params);
+  DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+  unsigned NumParam = Proto->getNumParams();
 
+  // [C++2b]
+  // A conversion function shall have no non-object parameters.
+  if (NumParam == 1) {
+    DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+    if (const auto *First =
+            dyn_cast_if_present<ParmVarDecl>(FTI.Params[0].Param);
+        First && First->isExplicitObjectParameter())
+      NumParam--;
+  }
+
+  if (NumParam != 0) {
+    Diag(D.getIdentifierLoc(), diag::err_conv_function_with_params);
     // Delete the parameters.
-    D.getFunctionTypeInfo().freeParams();
+    FTI.freeParams();
     D.setInvalidType();
   } else if (Proto->isVariadic()) {
     Diag(D.getIdentifierLoc(), diag::err_conv_function_variadic);
     D.setInvalidType();
   }
 
   // Diagnose "&operator bool()" and other such nonsense.  This
   // is actually a gcc extension which we don't support.
Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
 
   if (FunctionTemplateDecl *ConversionTemplate
                                 = Conversion->getDescribedFunctionTemplate())
     return ConversionTemplate;
 
   return Conversion;
 }
 
+void Sema::CheckExplicitObjectMemberFunction(DeclContext *DC, Declarator &D,
+                                             DeclarationName Name, QualType R) {
+  CheckExplicitObjectMemberFunction(D, Name, R, false, DC);
+}
+
+void Sema::CheckExplicitObjectLambda(Declarator &D) {
+  CheckExplicitObjectMemberFunction(D, {}, {}, true);
+}
+
+void Sema::CheckExplicitObjectMemberFunction(Declarator &D,
+                                             DeclarationName Name, QualType R,
+                                             bool IsLambda, DeclContext *DC) {
+  if (!D.isFunctionDeclarator())
+    return;
+
+  DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+  if (FTI.NumParams == 0)
+    return;
+  ParmVarDecl *ExplicitObjectParam = nullptr;
+  for (unsigned Idx = 0; Idx < FTI.NumParams; Idx++) {
+    const auto &ParamInfo = FTI.Params[Idx];
+    if (!ParamInfo.Param)
+      continue;
+    ParmVarDecl *Param = cast<ParmVarDecl>(ParamInfo.Param);
+    if (!Param->isExplicitObjectParameter())
+      continue;
+    if (Idx == 0) {
+      ExplicitObjectParam = Param;
+      continue;
+    } else {
+      Diag(Param->getLocation(),
+           diag::err_explicit_object_parameter_must_be_first)
+          << IsLambda << Param->getSourceRange();
+    }
+  }
+  if (!ExplicitObjectParam)
+    return;
+
+  if (ExplicitObjectParam->hasDefaultArg()) {
+    Diag(ExplicitObjectParam->getLocation(),
+         diag::err_explicit_object_default_arg)
+        << ExplicitObjectParam->getSourceRange();
+  }
+
+  if (D.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_static) {
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_nonmember)
+        << D.getSourceRange() << /*static=*/0 << IsLambda;
+    return;
+  }
+
+  if (D.getDeclSpec().isVirtualSpecified()) {
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_nonmember)
+        << D.getSourceRange() << /*virtual=*/1 << IsLambda;
+  }
+
+  if (FTI.hasMutableQualifier()) {
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_qualifiers)
+        << D.getSourceRange() << IsLambda << /*mutable=*/1;
+  }
+
+  if (IsLambda)
+    return;
+
+  if (!DC || !DC->isRecord()) {
+    Diag(ExplicitObjectParam->getLocation(),
+         diag::err_explicit_object_parameter_nonmember)
+        << D.getSourceRange() << /*non-member=*/2 << IsLambda;
+    return;
+  }
+
+  if (FTI.hasMethodTypeQualifiers()) {
+    if (FTI.getConstQualifierLoc().isValid())
+      Diag(FTI.getConstQualifierLoc(),
+           diag::err_explicit_object_parameter_qualifiers)
+          << /*function=*/0 << /*const=*/0 << D.getSourceRange();
+    else
+      // Generic message for the less common qualifiers
+      Diag(D.getBeginLoc(), diag::err_explicit_object_parameter_qualifiers)
+          << /*function=*/0 << /*qualifiers=*/3 << D.getSourceRange();
+  }
+  if (FTI.hasRefQualifier())
+    Diag(FTI.getRefQualifierLoc(),
+         diag::err_explicit_object_parameter_qualifiers)
+        << /*function=*/0 << /*reference=*/2 << D.getSourceRange();
+
+  // CWG2674: constructors and destructors cannot have explicit parameters.
+  if (Name.getNameKind() == DeclarationName::CXXConstructorName ||
+      Name.getNameKind() == DeclarationName::CXXDestructorName)
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_constructor)
+        << (Name.getNameKind() == DeclarationName::CXXDestructorName)
+        << D.getSourceRange();
+}
+
 namespace {
 /// Utility class to accumulate and print a diagnostic listing the invalid
 /// specifier(s) on a declaration.
 struct BadSpecifierDiagnoser {
   BadSpecifierDiagnoser(Sema &S, SourceLocation Loc, unsigned DiagID)
       : S(S), Diagnostic(S.Diag(Loc, DiagID)) {}
   ~BadSpecifierDiagnoser() {
     Diagnostic << Specifiers;
void Sema::DefineImplicitCopyAssignment(SourceLocation CurrentLocation,
   //   order of their declaration in the base-specifier-list, and then the
   //   immediate non-static data members of X are assigned, in the order in
   //   which they were declared in the class definition.
 
   // The statements that form the synthesized function body.
   SmallVector<Stmt*, 8> Statements;
 
   // The parameter for the "other" object, which we are copying from.
-  ParmVarDecl *Other = CopyAssignOperator->getParamDecl(0);
+  ParmVarDecl *Other = CopyAssignOperator->getNonObjectParameter(0);
   Qualifiers OtherQuals = Other->getType().getQualifiers();
   QualType OtherRefType = Other->getType();
-  if (const LValueReferenceType *OtherRef
-                                = OtherRefType->getAs<LValueReferenceType>()) {
-    OtherRefType = OtherRef->getPointeeType();
+  if (OtherRefType->isLValueReferenceType()) {
+    OtherRefType = OtherRefType->getPointeeType();
     OtherQuals = OtherRefType.getQualifiers();
   }
 
   // Our location for everything implicitly-generated.
   SourceLocation Loc = CopyAssignOperator->getEndLoc().isValid()
                            ? CopyAssignOperator->getEndLoc()
                            : CopyAssignOperator->getLocation();
 
   // Builds a DeclRefExpr for the "other" object.
   RefBuilder OtherRef(Other, OtherRefType);
 
-  // Builds the "this" pointer.
-  ThisBuilder This;
+  // Builds the function object parameter.
+  std::optional<ThisBuilder> This;
+  std::optional<DerefBuilder> DerefThis;
+  std::optional<RefBuilder> ExplicitObject;
+  bool IsArrow = false;
+  QualType ObjectType;
+  if (CopyAssignOperator->isExplicitObjectMemberFunction()) {
+    ObjectType = CopyAssignOperator->getParamDecl(0)->getType();
+    if (ObjectType->isReferenceType())
+      ObjectType = ObjectType->getPointeeType();
+    ExplicitObject.emplace(CopyAssignOperator->getParamDecl(0), ObjectType);
+  } else {
+    ObjectType = getCurrentThisType();
+    This.emplace();
+    DerefThis.emplace(*This);
+    IsArrow = !LangOpts.HLSL;
+  }
+  ExprBuilder &ObjectParameter =
+      ExplicitObject ? (ExprBuilder &)*ExplicitObject : (ExprBuilder &)*This;
 
   // Assign base classes.
   bool Invalid = false;
   for (auto &Base : ClassDecl->bases()) {
     // Form the assignment:
     //   static_cast<Base*>(this)->Base::operator=(static_cast<Base&>(other));
     QualType BaseType = Base.getType().getUnqualifiedType();
     if (!BaseType->isRecordType()) {
       Invalid = true;
       continue;
     }
 
     CXXCastPath BasePath;
     BasePath.push_back(&Base);
 
     // Construct the "from" expression, which is an implicit cast to the
     // appropriately-qualified base type.
     CastBuilder From(OtherRef, Context.getQualifiedType(BaseType, OtherQuals),
                      VK_LValue, BasePath);
 
     // Dereference "this".
-    DerefBuilder DerefThis(This);
-    CastBuilder To(DerefThis,
-                   Context.getQualifiedType(
-                       BaseType, CopyAssignOperator->getMethodQualifiers()),
-                   VK_LValue, BasePath);
+    CastBuilder To(
+        ExplicitObject ? (ExprBuilder &)*ExplicitObject
+                       : (ExprBuilder &)*DerefThis,
+        Context.getQualifiedType(BaseType, ObjectType.getQualifiers()),
 
     // Build the copy.
     StmtResult Copy = buildSingleCopyAssign(*this, Loc, BaseType,
                                             To, From,
                                             /*CopyingBaseSubobject=*/true,
                                             /*Copying=*/true);
     if (Copy.isInvalid()) {
       CopyAssignOperator->setInvalidDecl();
for (auto *Field : ClassDecl->fields()) {
     // Build references to the field in the object we're copying from and to.
     CXXScopeSpec SS; // Intentionally empty
     LookupResult MemberLookup(*this, Field->getDeclName(), Loc,
                               LookupMemberName);
     MemberLookup.addDecl(Field);
     MemberLookup.resolveKind();
 
     MemberBuilder From(OtherRef, OtherRefType, /*IsArrow=*/false, MemberLookup);
-
-    MemberBuilder To(This, getCurrentThisType(), /*IsArrow=*/!LangOpts.HLSL,
-                     MemberLookup);
-
+    MemberBuilder To(ObjectParameter, ObjectType, IsArrow, MemberLookup);
     // Build the copy of this field.
     StmtResult Copy = buildSingleCopyAssign(*this, Loc, FieldType,
                                             To, From,
                                             /*CopyingBaseSubobject=*/false,
                                             /*Copying=*/true);
     if (Copy.isInvalid()) {
       CopyAssignOperator->setInvalidDecl();
       return;
     }
 
     // Success! Record the copy.
     Statements.push_back(Copy.getAs<Stmt>());
   }
 
   if (!Invalid) {
     // Add a "return *this;"
-    Expr *ThisExpr = nullptr;
-    if (!LangOpts.HLSL) {
-      ExprResult ThisObj =
-          CreateBuiltinUnaryOp(Loc, UO_Deref, This.build(*this, Loc));
-      ThisExpr = ThisObj.get();
-    } else {
-      ThisExpr = This.build(*this, Loc);
-    }
-
+    Expr *ThisExpr = (ExplicitObject  ? (ExprBuilder &)*ExplicitObject
+                      : LangOpts.HLSL ? (ExprBuilder &)*This
+                                      : (ExprBuilder &)*DerefThis)
+                         .build(*this, Loc);
     StmtResult Return = BuildReturnStmt(Loc, ThisExpr);
     if (Return.isInvalid())
       Invalid = true;
     else
       Statements.push_back(Return.getAs<Stmt>());
   }
 
   if (Invalid) {
void Sema::DefineImplicitMoveAssignment(SourceLocation CurrentLocation,
 
   // Add a context note for diagnostics produced after this point.
   Scope.addContextNote(CurrentLocation);
 
   // The statements that form the synthesized function body.
   SmallVector<Stmt*, 8> Statements;
 
   // The parameter for the "other" object, which we are move from.
-  ParmVarDecl *Other = MoveAssignOperator->getParamDecl(0);
+  ParmVarDecl *Other = MoveAssignOperator->getNonObjectParameter(0);
   QualType OtherRefType =
       Other->getType()->castAs<RValueReferenceType>()->getPointeeType();
 
   // Our location for everything implicitly-generated.
   SourceLocation Loc = MoveAssignOperator->getEndLoc().isValid()
                            ? MoveAssignOperator->getEndLoc()
                            : MoveAssignOperator->getLocation();
 
   // Builds a reference to the "other" object.
   RefBuilder OtherRef(Other, OtherRefType);
   // Cast to rvalue.
   MoveCastBuilder MoveOther(OtherRef);
 
-  // Builds the "this" pointer.
-  ThisBuilder This;
+  // Builds the function object parameter.
+  std::optional<ThisBuilder> This;
+  std::optional<DerefBuilder> DerefThis;
+  std::optional<RefBuilder> ExplicitObject;
+  QualType ObjectType;
+  if (MoveAssignOperator->isExplicitObjectMemberFunction()) {
+    ObjectType = MoveAssignOperator->getParamDecl(0)->getType();
+    if (ObjectType->isReferenceType())
+      ObjectType = ObjectType->getPointeeType();
+    ExplicitObject.emplace(MoveAssignOperator->getParamDecl(0), ObjectType);
+  } else {
+    ObjectType = getCurrentThisType();
+    This.emplace();
+    DerefThis.emplace(*This);
+  }
+  ExprBuilder &ObjectParameter =
+      ExplicitObject ? (ExprBuilder &)*ExplicitObject : (ExprBuilder &)*This;
 
   // Assign base classes.
   bool Invalid = false;
   for (auto &Base : ClassDecl->bases()) {
     // C++11 [class.copy]p28:
     //   It is unspecified whether subobjects representing virtual base classes
     //   are assigned more than once by the implicitly-defined copy assignment
     //   operator.
for (auto &Base : ClassDecl->bases()) {
 
     CXXCastPath BasePath;
     BasePath.push_back(&Base);
 
     // Construct the "from" expression, which is an implicit cast to the
     // appropriately-qualified base type.
     CastBuilder From(OtherRef, BaseType, VK_XValue, BasePath);
 
-    // Dereference "this".
-    DerefBuilder DerefThis(This);
-
     // Implicitly cast "this" to the appropriately-qualified base type.
-    CastBuilder To(DerefThis,
-                   Context.getQualifiedType(
-                       BaseType, MoveAssignOperator->getMethodQualifiers()),
-                   VK_LValue, BasePath);
+    // Dereference "this".
+    CastBuilder To(
+        ExplicitObject ? (ExprBuilder &)*ExplicitObject
+                       : (ExprBuilder &)*DerefThis,
+        Context.getQualifiedType(BaseType, ObjectType.getQualifiers()),
 
     // Build the move.
     StmtResult Move = buildSingleCopyAssign(*this, Loc, BaseType,
                                             To, From,
                                             /*CopyingBaseSubobject=*/true,
                                             /*Copying=*/false);
     if (Move.isInvalid()) {
       MoveAssignOperator->setInvalidDecl();
for (auto *Field : ClassDecl->fields()) {
 
     // Build references to the field in the object we're copying from and to.
     LookupResult MemberLookup(*this, Field->getDeclName(), Loc,
                               LookupMemberName);
     MemberLookup.addDecl(Field);
     MemberLookup.resolveKind();
     MemberBuilder From(MoveOther, OtherRefType,
                        /*IsArrow=*/false, MemberLookup);
-    MemberBuilder To(This, getCurrentThisType(),
-                     /*IsArrow=*/true, MemberLookup);
+    MemberBuilder To(ObjectParameter, ObjectType, /*IsArrow=*/!ExplicitObject,
+                     MemberLookup);
 
     assert(!From.build(*this, Loc)->isLValue() && // could be xvalue or prvalue
         "Member reference with rvalue base must be rvalue except for reference "
         "members, which aren't allowed for move assignment.");
 
     // Build the move of this field.
     StmtResult Move = buildSingleCopyAssign(*this, Loc, FieldType,
                                             To, From,
                                             /*CopyingBaseSubobject=*/false,
                                             /*Copying=*/false);
     if (Move.isInvalid()) {
       MoveAssignOperator->setInvalidDecl();
       return;
     }
 
     // Success! Record the copy.
     Statements.push_back(Move.getAs<Stmt>());
   }
 
   if (!Invalid) {
     // Add a "return *this;"
-    ExprResult ThisObj =
-        CreateBuiltinUnaryOp(Loc, UO_Deref, This.build(*this, Loc));
+    Expr *ThisExpr = (ExplicitObject ? (ExprBuilder &)*ExplicitObject
+                                     : (ExprBuilder &)*DerefThis)
+                         .build(*this, Loc);
 
-    StmtResult Return = BuildReturnStmt(Loc, ThisObj.get());
+    StmtResult Return = BuildReturnStmt(Loc, ThisExpr);
     if (Return.isInvalid())
       Invalid = true;
     else
       Statements.push_back(Return.getAs<Stmt>());
   }
 
   if (Invalid) {
     MoveAssignOperator->setInvalidDecl();
void Sema::DefineImplicitLambdaToFunctionPointerConversion(
 
   QualType ConvRT = Conv->getType()->castAs<FunctionType>()->getReturnType();
   CallingConv CC =
       ConvRT->getPointeeType()->castAs<FunctionType>()->getCallConv();
 
   CXXRecordDecl *Lambda = Conv->getParent();
   FunctionDecl *CallOp = Lambda->getLambdaCallOperator();
   FunctionDecl *Invoker =
-      CallOp->isStatic() ? CallOp : Lambda->getLambdaStaticInvoker(CC);
+      CallOp->hasCXXExplicitFunctionObjectParameter() || CallOp->isStatic()
+          ? CallOp
+          : Lambda->getLambdaStaticInvoker(CC);
 
   if (auto *TemplateArgs = Conv->getTemplateSpecializationArgs()) {
     CallOp = InstantiateFunctionDeclaration(
         CallOp->getDescribedFunctionTemplate(), TemplateArgs, CurrentLocation);
     if (!CallOp)
       return;
 
     if (CallOp != Invoker) {
#include "clang/Basic/OperatorKinds.def"
   bool CanBeUnaryOperator = OperatorUses[Op][0];
   bool CanBeBinaryOperator = OperatorUses[Op][1];
   bool MustBeMemberOperator = OperatorUses[Op][2];
 
   // C++ [over.oper]p8:
   //   [...] Operator functions cannot have more or fewer parameters
   //   than the number required for the corresponding operator, as
   //   described in the rest of this subclause.
-  unsigned NumParams = FnDecl->getNumParams()
-                     + (isa<CXXMethodDecl>(FnDecl)? 1 : 0);
+  unsigned NumParams = FnDecl->getNumParams() +
+                       (isa<CXXMethodDecl>(FnDecl) &&
+                                !FnDecl->hasCXXExplicitFunctionObjectParameter()
+                            ? 1
+                            : 0);
   if (Op != OO_Call && Op != OO_Subscript &&
       ((NumParams == 1 && !CanBeUnaryOperator) ||
        (NumParams == 2 && !CanBeBinaryOperator) || (NumParams < 1) ||
        (NumParams > 2))) {
     // We have the wrong number of parameters.
     unsigned ErrorKind;
     if (CanBeUnaryOperator && CanBeBinaryOperator) {
       ErrorKind = 2;  // 2 -> unary or binary.
bool Sema::CheckOverridingFunctionAttributes(const CXXMethodDecl *New,
 
   Diag(New->getLocation(),
        diag::err_conflicting_overriding_cc_attributes)
     << New->getDeclName() << New->getType() << Old->getType();
   Diag(Old->getLocation(), diag::note_overridden_virtual_function);
   return true;
 }
 
+bool Sema::CheckExplicitObjectOverride(CXXMethodDecl *New,
+                                       const CXXMethodDecl *Old) {
+  // CWG2553
+  // A virtual function shall not be an explicit object member function.
+  if (!New->isExplicitObjectMemberFunction())
+    return true;
+  Diag(New->getParamDecl(0)->getBeginLoc(),
+       diag::err_explicit_object_parameter_nonmember)
+      << New->getSourceRange() << /*virtual*/ 1 << /*IsLambda*/ false;
+  Diag(Old->getLocation(), diag::note_overridden_virtual_function);
+  New->setInvalidDecl();
+  return false;
+}
+
 bool Sema::CheckOverridingFunctionReturnType(const CXXMethodDecl *New,
                                              const CXXMethodDecl *Old) {
   QualType NewTy = New->getType()->castAs<FunctionType>()->getReturnType();
   QualType OldTy = Old->getType()->castAs<FunctionType>()->getReturnType();
 
   if (Context.hasSameType(NewTy, OldTy) ||
       NewTy->isDependentType() || OldTy->isDependentType())
     return false;

clang/lib/Sema/SemaExceptionSpec.cpp

case AR_delayed:
     llvm_unreachable("access check delayed in non-declaration");
   }
   llvm_unreachable("unexpected access check result");
 }
 
 /// CheckExceptionSpecSubset - Check whether the second function type's
 /// exception specification is a subset (or equivalent) of the first function
 /// type. This is used by override and pointer assignment checks.
-bool Sema::CheckExceptionSpecSubset(const PartialDiagnostic &DiagID,
-                                    const PartialDiagnostic &NestedDiagID,
-                                    const PartialDiagnostic &NoteID,
-                                    const PartialDiagnostic &NoThrowDiagID,
-                                    const FunctionProtoType *Superset,
-                                    SourceLocation SuperLoc,
-                                    const FunctionProtoType *Subset,
-                                    SourceLocation SubLoc) {
+bool Sema::CheckExceptionSpecSubset(
+    const PartialDiagnostic &DiagID, const PartialDiagnostic &NestedDiagID,
+    const PartialDiagnostic &NoteID, const PartialDiagnostic &NoThrowDiagID,
+    const FunctionProtoType *Superset, bool SkipSupersetFirstParameter,
+    SourceLocation SuperLoc, const FunctionProtoType *Subset,
+    bool SkipSubsetFirstParameter, SourceLocation SubLoc) {
 
   // Just auto-succeed under -fno-exceptions.
   if (!getLangOpts().CXXExceptions)
     return false;
 
   // FIXME: As usual, we could be more specific in our error messages, but
   // that better waits until we've got types with source locations.
 
bool Sema::CheckExceptionSpecSubset(
 
   CanThrowResult SuperCanThrow = Superset->canThrow();
   CanThrowResult SubCanThrow = Subset->canThrow();
 
   // If the superset contains everything or the subset contains nothing, we're
   // done.
   if ((SuperCanThrow == CT_Can && SuperEST != EST_Dynamic) ||
       SubCanThrow == CT_Cannot)
-    return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset, SuperLoc,
-                                   Subset, SubLoc);
+    return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset,
+                                   SkipSupersetFirstParameter, SuperLoc, Subset,
+                                   SkipSubsetFirstParameter, SubLoc);
 
   // Allow __declspec(nothrow) to be missing on redeclaration as an extension in
   // some cases.
   if (NoThrowDiagID.getDiagID() != 0 && SubCanThrow == CT_Can &&
       SuperCanThrow == CT_Cannot && SuperEST == EST_NoThrow) {
     Diag(SubLoc, NoThrowDiagID);
     if (NoteID.getDiagID() != 0)
       Diag(SuperLoc, NoteID);
for (QualType SubI : Subset->exceptions()) {
     if (!Contained) {
       Diag(SubLoc, DiagID);
       if (NoteID.getDiagID() != 0)
         Diag(SuperLoc, NoteID);
       return true;
     }
   }
   // We've run half the gauntlet.
-  return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset, SuperLoc,
-                                 Subset, SubLoc);
+  return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset,
+                                 SkipSupersetFirstParameter, SuperLoc, Subset,
+                                 SkipSupersetFirstParameter, SubLoc);
 }
 
 static bool
 CheckSpecForTypesEquivalent(Sema &S, const PartialDiagnostic &DiagID,
                             const PartialDiagnostic &NoteID, QualType Target,
                             SourceLocation TargetLoc, QualType Source,
                             SourceLocation SourceLoc) {
   const FunctionProtoType *TFunc = GetUnderlyingFunction(Target);
   if (!TFunc)
     return false;
   const FunctionProtoType *SFunc = GetUnderlyingFunction(Source);
   if (!SFunc)
     return false;
 
   return S.CheckEquivalentExceptionSpec(DiagID, NoteID, TFunc, TargetLoc,
                                         SFunc, SourceLoc);
 }
 
 /// CheckParamExceptionSpec - Check if the parameter and return types of the
 /// two functions have equivalent exception specs. This is part of the
 /// assignment and override compatibility check. We do not check the parameters
 /// of parameter function pointers recursively, as no sane programmer would
 /// even be able to write such a function type.
-bool Sema::CheckParamExceptionSpec(const PartialDiagnostic &DiagID,
-                                   const PartialDiagnostic &NoteID,
-                                   const FunctionProtoType *Target,
-                                   SourceLocation TargetLoc,
-                                   const FunctionProtoType *Source,
-                                   SourceLocation SourceLoc) {
+bool Sema::CheckParamExceptionSpec(
+    const PartialDiagnostic &DiagID, const PartialDiagnostic &NoteID,
+    const FunctionProtoType *Target, bool SkipTargetFirstParameter,
+    SourceLocation TargetLoc, const FunctionProtoType *Source,
+    bool SkipSourceFirstParameter, SourceLocation SourceLoc) {
   auto RetDiag = DiagID;
   RetDiag << 0;
   if (CheckSpecForTypesEquivalent(
           *this, RetDiag, PDiag(),
           Target->getReturnType(), TargetLoc, Source->getReturnType(),
           SourceLoc))
     return true;
 
   // We shouldn't even be testing this unless the arguments are otherwise
   // compatible.
-  assert(Target->getNumParams() == Source->getNumParams() &&
+  assert((Target->getNumParams() - (unsigned)SkipTargetFirstParameter) ==
+             (Source->getNumParams() - (unsigned)SkipSourceFirstParameter) &&
          "Functions have different argument counts.");
   for (unsigned i = 0, E = Target->getNumParams(); i != E; ++i) {
     auto ParamDiag = DiagID;
     ParamDiag << 1;
     if (CheckSpecForTypesEquivalent(
             *this, ParamDiag, PDiag(),
-            Target->getParamType(i), TargetLoc, Source->getParamType(i),
+            Target->getParamType(i + (SkipTargetFirstParameter ? 1 : 0)),
+            TargetLoc, Source->getParamType(SkipSourceFirstParameter ? 1 : 0),
             SourceLoc))
       return true;
   }
   return false;
 }
 
 bool Sema::CheckExceptionSpecCompatibility(Expr *From, QualType ToType) {
   // First we check for applicability.
bool Sema::CheckExceptionSpecCompatibility(Expr *From, QualType ToType) {
   //
   // FIXME: If there is a nested dependent exception specification, we should
   // not be checking it here. This is fine:
   //   template<typename T> void f() {
   //     void (*p)(void (*) throw(T));
   //     void (*q)(void (*) throw(int)) = p;
   //   }
   // ... because it might be instantiated with T=int.
-  return CheckExceptionSpecSubset(
-             PDiag(DiagID), PDiag(NestedDiagID), PDiag(), PDiag(), ToFunc,
-             From->getSourceRange().getBegin(), FromFunc, SourceLocation()) &&
+  return CheckExceptionSpecSubset(PDiag(DiagID), PDiag(NestedDiagID), PDiag(),
+                                  PDiag(), ToFunc, 0,
+                                  From->getSourceRange().getBegin(), FromFunc,
+                                  0, SourceLocation()) &&
          !getLangOpts().CPlusPlus17;
 }
 
 bool Sema::CheckOverridingFunctionExceptionSpec(const CXXMethodDecl *New,
                                                 const CXXMethodDecl *Old) {
   // If the new exception specification hasn't been parsed yet, skip the check.
   // We'll get called again once it's been parsed.
   if (New->getType()->castAs<FunctionProtoType>()->getExceptionSpecType() ==
bool Sema::CheckOverridingFunctionExceptionSpec(const CXXMethodDecl *New,
   if (exceptionSpecNotKnownYet(Old) || exceptionSpecNotKnownYet(New)) {
     DelayedOverridingExceptionSpecChecks.push_back({New, Old});
     return false;
   }
 
   unsigned DiagID = diag::err_override_exception_spec;
   if (getLangOpts().MSVCCompat)
     DiagID = diag::ext_override_exception_spec;
-  return CheckExceptionSpecSubset(PDiag(DiagID),
-                                  PDiag(diag::err_deep_exception_specs_differ),
-                                  PDiag(diag::note_overridden_virtual_function),
-                                  PDiag(diag::ext_override_exception_spec),
-                                  Old->getType()->castAs<FunctionProtoType>(),
-                                  Old->getLocation(),
-                                  New->getType()->castAs<FunctionProtoType>(),
-                                  New->getLocation());
+  return CheckExceptionSpecSubset(
+      PDiag(DiagID), PDiag(diag::err_deep_exception_specs_differ),
+      Old->hasCXXExplicitFunctionObjectParameter(), Old->getLocation(),
+      New->hasCXXExplicitFunctionObjectParameter(), New->getLocation());
 }
 
 static CanThrowResult canSubStmtsThrow(Sema &Self, const Stmt *S) {
   CanThrowResult R = CT_Cannot;
   for (const Stmt *SubStmt : S->children()) {
     if (!SubStmt)
       continue;
     R = mergeCanThrow(R, Self.canThrow(SubStmt));

clang/lib/Sema/SemaExpr.cpp

bool Sema::DiagnoseDependentMemberLookup(const LookupResult &R) {
   // Return true if we are inside a default argument instantiation
   // and the found name refers to an instance member function, otherwise
   // the caller will try to create an implicit member call and this is wrong
   // for default arguments.
   //
   // FIXME: Is this special case necessary? We could allow the caller to
   // diagnose this.
   if (isDefaultArgument && ((*R.begin())->isCXXInstanceMember())) {
-    Diag(R.getNameLoc(), diag::err_member_call_without_object);
+    Diag(R.getNameLoc(), diag::err_member_call_without_object) << 0;
     return true;
   }
 
   // Tell the callee to try to recover.
   return false;
 }
 
 /// Diagnose an empty lookup.
if (FromPtrType) {
       DestType = Context.getPointerType(DestRecordType);
       FromRecordType = FromPtrType->getPointeeType();
       PointerConversions = true;
     } else {
       DestType = DestRecordType;
       FromRecordType = FromType;
     }
   } else if (const auto *Method = dyn_cast<CXXMethodDecl>(Member)) {
-    if (Method->isStatic())
+    if (!Method->isImplicitObjectMemberFunction())
       return From;
 
     DestType = Method->getThisType();
     DestRecordType = DestType->getPointeeType();
 
     if (FromType->getAs<PointerType>()) {
       FromRecordType = FromType->getPointeeType();
       PointerConversions = true;
Sema::ConvertArgumentsForCall(CallExpr *Call, Expr *Fn,
   // Bail out early if calling a builtin with custom typechecking.
   if (FDecl)
     if (unsigned ID = FDecl->getBuiltinID())
       if (Context.BuiltinInfo.hasCustomTypechecking(ID))
         return false;
 
   // C99 6.5.2.2p7 - the arguments are implicitly converted, as if by
   // assignment, to the types of the corresponding parameter, ...
+  bool HasExplicitObjectParameter =
+      FDecl && FDecl->hasCXXExplicitFunctionObjectParameter();
+  unsigned ExplicitObjectParameterOffset = HasExplicitObjectParameter ? 1 : 0;
   unsigned NumParams = Proto->getNumParams();
   bool Invalid = false;
   unsigned MinArgs = FDecl ? FDecl->getMinRequiredArguments() : NumParams;
   unsigned FnKind = Fn->getType()->isBlockPointerType()
                        ? 1 /* block */
                        : (IsExecConfig ? 3 /* kernel function (exec config) */
                                        : 0 /* function */);
 
   // If too few arguments are available (and we don't have default
   // arguments for the remaining parameters), don't make the call.
   if (Args.size() < NumParams) {
     if (Args.size() < MinArgs) {
       TypoCorrection TC;
       if (FDecl && (TC = TryTypoCorrectionForCall(*this, Fn, FDecl, Args))) {
         unsigned diag_id =
             MinArgs == NumParams && !Proto->isVariadic()
                 ? diag::err_typecheck_call_too_few_args_suggest
                 : diag::err_typecheck_call_too_few_args_at_least_suggest;
-        diagnoseTypo(TC, PDiag(diag_id) << FnKind << MinArgs
-                                        << static_cast<unsigned>(Args.size())
-                                        << TC.getCorrectionRange());
-      } else if (MinArgs == 1 && FDecl && FDecl->getParamDecl(0)->getDeclName())
+        diagnoseTypo(
+            TC, PDiag(diag_id)
+                    << FnKind << MinArgs - ExplicitObjectParameterOffset
+                    << static_cast<unsigned>(Args.size()) -
+                           ExplicitObjectParameterOffset
+                    << HasExplicitObjectParameter << TC.getCorrectionRange());
+      } else if (MinArgs - ExplicitObjectParameterOffset == 1 && FDecl &&
+                 FDecl->getParamDecl(ExplicitObjectParameterOffset)
+                     ->getDeclName())
         Diag(RParenLoc,
              MinArgs == NumParams && !Proto->isVariadic()
                  ? diag::err_typecheck_call_too_few_args_one
                  : diag::err_typecheck_call_too_few_args_at_least_one)
-            << FnKind << FDecl->getParamDecl(0) << Fn->getSourceRange();
+            << FnKind << FDecl->getParamDecl(ExplicitObjectParameterOffset)
+            << HasExplicitObjectParameter << Fn->getSourceRange();
       else
         Diag(RParenLoc, MinArgs == NumParams && !Proto->isVariadic()
                             ? diag::err_typecheck_call_too_few_args
                             : diag::err_typecheck_call_too_few_args_at_least)
-            << FnKind << MinArgs << static_cast<unsigned>(Args.size())
-            << Fn->getSourceRange();
+            << FnKind << MinArgs - ExplicitObjectParameterOffset
+            << static_cast<unsigned>(Args.size()) -
+                   ExplicitObjectParameterOffset
+            << HasExplicitObjectParameter << Fn->getSourceRange();
 
       // Emit the location of the prototype.
       if (!TC && FDecl && !FDecl->getBuiltinID() && !IsExecConfig)
         Diag(FDecl->getLocation(), diag::note_callee_decl)
             << FDecl << FDecl->getParametersSourceRange();
 
       return true;
     }
     // We reserve space for the default arguments when we create
     // the call expression, before calling ConvertArgumentsForCall.
     assert((Call->getNumArgs() == NumParams) &&
            "We should have reserved space for the default arguments before!");
   }
 
   // If too many are passed and not variadic, error on the extras and drop
   // them.
   if (Args.size() > NumParams) {
     if (!Proto->isVariadic()) {
       TypoCorrection TC;
       if (FDecl && (TC = TryTypoCorrectionForCall(*this, Fn, FDecl, Args))) {
         unsigned diag_id =
             MinArgs == NumParams && !Proto->isVariadic()
                 ? diag::err_typecheck_call_too_many_args_suggest
                 : diag::err_typecheck_call_too_many_args_at_most_suggest;
-        diagnoseTypo(TC, PDiag(diag_id) << FnKind << NumParams
-                                        << static_cast<unsigned>(Args.size())
-                                        << TC.getCorrectionRange());
-      } else if (NumParams == 1 && FDecl &&
-                 FDecl->getParamDecl(0)->getDeclName())
+        diagnoseTypo(
+            TC, PDiag(diag_id)
+                    << FnKind << NumParams - ExplicitObjectParameterOffset
+                    << static_cast<unsigned>(Args.size()) -
+                           ExplicitObjectParameterOffset
+                    << HasExplicitObjectParameter << TC.getCorrectionRange());
+      } else if (NumParams - ExplicitObjectParameterOffset == 1 && FDecl &&
+                 FDecl->getParamDecl(ExplicitObjectParameterOffset)
+                     ->getDeclName())
         Diag(Args[NumParams]->getBeginLoc(),
              MinArgs == NumParams
                  ? diag::err_typecheck_call_too_many_args_one
                  : diag::err_typecheck_call_too_many_args_at_most_one)
-            << FnKind << FDecl->getParamDecl(0)
-            << static_cast<unsigned>(Args.size()) << Fn->getSourceRange()
+            << FnKind << FDecl->getParamDecl(ExplicitObjectParameterOffset)
+            << static_cast<unsigned>(Args.size()) -
+                   ExplicitObjectParameterOffset
+            << HasExplicitObjectParameter << Fn->getSourceRange()
             << SourceRange(Args[NumParams]->getBeginLoc(),
                            Args.back()->getEndLoc());
       else
         Diag(Args[NumParams]->getBeginLoc(),
              MinArgs == NumParams
                  ? diag::err_typecheck_call_too_many_args
                  : diag::err_typecheck_call_too_many_args_at_most)
-            << FnKind << NumParams << static_cast<unsigned>(Args.size())
-            << Fn->getSourceRange()
+            << FnKind << NumParams - ExplicitObjectParameterOffset
+            << static_cast<unsigned>(Args.size()) -
+                   ExplicitObjectParameterOffset
+            << HasExplicitObjectParameter << Fn->getSourceRange()
             << SourceRange(Args[NumParams]->getBeginLoc(),
                            Args.back()->getEndLoc());
 
       // Emit the location of the prototype.
       if (!TC && FDecl && !FDecl->getBuiltinID() && !IsExecConfig)
         Diag(FDecl->getLocation(), diag::note_callee_decl)
             << FDecl << FDecl->getParametersSourceRange();
 
for (unsigned i = 0, e = Args.size(); i != e; i++) {
         return ExprError();
 
       TheCall->setArg(i, Arg);
     }
     TheCall->computeDependence();
   }
 
   if (CXXMethodDecl *Method = dyn_cast_or_null<CXXMethodDecl>(FDecl))
-    if (!Method->isStatic())
+    if (Method->isImplicitObjectMemberFunction())
       return ExprError(Diag(LParenLoc, diag::err_member_call_without_object)
-        << Fn->getSourceRange());
+                       << Fn->getSourceRange() << 0);
 
   // Check for sentinels
   if (NDecl)
     DiagnoseSentinelCalls(NDecl, LParenLoc, Args);
 
   // Warn for unions passing across security boundary (CMSE).
   if (FuncT != nullptr && FuncT->getCmseNSCallAttr()) {
     for (unsigned i = 0, e = Args.size(); i != e; i++) {

 /// Diagnose invalid operand for address of operations.
 ///
 /// \param Type The type of operand which cannot have its address taken.
 static void diagnoseAddressOfInvalidType(Sema &S, SourceLocation Loc,
                                          Expr *E, unsigned Type) {
   S.Diag(Loc, diag::err_typecheck_address_of) << Type << E->getSourceRange();
 }
 
+bool Sema::DiagnoseMissingQualifiersInAddressOfOperand(
+    SourceLocation OpLoc, const Expr *Op, const CXXMethodDecl *MD) {
+  const auto *DRE = cast<DeclRefExpr>(Op->IgnoreParens());
+
+  if (Op != DRE)
+    return Diag(OpLoc, diag::err_parens_pointer_member_function)
+           << Op->getSourceRange();
+
+  // Taking the address of a dtor is illegal per C++ [class.dtor]p2.
+  if (isa<CXXDestructorDecl>(MD))
+    return Diag(OpLoc, diag::err_typecheck_addrof_dtor)
+           << DRE->getSourceRange();
+
+  if (DRE->getQualifier())
+    return false;
+
+  if (MD->getParent()->getName().empty())
+    return Diag(OpLoc, diag::err_unqualified_pointer_member_function)
+           << DRE->getSourceRange();
+
+  SmallString<32> Str;
+  StringRef Qual = (MD->getParent()->getName() + "::").toStringRef(Str);
+  return Diag(OpLoc, diag::err_unqualified_pointer_member_function)
+         << DRE->getSourceRange()
+         << FixItHint::CreateInsertion(DRE->getSourceRange().getBegin(), Qual);
+}
+
 /// CheckAddressOfOperand - The operand of & must be either a function
 /// designator or an lvalue designating an object. If it is an lvalue, the
 /// object cannot be declared with storage class register or be a bit field.
 /// Note: The usual conversions are *not* applied to the operand of the &
 /// operator (C99 6.3.2.1p[2-4]), and its result is never an lvalue.
 /// In C++, the operand might be an overloaded function name, in which case
 /// we allow the '&' but retain the overloaded-function type.
 QualType Sema::CheckAddressOfOperand(ExprResult &OrigOp, SourceLocation OpLoc) {
if (lval == Expr::LV_ClassTemporary || lval == Expr::LV_ArrayTemporary) {
     if (!isa<DeclRefExpr>(op)) {
       Diag(OpLoc, diag::err_invalid_form_pointer_member_function)
         << OrigOp.get()->getSourceRange();
       return QualType();
     }
     DeclRefExpr *DRE = cast<DeclRefExpr>(op);
     CXXMethodDecl *MD = cast<CXXMethodDecl>(DRE->getDecl());
 
-    // The id-expression was parenthesized.
-    if (OrigOp.get() != DRE) {
-      Diag(OpLoc, diag::err_parens_pointer_member_function)
-        << OrigOp.get()->getSourceRange();
-
-    // The method was named without a qualifier.
-    } else if (!DRE->getQualifier()) {
-      if (MD->getParent()->getName().empty())
-        Diag(OpLoc, diag::err_unqualified_pointer_member_function)
-          << op->getSourceRange();
-      else {
-        SmallString<32> Str;
-        StringRef Qual = (MD->getParent()->getName() + "::").toStringRef(Str);
-        Diag(OpLoc, diag::err_unqualified_pointer_member_function)
-          << op->getSourceRange()
-          << FixItHint::CreateInsertion(op->getSourceRange().getBegin(), Qual);
-      }
-    }
-
-    // Taking the address of a dtor is illegal per C++ [class.dtor]p2.
-    if (isa<CXXDestructorDecl>(MD))
-      Diag(OpLoc, diag::err_typecheck_addrof_dtor) << op->getSourceRange();
+    DiagnoseMissingQualifiersInAddressOfOperand(OpLoc, OrigOp.get(), MD);
 
     QualType MPTy = Context.getMemberPointerType(
         op->getType(), Context.getTypeDeclType(MD->getParent()).getTypePtr());
     // Under the MS ABI, lock down the inheritance model now.
     if (Context.getTargetInfo().getCXXABI().isMicrosoft())
       (void)isCompleteType(OpLoc, MPTy);
     return MPTy;
   } else if (lval != Expr::LV_Valid && lval != Expr::LV_IncompleteVoidType) {
     // C99 6.5.3.2p1
     // The operand must be either an l-value or a function designator
     if (!op->getType()->isFunctionType()) {
       // Use a special diagnostic for loads from property references.
       if (isa<PseudoObjectExpr>(op)) {
         AddressOfError = AO_Property_Expansion;
       } else {
         Diag(OpLoc, diag::err_typecheck_invalid_lvalue_addrof)
           << op->getType() << op->getSourceRange();
         return QualType();
       }
+    } else if (const auto *DRE = dyn_cast<DeclRefExpr>(op)) {
+      if (const auto *MD = dyn_cast_or_null<CXXMethodDecl>(DRE->getDecl()))
+        DiagnoseMissingQualifiersInAddressOfOperand(OpLoc, OrigOp.get(), MD);
     }
+
   } else if (op->getObjectKind() == OK_BitField) { // C99 6.5.3.2p1
     // The operand cannot be a bit-field
     AddressOfError = AO_Bit_Field;
   } else if (op->getObjectKind() == OK_VectorComponent) {
     // The operand cannot be an element of a vector
     AddressOfError = AO_Vector_Element;
   } else if (op->getObjectKind() == OK_MatrixComponent) {
     // The operand cannot be an element of a matrix.
if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E)) {
 
     ValueDecl *VD = DRE->getDecl();
     if (!VD->isCXXClassMember())
       return false;
 
     if (isa<FieldDecl>(VD) || isa<IndirectFieldDecl>(VD))
       return true;
     if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(VD))
-      return Method->isInstance();
+      return Method->isImplicitObjectMemberFunction();
 
     return false;
   }
 
   if (UnresolvedLookupExpr *ULE = dyn_cast<UnresolvedLookupExpr>(E)) {
     if (!ULE->getQualifier())
       return false;
 
     for (NamedDecl *D : ULE->decls()) {
       if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
-        if (Method->isInstance())
+        if (Method->isImplicitObjectMemberFunction())
           return true;
       } else {
         // Overload set does not contain methods.
         break;
       }
     }
 
     return false;
if (CSI->CaptureMap.count(Var)) {
     // Compute the type of an expression that refers to this variable.
     DeclRefType = CaptureType.getNonReferenceType();
 
     // Similarly to mutable captures in lambda, all the OpenMP captures by copy
     // are mutable in the sense that user can change their value - they are
     // private instances of the captured declarations.
     const Capture &Cap = CSI->getCapture(Var);
     if (Cap.isCopyCapture() &&
-        !(isa<LambdaScopeInfo>(CSI) && cast<LambdaScopeInfo>(CSI)->Mutable) &&
+        !(isa<LambdaScopeInfo>(CSI) &&
+          !cast<LambdaScopeInfo>(CSI)->lambdaCaptureShouldBeConst()) &&
         !(isa<CapturedRegionScopeInfo>(CSI) &&
           cast<CapturedRegionScopeInfo>(CSI)->CapRegionKind == CR_OpenMP))
       DeclRefType.addConst();
     return true;
   }
   return false;
 }
 
if (ByRef)
     DeclRefType = CaptureType.getNonReferenceType();
   else {
     // C++ [expr.prim.lambda]p5:
     //   The closure type for a lambda-expression has a public inline
     //   function call operator [...]. This function call operator is
     //   declared const (9.3.1) if and only if the lambda-expression's
     //   parameter-declaration-clause is not followed by mutable.
     DeclRefType = CaptureType.getNonReferenceType();
-    if (!LSI->Mutable && !CaptureType->isReferenceType())
+    bool Const = LSI->lambdaCaptureShouldBeConst();
+    if (Const && !CaptureType->isReferenceType())
       DeclRefType.addConst();
   }
 
   // Add the capture.
   if (BuildAndDiagnose)
     LSI->addCapture(Var, /*isBlock=*/false, ByRef, RefersToCapturedVariable,
                     Loc, EllipsisLoc, CaptureType, Invalid);
 

 
 /// Mark a variable referenced, and check whether it is odr-used
 /// (C++ [basic.def.odr]p2, C99 6.9p3).  Note that this should not be
 /// used directly for normal expressions referring to VarDecl.
 void Sema::MarkVariableReferenced(SourceLocation Loc, VarDecl *Var) {
   DoMarkVarDeclReferenced(*this, Loc, Var, nullptr, RefsMinusAssignments);
 }
 
+// C++ [temp.dep.expr]p3:
+//   An id-expression is type-dependent if it contains:
+//     - an identifier associated by name lookup with an entity captured by copy
+//       in a lambda-expression that has an explicit object parameter whose type
+//       is dependent ([dcl.fct]),
+static void FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(
+    Sema &SemaRef, ValueDecl *D, Expr *E) {
+  DeclRefExpr *ID = dyn_cast<DeclRefExpr>(E);
+  if (!ID || ID->isTypeDependent())
+    return;
+
+  auto IsDependent = [&]() {
+    const LambdaScopeInfo *LSI = SemaRef.getCurLambda();
+    if (!LSI)
+      return false;
+    if (!LSI->ExplicitObjectParameter ||
+        !LSI->ExplicitObjectParameter->getType()->isDependentType())
+      return false;
+    if (!LSI->CaptureMap.count(D))
+      return false;
+    const Capture &Cap = LSI->getCapture(D);
+    return !Cap.isCopyCapture();
+  }();
+
+  ID->setCapturedByCopyInLambdaWithExplicitObjectParameter(
+      IsDependent, SemaRef.getASTContext());
+}
+
 static void
 MarkExprReferenced(Sema &SemaRef, SourceLocation Loc, Decl *D, Expr *E,
                    bool MightBeOdrUse,
                    llvm::DenseMap<const VarDecl *, int> &RefsMinusAssignments) {
   if (SemaRef.isInOpenMPDeclareTargetContext())
     SemaRef.checkDeclIsAllowedInOpenMPTarget(E, D);
 
   if (VarDecl *Var = dyn_cast<VarDecl>(D)) {
     DoMarkVarDeclReferenced(SemaRef, Loc, Var, E, RefsMinusAssignments);
+    if (SemaRef.getLangOpts().CPlusPlus)
+      FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(SemaRef,
+                                                                       Var, E);
     return;
   }
 
   if (BindingDecl *Decl = dyn_cast<BindingDecl>(D)) {
     DoMarkBindingDeclReferenced(SemaRef, Loc, Decl, E);
+    if (SemaRef.getLangOpts().CPlusPlus)
+      FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(SemaRef,
+                                                                       Decl, E);
     return;
   }
-
   SemaRef.MarkAnyDeclReferenced(Loc, D, MightBeOdrUse);
 
   // If this is a call to a method via a cast, also mark the method in the
   // derived class used in case codegen can devirtualize the call.
   const MemberExpr *ME = dyn_cast<MemberExpr>(E);
   if (!ME)
     return;
   CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(ME->getMemberDecl());

clang/lib/Sema/SemaExprCXX.cpp

for (int I = FunctionScopes.size();
     CurLSI = cast<LambdaScopeInfo>(FunctionScopes[I]);
 
     if (!CurLSI->isCXXThisCaptured())
         continue;
 
     auto C = CurLSI->getCXXThisCapture();
 
     if (C.isCopyCapture()) {
-      if (!CurLSI->Mutable)
+      if (CurLSI->lambdaCaptureShouldBeConst())
         ClassType.addConst();
       return ASTCtx.getPointerType(ClassType);
     }
   }
 
   // 2) We've run out of ScopeInfos but check 1. if CurDC is a lambda (which
   //    can happen during instantiation of its nested generic lambda call
   //    operator); 2. if we're in a lambda scope (lambda body).
static QualType adjustCVQualifiersForCXXThisWithinLambda(
   return ASTCtx.getPointerType(ClassType);
 }
 
 QualType Sema::getCurrentThisType() {
   DeclContext *DC = getFunctionLevelDeclContext();
   QualType ThisTy = CXXThisTypeOverride;
 
   if (CXXMethodDecl *method = dyn_cast<CXXMethodDecl>(DC)) {
-    if (method && method->isInstance())
+    if (method && method->isImplicitObjectMemberFunction())
       ThisTy = method->getThisType();
   }
 
-  if (ThisTy.isNull() && isLambdaCallOperator(CurContext) &&
+  if (ThisTy.isNull() && isLambdaCallWithImplicitObjectParameter(CurContext) &&
       inTemplateInstantiation() && isa<CXXRecordDecl>(DC)) {
 
     // This is a lambda call operator that is being instantiated as a default
     // initializer. DC must point to the enclosing class type, so we can recover
     // the 'this' type from it.
     QualType ClassTy = Context.getTypeDeclType(cast<CXXRecordDecl>(DC));
     // There are no cv-qualifiers for 'this' within default initializers,
     // per [expr.prim.general]p4.
bool Sema::CheckCXXThisCapture(SourceLocation Loc, const bool Explicit,
   }
   return false;
 }
 
 ExprResult Sema::ActOnCXXThis(SourceLocation Loc) {
   /// C++ 9.3.2: In the body of a non-static member function, the keyword this
   /// is a non-lvalue expression whose value is the address of the object for
   /// which the function is called.
-
   QualType ThisTy = getCurrentThisType();
-  if (ThisTy.isNull())
-    return Diag(Loc, diag::err_invalid_this_use);
+
+  if (ThisTy.isNull()) {
+    DeclContext *DC = getFunctionLevelDeclContext();
+
+    if (const auto *Method = dyn_cast<CXXMethodDecl>(DC);
+        Method && Method->isExplicitObjectMemberFunction()) {
+      return Diag(Loc, diag::err_invalid_this_use) << 1;
+    }
+
+    if (isLambdaCallWithExplicitObjectParameter(CurContext))
+      return Diag(Loc, diag::err_invalid_this_use) << 1;
+
+    return Diag(Loc, diag::err_invalid_this_use) << 0;
+  }
+
   return BuildCXXThisExpr(Loc, ThisTy, /*IsImplicit=*/false);
 }
 
 Expr *Sema::BuildCXXThisExpr(SourceLocation Loc, QualType Type,
                              bool IsImplicit) {
   if (getLangOpts().HLSL && Type.getTypePtr()->isPointerType()) {
     auto *This = new (Context)
         CXXThisExpr(Loc, Type.getTypePtr()->getPointeeType(), IsImplicit);
     This->setValueKind(ExprValueKind::VK_LValue);
if (Context.hasSameType(FromType, Context.OverloadTy)) {
     FunctionDecl *Fn = ResolveAddressOfOverloadedFunction(From, ToType,
                                                           true, Found);
     if (!Fn)
       return ExprError();
 
     if (DiagnoseUseOfDecl(Fn, From->getBeginLoc()))
       return ExprError();
 
-    From = FixOverloadedFunctionReference(From, Found, Fn);
+    ExprResult Res = FixOverloadedFunctionReference(From, Found, Fn);
+    if (Res.isInvalid())
+      return ExprError();
 
     // We might get back another placeholder expression if we resolved to a
     // builtin.
-    ExprResult Checked = CheckPlaceholderExpr(From);
-    if (Checked.isInvalid())
+    Res = CheckPlaceholderExpr(Res.get());
+    if (Res.isInvalid())
       return ExprError();
 
-    From = Checked.get();
+    From = Res.get();
     FromType = From->getType();
   }
 
   // If we're converting to an atomic type, first convert to the corresponding
   // non-atomic type.
   QualType ToAtomicType;
   if (const AtomicType *ToAtomic = ToType->getAs<AtomicType>()) {
     ToAtomicType = ToType;
ExprResult Sema::ActOnPseudoDestructorExpr(Scope *S, Expr *Base,
   TypeSourceInfo *DestructedTypeInfo = TLB.getTypeSourceInfo(Context, T);
   PseudoDestructorTypeStorage Destructed(DestructedTypeInfo);
 
   return BuildPseudoDestructorExpr(Base, OpLoc, OpKind, CXXScopeSpec(),
                                    nullptr, SourceLocation(), TildeLoc,
                                    Destructed);
 }
 
-ExprResult Sema::BuildCXXMemberCallExpr(Expr *E, NamedDecl *FoundDecl,
-                                        CXXConversionDecl *Method,
-                                        bool HadMultipleCandidates) {
-  // Convert the expression to match the conversion function's implicit object
-  // parameter.
-  ExprResult Exp = PerformObjectArgumentInitialization(E, /*Qualifier=*/nullptr,
-                                          FoundDecl, Method);
-  if (Exp.isInvalid())
-    return true;
-
-  if (Method->getParent()->isLambda() &&
-      Method->getConversionType()->isBlockPointerType()) {
-    // This is a lambda conversion to block pointer; check if the argument
-    // was a LambdaExpr.
-    Expr *SubE = E;
-    CastExpr *CE = dyn_cast<CastExpr>(SubE);
-    if (CE && CE->getCastKind() == CK_NoOp)
-      SubE = CE->getSubExpr();
-    SubE = SubE->IgnoreParens();
-    if (CXXBindTemporaryExpr *BE = dyn_cast<CXXBindTemporaryExpr>(SubE))
-      SubE = BE->getSubExpr();
-    if (isa<LambdaExpr>(SubE)) {
-      // For the conversion to block pointer on a lambda expression, we
-      // construct a special BlockLiteral instead; this doesn't really make
-      // a difference in ARC, but outside of ARC the resulting block literal
-      // follows the normal lifetime rules for block literals instead of being
-      // autoreleased.
-      PushExpressionEvaluationContext(
-          ExpressionEvaluationContext::PotentiallyEvaluated);
-      ExprResult BlockExp = BuildBlockForLambdaConversion(
-          Exp.get()->getExprLoc(), Exp.get()->getExprLoc(), Method, Exp.get());
-      PopExpressionEvaluationContext();
-
-      // FIXME: This note should be produced by a CodeSynthesisContext.
-      if (BlockExp.isInvalid())
-        Diag(Exp.get()->getExprLoc(), diag::note_lambda_to_block_conv);
-      return BlockExp;
-    }
-  }
-
-  MemberExpr *ME =
-      BuildMemberExpr(Exp.get(), /*IsArrow=*/false, SourceLocation(),
-                      NestedNameSpecifierLoc(), SourceLocation(), Method,
-                      DeclAccessPair::make(FoundDecl, FoundDecl->getAccess()),
-                      HadMultipleCandidates, DeclarationNameInfo(),
-                      Context.BoundMemberTy, VK_PRValue, OK_Ordinary);
-
-  QualType ResultType = Method->getReturnType();
-  ExprValueKind VK = Expr::getValueKindForType(ResultType);
-  ResultType = ResultType.getNonLValueExprType(Context);
-
-  CXXMemberCallExpr *CE = CXXMemberCallExpr::Create(
-      Context, ME, /*Args=*/{}, ResultType, VK, Exp.get()->getEndLoc(),
-      CurFPFeatureOverrides());
-
-  if (CheckFunctionCall(Method, CE,
-                        Method->getType()->castAs<FunctionProtoType>()))
-    return ExprError();
-
-  return CheckForImmediateInvocation(CE, CE->getMethodDecl());
-}
-
 ExprResult Sema::BuildCXXNoexceptExpr(SourceLocation KeyLoc, Expr *Operand,
                                       SourceLocation RParen) {
   // If the operand is an unresolved lookup expression, the expression is ill-
   // formed per [over.over]p1, because overloaded function names cannot be used
   // without arguments except in explicit contexts.
   ExprResult R = CheckPlaceholderExpr(Operand);
   if (R.isInvalid())
     return R;

clang/lib/Sema/SemaExprMember.cpp

enum IMAKind {
   /// The reference is definitely not an instance member access.
   IMA_Static,
 
   /// The reference may be an implicit instance member access.
   IMA_Mixed,
 
   /// The reference may be to an instance member, but it might be invalid if
   /// so, because the context is not an instance method.
-  IMA_Mixed_StaticContext,
+  IMA_Mixed_StaticOrExplicitContext,
 
   /// The reference may be to an instance member, but it is invalid if
   /// so, because the context is from an unrelated class.
   IMA_Mixed_Unrelated,
 
   /// The reference is definitely an implicit instance member access.
   IMA_Instance,
 
   /// The reference may be to an unresolved using declaration.
   IMA_Unresolved,
 
   /// The reference is a contextually-permitted abstract member reference.
   IMA_Abstract,
 
   /// The reference may be to an unresolved using declaration and the
   /// context is not an instance method.
-  IMA_Unresolved_StaticContext,
+  IMA_Unresolved_StaticOrExplicitContext,
 
   // The reference refers to a field which is not a member of the containing
   // class, which is allowed because we're in C++11 mode and the context is
   // unevaluated.
   IMA_Field_Uneval_Context,
 
   /// All possible referrents are instance members and the current
   /// context is not an instance method.
-  IMA_Error_StaticContext,
+  IMA_Error_StaticOrExplicitContext,
 
   /// All possible referrents are instance members of an unrelated
   /// class.
   IMA_Error_Unrelated
 };
 
 /// The given lookup names class member(s) and is not being used for
 /// an address-of-member expression.  Classify the type of access
 /// according to whether it's possible that this reference names an
 /// instance member.  This is best-effort in dependent contexts; it is okay to
 /// conservatively answer "yes", in which case some errors will simply
 /// not be caught until template-instantiation.
 static IMAKind ClassifyImplicitMemberAccess(Sema &SemaRef,
                                             const LookupResult &R) {
   assert(!R.empty() && (*R.begin())->isCXXClassMember());
 
   DeclContext *DC = SemaRef.getFunctionLevelDeclContext();
 
-  bool isStaticContext = SemaRef.CXXThisTypeOverride.isNull() &&
-    (!isa<CXXMethodDecl>(DC) || cast<CXXMethodDecl>(DC)->isStatic());
+  bool isStaticOrExplicitContext =
+      SemaRef.CXXThisTypeOverride.isNull() &&
+      (!isa<CXXMethodDecl>(DC) || cast<CXXMethodDecl>(DC)->isStatic() ||
+       cast<CXXMethodDecl>(DC)->isExplicitObjectMemberFunction());
 
   if (R.isUnresolvableResult())
-    return isStaticContext ? IMA_Unresolved_StaticContext : IMA_Unresolved;
+    return isStaticOrExplicitContext ? IMA_Unresolved_StaticOrExplicitContext
+                                     : IMA_Unresolved;
 
   // Collect all the declaring classes of instance members we find.
   bool hasNonInstance = false;
   bool isField = false;
   BaseSet Classes;
   for (NamedDecl *D : R) {
     // Look through any using decls.
     D = D->getUnderlyingDecl();
static IMAKind ClassifyImplicitMemberAccess(Sema &SemaRef,
   case Sema::ExpressionEvaluationContext::ImmediateFunctionContext:
   case Sema::ExpressionEvaluationContext::PotentiallyEvaluated:
   case Sema::ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed:
     break;
   }
 
   // If the current context is not an instance method, it can't be
   // an implicit member reference.
-  if (isStaticContext) {
+  if (isStaticOrExplicitContext) {
     if (hasNonInstance)
-      return IMA_Mixed_StaticContext;
+      return IMA_Mixed_StaticOrExplicitContext;
 
     return AbstractInstanceResult ? AbstractInstanceResult
-                                  : IMA_Error_StaticContext;
+                                  : IMA_Error_StaticOrExplicitContext;
   }
 
   CXXRecordDecl *contextClass;
   if (auto *MD = dyn_cast<CXXMethodDecl>(DC))
     contextClass = MD->getParent()->getCanonicalDecl();
   else if (auto *RD = dyn_cast<CXXRecordDecl>(DC))
     contextClass = RD;
   else
     return AbstractInstanceResult ? AbstractInstanceResult
-                                  : IMA_Error_StaticContext;
+                                  : IMA_Error_StaticOrExplicitContext;
 
   // [class.mfct.non-static]p3:
   // ...is used in the body of a non-static member function of class X,
   // if name lookup (3.4.1) resolves the name in the id-expression to a
   // non-static non-type member of some class C [...]
   // ...if C is not X or a base class of X, the class member access expression
   // is ill-formed.
   if (R.getNamingClass() &&
static void diagnoseInstanceReference(Sema &SemaRef,
   Rep = Rep->getUnderlyingDecl();
 
   DeclContext *FunctionLevelDC = SemaRef.getFunctionLevelDeclContext();
   CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FunctionLevelDC);
   CXXRecordDecl *ContextClass = Method ? Method->getParent() : nullptr;
   CXXRecordDecl *RepClass = dyn_cast<CXXRecordDecl>(Rep->getDeclContext());
 
   bool InStaticMethod = Method && Method->isStatic();
+  bool InExplicitObjectMethod =
+      Method && Method->isExplicitObjectMemberFunction();
   bool IsField = isa<FieldDecl>(Rep) || isa<IndirectFieldDecl>(Rep);
 
+  std::string Replacement;
+  if (InExplicitObjectMethod) {
+    DeclarationName N = Method->getParamDecl(0)->getDeclName();
+    if (!N.isEmpty()) {
+      Replacement.append(N.getAsString());
+      Replacement.append(".");
+    }
+  }
   if (IsField && InStaticMethod)
     // "invalid use of member 'x' in static member function"
-    SemaRef.Diag(Loc, diag::err_invalid_member_use_in_static_method)
-        << Range << nameInfo.getName();
-  else if (ContextClass && RepClass && SS.isEmpty() && !InStaticMethod &&
-           !RepClass->Equals(ContextClass) && RepClass->Encloses(ContextClass))
+    SemaRef.Diag(Loc, diag::err_invalid_member_use_in_method)
+        << Range << nameInfo.getName() << /*static*/ 0;
+  else if (IsField && InExplicitObjectMethod) {
+    auto Diag = SemaRef.Diag(Loc, diag::err_invalid_member_use_in_method)
+                << Range << nameInfo.getName() << /*explicit*/ 1;
+    if (!Replacement.empty())
+      Diag << FixItHint::CreateInsertion(Loc, Replacement);
+  } else if (ContextClass && RepClass && SS.isEmpty() &&
+             !InExplicitObjectMethod && !InStaticMethod &&
+             !RepClass->Equals(ContextClass) &&
+             RepClass->Encloses(ContextClass))
     // Unqualified lookup in a non-static member function found a member of an
     // enclosing class.
     SemaRef.Diag(Loc, diag::err_nested_non_static_member_use)
       << IsField << RepClass << nameInfo.getName() << ContextClass << Range;
   else if (IsField)
     SemaRef.Diag(Loc, diag::err_invalid_non_static_member_use)
       << nameInfo.getName() << Range;
-  else
+  else if (!InExplicitObjectMethod)
     SemaRef.Diag(Loc, diag::err_member_call_without_object)
-      << Range;
+        << Range << /*static*/ 0;
+  else {
+    const auto *Callee = dyn_cast<CXXMethodDecl>(Rep);
+    auto Diag = SemaRef.Diag(Loc, diag::err_member_call_without_object)
+                << Range << Callee->isExplicitObjectMemberFunction();
+    if (!Replacement.empty())
+      Diag << FixItHint::CreateInsertion(Loc, Replacement);
+  }
 }
 
 /// Builds an expression which might be an implicit member expression.
 ExprResult Sema::BuildPossibleImplicitMemberExpr(
     const CXXScopeSpec &SS, SourceLocation TemplateKWLoc, LookupResult &R,
     const TemplateArgumentListInfo *TemplateArgs, const Scope *S,
     UnresolvedLookupExpr *AsULE) {
   switch (ClassifyImplicitMemberAccess(*this, R)) {
   case IMA_Instance:
     return BuildImplicitMemberExpr(SS, TemplateKWLoc, R, TemplateArgs, true, S);
 
   case IMA_Mixed:
   case IMA_Mixed_Unrelated:
   case IMA_Unresolved:
     return BuildImplicitMemberExpr(SS, TemplateKWLoc, R, TemplateArgs, false,
                                    S);
 
   case IMA_Field_Uneval_Context:
     Diag(R.getNameLoc(), diag::warn_cxx98_compat_non_static_member_use)
       << R.getLookupNameInfo().getName();
     [[fallthrough]];
   case IMA_Static:
   case IMA_Abstract:
-  case IMA_Mixed_StaticContext:
-  case IMA_Unresolved_StaticContext:
+  case IMA_Mixed_StaticOrExplicitContext:
+  case IMA_Unresolved_StaticOrExplicitContext:
     if (TemplateArgs || TemplateKWLoc.isValid())
       return BuildTemplateIdExpr(SS, TemplateKWLoc, R, false, TemplateArgs);
     return AsULE ? AsULE : BuildDeclarationNameExpr(SS, R, false);
 
-  case IMA_Error_StaticContext:
+  case IMA_Error_StaticOrExplicitContext:
   case IMA_Error_Unrelated:
     diagnoseInstanceReference(*this, SS, R.getRepresentativeDecl(),
                               R.getLookupNameInfo());
     return ExprError();
   }
 
   llvm_unreachable("unexpected instance member access kind");
 }

clang/lib/Sema/SemaExprObjC.cpp

bool Sema::CheckMessageArgumentTypes(
   unsigned NumNamedArgs = Sel.getNumArgs();
   // Method might have more arguments than selector indicates. This is due
   // to addition of c-style arguments in method.
   if (Method->param_size() > Sel.getNumArgs())
     NumNamedArgs = Method->param_size();
   // FIXME. This need be cleaned up.
   if (Args.size() < NumNamedArgs) {
     Diag(SelLoc, diag::err_typecheck_call_too_few_args)
-      << 2 << NumNamedArgs << static_cast<unsigned>(Args.size());
+        << 2 << NumNamedArgs << static_cast<unsigned>(Args.size())
+        << /*is non object*/ 0;
     return false;
   }
 
   // Compute the set of type arguments to be substituted into each parameter
   // type.
   std::optional<ArrayRef<QualType>> typeArgs =
       ReceiverType->getObjCSubstitutions(Method->getDeclContext());
   bool IsError = false;
for (unsigned i = NumNamedArgs, e = Args.size(); i < e; ++i) {
       Args[i] = Arg.get();
     }
   } else {
     // Check for extra arguments to non-variadic methods.
     if (Args.size() != NumNamedArgs) {
       Diag(Args[NumNamedArgs]->getBeginLoc(),
            diag::err_typecheck_call_too_many_args)
           << 2 /*method*/ << NumNamedArgs << static_cast<unsigned>(Args.size())
-          << Method->getSourceRange()
+          << Method->getSourceRange() << /*is non object*/ 0
           << SourceRange(Args[NumNamedArgs]->getBeginLoc(),
                          Args.back()->getEndLoc());
     }
   }
 
   DiagnoseSentinelCalls(Method, SelLoc, Args);
 
   // Do additional checkings on method.

clang/lib/Sema/SemaInit.cpp

 }
 
 static bool shouldTrackImplicitObjectArg(const CXXMethodDecl *Callee) {
   if (auto *Conv = dyn_cast_or_null<CXXConversionDecl>(Callee))
     if (isRecordWithAttr<PointerAttr>(Conv->getConversionType()))
       return true;
   if (!isInStlNamespace(Callee->getParent()))
     return false;
-  if (!isRecordWithAttr<PointerAttr>(Callee->getThisObjectType()) &&
-      !isRecordWithAttr<OwnerAttr>(Callee->getThisObjectType()))
+  if (!isRecordWithAttr<PointerAttr>(
+          Callee->getFunctionObjectParameterType()) &&
+      !isRecordWithAttr<OwnerAttr>(Callee->getFunctionObjectParameterType()))
     return false;
   if (Callee->getReturnType()->isPointerType() ||
       isRecordWithAttr<PointerAttr>(Callee->getReturnType())) {
     if (!Callee->getIdentifier())
       return false;
     return llvm::StringSwitch<bool>(Callee->getName())
         .Cases("begin", "rbegin", "cbegin", "crbegin", true)
         .Cases("end", "rend", "cend", "crend", true)
static bool implicitObjectParamIsLifetimeBound(const FunctionDecl *FD) {
   OverloadedOperatorKind OO = FD->getDeclName().getCXXOverloadedOperator();
   if (OO == OO_Equal || isCompoundAssignmentOperator(OO)) {
     QualType RetT = FD->getReturnType();
     if (RetT->isLValueReferenceType()) {
       ASTContext &Ctx = FD->getASTContext();
       QualType LHST;
       auto *MD = dyn_cast<CXXMethodDecl>(FD);
       if (MD && MD->isCXXInstanceMember())
-        LHST = Ctx.getLValueReferenceType(MD->getThisObjectType());
+        LHST = Ctx.getLValueReferenceType(MD->getFunctionObjectParameterType());
       else
         LHST = MD->getParamDecl(0)->getType();
       if (Ctx.hasSameType(RetT, LHST))
         return true;
     }
   }
 
   return false;

clang/lib/Sema/SemaLambda.cpp

if (Result->isUndeducedType()) {
       Result = S.SubstAutoTypeDependent(Result);
       MethodType = S.Context.getFunctionType(Result, FPT->getParamTypes(),
                                              FPT->getExtProtoInfo());
     }
   }
   return MethodType;
 }
 
+// [C++2b] [expr.prim.lambda.closure] p4
+//  Given a lambda with a lambda-capture, the type of the explicit object
+//  parameter, if any, of the lambda's function call operator (possibly
+//  instantiated from a function call operator template) shall be either:
+//  - the closure type,
+//  - class type derived from the closure type, or
+//  - a reference to a possibly cv-qualified such type.
+void Sema::DiagnoseInvalidExplicitObjectParameterInLambda(
+    CXXMethodDecl *Method) {
+  if (!isLambdaCallWithExplicitObjectParameter(Method))
+    return;
+  CXXRecordDecl *RD = Method->getParent();
+  if (Method->getType()->isDependentType())
+    return;
+  if (RD->getLambdaCaptureDefault() == LambdaCaptureDefault::LCD_None &&
+      RD->capture_size() == 0)
+    return;
+  QualType ExplicitObjectParameterType = Method->getParamDecl(0)
+                                             ->getType()
+                                             .getNonReferenceType()
+                                             .getUnqualifiedType()
+                                             .getDesugaredType(getASTContext());
+  QualType LambdaType = getASTContext().getRecordType(RD);
+  if (LambdaType == ExplicitObjectParameterType)
+    return;
+  if (IsDerivedFrom(RD->getLocation(), ExplicitObjectParameterType, LambdaType))
+    return;
+  Diag(Method->getParamDecl(0)->getLocation(),
+       diag::err_invalid_explicit_object_type_in_lambda)
+      << ExplicitObjectParameterType;
+}
+
 void Sema::handleLambdaNumbering(
     CXXRecordDecl *Class, CXXMethodDecl *Method,
     std::optional<CXXRecordDecl::LambdaNumbering> NumberingOverride) {
   if (NumberingOverride) {
     Class->setLambdaNumbering(*NumberingOverride);
     return;
   }
 
static TypeSourceInfo *getLambdaType(Sema &S, LambdaIntroducer &Intro,
   bool IsLambdaStatic =
       ParamInfo.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_static;
 
   TypeSourceInfo *MethodTyInfo;
 
   if (ParamInfo.getNumTypeObjects() == 0) {
     MethodTyInfo = getDummyLambdaType(S, Loc);
   } else {
+    // Check explicit parameters
+    S.CheckExplicitObjectLambda(ParamInfo);
+
     DeclaratorChunk::FunctionTypeInfo &FTI = ParamInfo.getFunctionTypeInfo();
+
+    bool HasExplicitObjectParameter =
+        FTI.NumParams > 0 &&
+        cast<ParmVarDecl>(FTI.Params[0].Param)->isExplicitObjectParameter();
+
     ExplicitResultType = FTI.hasTrailingReturnType();
-    if (!FTI.hasMutableQualifier() && !IsLambdaStatic)
+    if (!FTI.hasMutableQualifier() && !IsLambdaStatic &&
+        !HasExplicitObjectParameter)
       FTI.getOrCreateMethodQualifiers().SetTypeQual(DeclSpec::TQ_const, Loc);
 
     if (ExplicitResultType && S.getLangOpts().HLSL) {
       QualType RetTy = FTI.getTrailingReturnType().get();
       if (!RetTy.isNull()) {
         // HLSL does not support specifying an address space on a lambda return
         // type.
         LangAS AddressSpace = RetTy.getAddressSpace();
if (Class->isGenericLambda()) {
     Class->addDecl(ConversionTemplate);
   } else
     Class->addDecl(Conversion);
 
   // If the lambda is not static, we need to add a static member
   // function that will be the result of the conversion with a
   // certain unique ID.
   // When it is static we just return the static call operator instead.
-  if (CallOperator->isInstance()) {
+  if (CallOperator->isImplicitObjectMemberFunction()) {
     DeclarationName InvokerName =
         &S.Context.Idents.get(getLambdaStaticInvokerName());
     // FIXME: Instead of passing in the CallOperator->getTypeSourceInfo()
     // we should get a prebuilt TrivialTypeSourceInfo from Context
     // using FunctionTy & Loc and get its TypeLoc as a FunctionProtoTypeLoc
     // then rewire the parameters accordingly, by hoisting up the InvokeParams
     // loop below and then use its Params to set Invoke->setParams(...) below.
     // This would avoid the 'const' qualifier of the calloperator from

clang/lib/Sema/SemaOverload.cpp

default:
     return std::nullopt;
   }
 }
 
 static bool FunctionsCorrespond(ASTContext &Ctx, const FunctionDecl *X,
                                 const FunctionDecl *Y) {
   if (!X || !Y)
     return false;
   if (X->getNumParams() != Y->getNumParams())
     return false;
   for (unsigned I = 0; I < X->getNumParams(); ++I)
     if (!Ctx.hasSameUnqualifiedType(X->getParamDecl(I)->getType(),
                                     Y->getParamDecl(I)->getType()))
       return false;
   if (auto *FTX = X->getDescribedFunctionTemplate()) {
     auto *FTY = Y->getDescribedFunctionTemplate();
     if (!FTY)
       return false;
     if (!Ctx.isSameTemplateParameterList(FTX->getTemplateParameters(),
                                          FTY->getTemplateParameters()))
       return false;
if (!allowsReversed(Op))
     return false;
   if (Op == OverloadedOperatorKind::OO_EqualEqual) {
     assert(OriginalArgs.size() == 2);
     if (!shouldAddReversedEqEq(
             S, OpLoc, /*FirstOperand in reversed args*/ OriginalArgs[1], FD))
       return false;
   }
   // Don't bother adding a reversed candidate that can never be a better
   // match than the non-reversed version.
-  return FD->getNumParams() != 2 ||
+  return FD->getNumNonObjectParams() != 2 ||
          !S.Context.hasSameUnqualifiedType(FD->getParamDecl(0)->getType(),
                                            FD->getParamDecl(1)->getType()) ||
          FD->hasAttr<EnableIfAttr>();
 }
 
 void OverloadCandidateSet::destroyCandidates() {
   for (iterator i = begin(), e = end(); i != e; ++i) {
     for (auto &C : i->Conversions)
       C.~ImplicitConversionSequence();
     if (!i->Viable && i->FailureKind == ovl_fail_bad_deduction)
if (New->getFriendObjectKind() && New->getQualifier() &&
     return Ovl_Match;
   }
 
   return Ovl_Overload;
 }
 
 bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
                       bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs,
-                      bool ConsiderRequiresClauses) {
+                      bool UseOverrideRules) {
   // C++ [basic.start.main]p2: This function shall not be overloaded.
   if (New->isMain())
     return false;
 
   // MSVCRT user defined entry points cannot be overloaded.
   if (New->isMSVCRTEntryPoint())
     return false;
 
if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
     return false;
 
   const FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
   const FunctionProtoType *NewType = cast<FunctionProtoType>(NewQType);
 
   // The signature of a function includes the types of its
   // parameters (C++ 1.3.10), which includes the presence or absence
   // of the ellipsis; see C++ DR 357).
-  if (OldQType != NewQType &&
-      (OldType->getNumParams() != NewType->getNumParams() ||
-       OldType->isVariadic() != NewType->isVariadic() ||
-       !FunctionParamTypesAreEqual(OldType, NewType)))
+  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
     return true;
 
   // For member-like friends, the enclosing class is part of the signature.
   if ((New->isMemberLikeConstrainedFriend() ||
        Old->isMemberLikeConstrainedFriend()) &&
       !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
     return true;
+  const auto *OldMethod = dyn_cast<CXXMethodDecl>(Old);
+  const auto *NewMethod = dyn_cast<CXXMethodDecl>(New);
+
+  int OldParamsOffset = 0;
+  int NewParamsOffset = 0;
+
+  // When determining if a method is an overload from a base class, act as if
+  // the implicit object parameter are of the same type.
+
+  auto NormalizeQualifiers = [&](const CXXMethodDecl *M, Qualifiers Q) {
+    if (M->isExplicitObjectMemberFunction())
+      return Q;
+
+    // We do not allow overloading based off of '__restrict'.
+    Q.removeRestrict();
+
+    // We may not have applied the implicit const for a constexpr member
+    // function yet (because we haven't yet resolved whether this is a static
+    // or non-static member function). Add it now, on the assumption that this
+    // is a redeclaration of OldMethod.
+    if (!getLangOpts().CPlusPlus14 && (M->isConstexpr() || M->isConsteval()) &&
+        !isa<CXXConstructorDecl>(NewMethod))
+      Q.addConst();
+    return Q;
+  };
+
+  auto CompareType = [&](QualType Base, QualType D) {
+    auto BS = Base.getNonReferenceType().getCanonicalType().split();
+    BS.Quals = NormalizeQualifiers(OldMethod, BS.Quals);
+
+    auto DS = D.getNonReferenceType().getCanonicalType().split();
+    DS.Quals = NormalizeQualifiers(NewMethod, DS.Quals);
+
+    if (BS.Quals != DS.Quals)
+      return false;
+
+    if (OldMethod->isImplicitObjectMemberFunction() &&
+        OldMethod->getParent() != NewMethod->getParent()) {
+      QualType ParentType =
+          Context.getTypeDeclType(OldMethod->getParent()).getCanonicalType();
+      if (ParentType.getTypePtr() != BS.Ty)
+        return false;
+      BS.Ty = DS.Ty;
+    }
+
+    if (BS.Ty != DS.Ty)
+      return false;
+
+    if (Base->isLValueReferenceType())
+      return D->isLValueReferenceType();
+    return Base->isRValueReferenceType() == D->isRValueReferenceType();
+  };
+
+  // If the function is a class member, its signature includes the
+  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
+  auto DiagnoseInconsistentRefQualifiers = [&]() {
+    if (LangOpts.CPlusPlus23)
+      return false;
+    if (OldMethod->getRefQualifier() == NewMethod->getRefQualifier())
+      return false;
+    if (OldMethod->isExplicitObjectMemberFunction() ||
+        NewMethod->isExplicitObjectMemberFunction())
+      return false;
+    if (!UseMemberUsingDeclRules && (OldMethod->getRefQualifier() == RQ_None ||
+                                     NewMethod->getRefQualifier() == RQ_None)) {
+      Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
+          << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
+      Diag(OldMethod->getLocation(), diag::note_previous_declaration);
+      return true;
+    }
+    return false;
+  };
+
+  if (OldMethod && OldMethod->isExplicitObjectMemberFunction())
+    OldParamsOffset++;
+  if (NewMethod && NewMethod->isExplicitObjectMemberFunction())
+    NewParamsOffset++;
+
+  if (OldType->getNumParams() - OldParamsOffset !=
+          NewType->getNumParams() - NewParamsOffset ||
+      !FunctionParamTypesAreEqual(
+          {OldType->param_type_begin() + OldParamsOffset,
+           OldType->param_type_end()},
+          {NewType->param_type_begin() + NewParamsOffset,
+           NewType->param_type_end()},
+          nullptr)) {
+    return true;
+  }
+
+  if (OldMethod && NewMethod && !OldMethod->isStatic() &&
+      !OldMethod->isStatic()) {
+    bool HaveCorrespondingObjectParameters = [&](const CXXMethodDecl *Old,
+                                                 const CXXMethodDecl *New) {
+      auto NewObjectType = New->getFunctionObjectParameterReferenceType();
+      auto OldObjectType = Old->getFunctionObjectParameterReferenceType();
+
+      auto IsImplicitWithNoRefQual = [](const CXXMethodDecl *F) {
+        return F->getRefQualifier() == RQ_None &&
+               !F->isExplicitObjectMemberFunction();
+      };
+
+      if (IsImplicitWithNoRefQual(Old) != IsImplicitWithNoRefQual(New) &&
+          CompareType(OldObjectType.getNonReferenceType(),
+                      NewObjectType.getNonReferenceType()))
+        return true;
+      return CompareType(OldObjectType, NewObjectType);
+    }(OldMethod, NewMethod);
+
+    if (!HaveCorrespondingObjectParameters) {
+      DiagnoseInconsistentRefQualifiers();
+      // CWG2554
+      // and, if at least one is an explicit object member function, ignoring
+      // object parameters
+      if (!UseOverrideRules || (!NewMethod->isExplicitObjectMemberFunction() &&
+                                !OldMethod->isExplicitObjectMemberFunction()))
+        return true;
+    }
+  }
 
   if (NewTemplate) {
     // C++ [temp.over.link]p4:
     //   The signature of a function template consists of its function
     //   signature, its return type and its template parameter list. The names
     //   of the template parameters are significant only for establishing the
     //   relationship between the template parameters and the rest of the
     //   signature.
     //
     // We check the return type and template parameter lists for function
if (NewTemplate) {
     if (UseMemberUsingDeclRules && ConstraintsInTemplateHead &&
         !SameTemplateParameterList)
       return true;
     if (!UseMemberUsingDeclRules &&
         (!SameTemplateParameterList || !SameReturnType))
       return true;
   }
 
-  if (ConsiderRequiresClauses) {
+  if (!UseOverrideRules) {
     Expr *NewRC = New->getTrailingRequiresClause(),
          *OldRC = Old->getTrailingRequiresClause();
     if ((NewRC != nullptr) != (OldRC != nullptr))
       return true;
 
     if (NewRC && !AreConstraintExpressionsEqual(Old, OldRC, New, NewRC))
         return true;
   }
 
-  // If the function is a class member, its signature includes the
-  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
-  //
-  // As part of this, also check whether one of the member functions
-  // is static, in which case they are not overloads (C++
-  // 13.1p2). While not part of the definition of the signature,
-  // this check is important to determine whether these functions
-  // can be overloaded.
-  CXXMethodDecl *OldMethod = dyn_cast<CXXMethodDecl>(Old);
-  CXXMethodDecl *NewMethod = dyn_cast<CXXMethodDecl>(New);
-  if (OldMethod && NewMethod &&
-      !OldMethod->isStatic() && !NewMethod->isStatic()) {
-    if (OldMethod->getRefQualifier() != NewMethod->getRefQualifier()) {
-      if (!UseMemberUsingDeclRules &&
-          (OldMethod->getRefQualifier() == RQ_None ||
-           NewMethod->getRefQualifier() == RQ_None)) {
-        // C++20 [over.load]p2:
-        //   - Member function declarations with the same name, the same
-        //     parameter-type-list, and the same trailing requires-clause (if
-        //     any), as well as member function template declarations with the
-        //     same name, the same parameter-type-list, the same trailing
-        //     requires-clause (if any), and the same template-head, cannot be
-        //     overloaded if any of them, but not all, have a ref-qualifier.
-        Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
-            << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
-        Diag(OldMethod->getLocation(), diag::note_previous_declaration);
-      }
-      return true;
-    }
-
-    // We may not have applied the implicit const for a constexpr member
-    // function yet (because we haven't yet resolved whether this is a static
-    // or non-static member function). Add it now, on the assumption that this
-    // is a redeclaration of OldMethod.
-    auto OldQuals = OldMethod->getMethodQualifiers();
-    auto NewQuals = NewMethod->getMethodQualifiers();
-    if (!getLangOpts().CPlusPlus14 && NewMethod->isConstexpr() &&
-        !isa<CXXConstructorDecl>(NewMethod))
-      NewQuals.addConst();
-    // We do not allow overloading based off of '__restrict'.
-    OldQuals.removeRestrict();
-    NewQuals.removeRestrict();
-    if (OldQuals != NewQuals)
+  if (NewMethod && OldMethod && OldMethod->isImplicitObjectMemberFunction() &&
+      NewMethod->isImplicitObjectMemberFunction()) {
+    if (DiagnoseInconsistentRefQualifiers())
       return true;
   }
 
   // Though pass_object_size is placed on parameters and takes an argument, we
   // consider it to be a function-level modifier for the sake of function
   // identity. Either the function has one or more parameters with
   // pass_object_size or it doesn't.
   if (functionHasPassObjectSizeParams(New) !=
if (FunctionDecl *Fn
             return false;
       }
 
       // Check if the "from" expression is taking the address of an overloaded
       // function and recompute the FromType accordingly. Take advantage of the
       // fact that non-static member functions *must* have such an address-of
       // expression.
       CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn);
-      if (Method && !Method->isStatic()) {
+      if (Method && !Method->isStatic() &&
+          !Method->isExplicitObjectMemberFunction()) {
         assert(isa<UnaryOperator>(From->IgnoreParens()) &&
                "Non-unary operator on non-static member address");
         assert(cast<UnaryOperator>(From->IgnoreParens())->getOpcode()
                == UO_AddrOf &&
                "Non-address-of operator on non-static member address");
         const Type *ClassType
           = S.Context.getTypeDeclType(Method->getParent()).getTypePtr();
         FromType = S.Context.getMemberPointerType(FromType, ClassType);
if (Context.getCanonicalType(FromFunctionType->getReturnType()) ==
                                        ConvertedType, IncompatibleObjC)) {
       // Okay, we have an Objective-C pointer conversion.
       HasObjCConversion = true;
     } else {
       // Function types are too different. Abort.
       return false;
     }
 
     // Check argument types.
     for (unsigned ArgIdx = 0, NumArgs = FromFunctionType->getNumParams();
          ArgIdx != NumArgs; ++ArgIdx) {
       QualType FromArgType = FromFunctionType->getParamType(ArgIdx);
       QualType ToArgType = ToFunctionType->getParamType(ArgIdx);
       if (Context.getCanonicalType(FromArgType)
             == Context.getCanonicalType(ToArgType)) {
         // Okay, the types match exactly. Nothing to do.
       } else if (isObjCPointerConversion(FromArgType, ToArgType,
                                          ConvertedType, IncompatibleObjC)) {
         // Okay, we have an Objective-C pointer conversion.
         HasObjCConversion = true;
       } else {

 
 /// FunctionParamTypesAreEqual - This routine checks two function proto types
 /// for equality of their parameter types. Caller has already checked that
 /// they have same number of parameters.  If the parameters are different,
 /// ArgPos will have the parameter index of the first different parameter.
 /// If `Reversed` is true, the parameters of `NewType` will be compared in
 /// reverse order. That's useful if one of the functions is being used as a C++20
 /// synthesized operator overload with a reversed parameter order.
-bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
-                                      const FunctionProtoType *NewType,
-                                      unsigned *ArgPos, bool Reversed) {
-  assert(OldType->getNumParams() == NewType->getNumParams() &&
+bool Sema::FunctionParamTypesAreEqual(ArrayRef<QualType> Old,
+                                      ArrayRef<QualType> New, unsigned *ArgPos,
+                                      bool Reversed) {
+  assert(llvm::size(Old) == llvm::size(New) &&
          "Can't compare parameters of functions with different number of "
          "parameters!");
-  for (size_t I = 0; I < OldType->getNumParams(); I++) {
+
+  for (auto &&[Idx, Type] : llvm::enumerate(Old)) {
     // Reverse iterate over the parameters of `OldType` if `Reversed` is true.
-    size_t J = Reversed ? (OldType->getNumParams() - I - 1) : I;
+    size_t J = Reversed ? (llvm::size(New) - Idx - 1) : Idx;
 
     // Ignore address spaces in pointee type. This is to disallow overloading
     // on __ptr32/__ptr64 address spaces.
-    QualType Old = Context.removePtrSizeAddrSpace(OldType->getParamType(I).getUnqualifiedType());
-    QualType New = Context.removePtrSizeAddrSpace(NewType->getParamType(J).getUnqualifiedType());
+    QualType OldType =
+        Context.removePtrSizeAddrSpace(Type.getUnqualifiedType());
+    QualType NewType =
+        Context.removePtrSizeAddrSpace((New.begin() + J)->getUnqualifiedType());
 
-    if (!Context.hasSameType(Old, New)) {
+    if (!Context.hasSameType(OldType, NewType)) {
       if (ArgPos)
-        *ArgPos = I;
+        *ArgPos = Idx;
       return false;
     }
   }
   return true;
 }
 
+bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
+                                      const FunctionProtoType *NewType,
+                                      unsigned *ArgPos, bool Reversed) {
+  return FunctionParamTypesAreEqual(OldType->param_types(),
+                                    NewType->param_types(), ArgPos, Reversed);
+}
+
 /// CheckPointerConversion - Check the pointer conversion from the
 /// expression From to the type ToType. This routine checks for
 /// ambiguous or inaccessible derived-to-base pointer
 /// conversions for which IsPointerConversion has already returned
 /// true. It returns true and produces a diagnostic if there was an
 /// error, or returns false otherwise.
 bool Sema::CheckPointerConversion(Expr *From, QualType ToType,
                                   CastKind &Kind,
ImplicitConversionSequence ICS =
     TryCopyInitialization(S, &TmpExpr, ToQTy, true, true, false);
 
   return !ICS.isBad();
 }
 
 /// TryObjectArgumentInitialization - Try to initialize the object
 /// parameter of the given member function (@c Method) from the
 /// expression @p From.
-static ImplicitConversionSequence
-TryObjectArgumentInitialization(Sema &S, SourceLocation Loc, QualType FromType,
-                                Expr::Classification FromClassification,
-                                CXXMethodDecl *Method,
-                                CXXRecordDecl *ActingContext) {
+static ImplicitConversionSequence TryObjectArgumentInitialization(
+    Sema &S, SourceLocation Loc, QualType FromType,
+    Expr::Classification FromClassification, CXXMethodDecl *Method,
+    const CXXRecordDecl *ActingContext, bool InOverloadResolution = false,
+    QualType ExplicitParameterType = QualType(),
+    bool SuppressUserConversion = false) {
+
+  // We need to have an object of class type.
+  if (const auto *PT = FromType->getAs<PointerType>()) {
+    FromType = PT->getPointeeType();
+
+    // When we had a pointer, it's implicitly dereferenced, so we
+    // better have an lvalue.
+    assert(FromClassification.isLValue());
+  }
+
+  auto ValueKindFromClassification = [](Expr::Classification C) {
+    if (C.isPRValue())
+      return clang::VK_PRValue;
+    if (C.isXValue())
+      return VK_XValue;
+    return clang::VK_LValue;
+  };
+
+  if (Method->isExplicitObjectMemberFunction()) {
+    if (ExplicitParameterType.isNull())
+      ExplicitParameterType = Method->getFunctionObjectParameterReferenceType();
+    OpaqueValueExpr TmpExpr(Loc, FromType.getNonReferenceType(),
+                            ValueKindFromClassification(FromClassification));
+    ImplicitConversionSequence ICS = TryCopyInitialization(
+        S, &TmpExpr, ExplicitParameterType, SuppressUserConversion,
+        /*InOverloadResolution=*/true, false);
+    if (ICS.isBad())
+      ICS.Bad.FromExpr = nullptr;
+    return ICS;
+  }
+
+  assert(FromType->isRecordType());
+
   QualType ClassType = S.Context.getTypeDeclType(ActingContext);
   // [class.dtor]p2: A destructor can be invoked for a const, volatile or
   //                 const volatile object.
   Qualifiers Quals = Method->getMethodQualifiers();
   if (isa<CXXDestructorDecl>(Method)) {
     Quals.addConst();
     Quals.addVolatile();
   }
 
   QualType ImplicitParamType = S.Context.getQualifiedType(ClassType, Quals);
 
   // Set up the conversion sequence as a "bad" conversion, to allow us
   // to exit early.
   ImplicitConversionSequence ICS;
 
-  // We need to have an object of class type.
-  if (const PointerType *PT = FromType->getAs<PointerType>()) {
-    FromType = PT->getPointeeType();
-
-    // When we had a pointer, it's implicitly dereferenced, so we
-    // better have an lvalue.
-    assert(FromClassification.isLValue());
-  }
-
-  assert(FromType->isRecordType());
-
   // C++0x [over.match.funcs]p4:
   //   For non-static member functions, the type of the implicit object
   //   parameter is
   //
   //     - "lvalue reference to cv X" for functions declared without a
   //        ref-qualifier or with the & ref-qualifier
   //     - "rvalue reference to cv X" for functions declared with the &&
   //        ref-qualifier
static ImplicitConversionSequence TryObjectArgumentInitialization(
   }
 
   // Check that we have either the same type or a derived type. It
   // affects the conversion rank.
   QualType ClassTypeCanon = S.Context.getCanonicalType(ClassType);
   ImplicitConversionKind SecondKind;
   if (ClassTypeCanon == FromTypeCanon.getLocalUnqualifiedType()) {
     SecondKind = ICK_Identity;
-  } else if (S.IsDerivedFrom(Loc, FromType, ClassType))
+  } else if (S.IsDerivedFrom(Loc, FromType, ClassType)) {
     SecondKind = ICK_Derived_To_Base;
-  else {
+  } else if (!Method->isExplicitObjectMemberFunction()) {
     ICS.setBad(BadConversionSequence::unrelated_class,
                FromType, ImplicitParamType);
     return ICS;
   }
 
   // Check the ref-qualifier.
   switch (Method->getRefQualifier()) {
   case RQ_None:
static ImplicitConversionSequence TryObjectArgumentInitialization(
   ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier
     = (Method->getRefQualifier() == RQ_None);
   return ICS;
 }
 
 /// PerformObjectArgumentInitialization - Perform initialization of
 /// the implicit object parameter for the given Method with the given
 /// expression.
-ExprResult
-Sema::PerformObjectArgumentInitialization(Expr *From,
-                                          NestedNameSpecifier *Qualifier,
-                                          NamedDecl *FoundDecl,
-                                          CXXMethodDecl *Method) {
+ExprResult Sema::PerformImplicitObjectArgumentInitialization(
+    Expr *From, NestedNameSpecifier *Qualifier, NamedDecl *FoundDecl,
   QualType FromRecordType, DestType;
   QualType ImplicitParamRecordType  =
     Method->getThisType()->castAs<PointerType>()->getPointeeType();
 
   Expr::Classification FromClassification;
   if (const PointerType *PT = From->getType()->getAs<PointerType>()) {
     FromRecordType = PT->getPointeeType();
     DestType = Method->getThisType();
ExprResult Sema::PerformContextuallyConvertToObjCPointer(Expr *From) {
   QualType Ty = Context.getObjCIdType();
   ImplicitConversionSequence ICS =
     TryContextuallyConvertToObjCPointer(*this, From);
   if (!ICS.isBad())
     return PerformImplicitConversion(From, Ty, ICS, AA_Converting);
   return ExprResult();
 }
 
+static QualType GetExplicitObjectType(Sema &S, const Expr *MemExprE) {
+  const Expr *Base = nullptr;
+  assert((isa<UnresolvedMemberExpr, MemberExpr>(MemExprE)) &&
+         "expected a member expression");
+
+  if (const auto M = dyn_cast<UnresolvedMemberExpr>(MemExprE);
+      M && !M->isImplicitAccess())
+    Base = M->getBase();
+  else if (const auto M = dyn_cast<MemberExpr>(MemExprE);
+           M && !M->isImplicitAccess())
+    Base = M->getBase();
+
+  QualType T = Base ? Base->getType() : S.getCurrentThisType();
+
+  if (T->isPointerType())
+    T = T->getPointeeType();
+
+  return T;
+}
+
+static Expr *GetExplicitObjectExpr(Sema &S, Expr *Obj,
+                                   const FunctionDecl *Fun) {
+  QualType ObjType = Obj->getType();
+  if (ObjType->isPointerType()) {
+    ObjType = ObjType->getPointeeType();
+    Obj = UnaryOperator::Create(S.getASTContext(), Obj, UO_Deref, ObjType,
+                                VK_LValue, OK_Ordinary, SourceLocation(),
+                                /*CanOverflow=*/false, FPOptionsOverride());
+  }
+  if (Obj->Classify(S.getASTContext()).isPRValue()) {
+    Obj = S.CreateMaterializeTemporaryExpr(
+        ObjType, Obj,
+        !Fun->getParamDecl(0)->getType()->isRValueReferenceType());
+  }
+  return Obj;
+}
+
+ExprResult Sema::InitializeExplicitObjectArgument(Sema &S, Expr *Obj,
+                                                  FunctionDecl *Fun) {
+  Obj = GetExplicitObjectExpr(S, Obj, Fun);
+  return S.PerformCopyInitialization(
+      InitializedEntity::InitializeParameter(S.Context, Fun->getParamDecl(0)),
+      Obj->getExprLoc(), Obj);
+}
+
+static void PrepareExplicitObjectArgument(Sema &S, CXXMethodDecl *Method,
+                                          Expr *Object, MultiExprArg &Args,
+                                          SmallVectorImpl<Expr *> &NewArgs) {
+  assert(Method->isExplicitObjectMemberFunction() &&
+         "Method is not an explicit member function");
+  assert(NewArgs.empty() && "NewArgs should be empty");
+  NewArgs.reserve(Args.size() + 1);
+  Expr *This = GetExplicitObjectExpr(S, Object, Method);
+  NewArgs.push_back(This);
+  NewArgs.append(Args.begin(), Args.end());
+  Args = NewArgs;
+}
+
 /// Determine whether the provided type is an integral type, or an enumeration
 /// type of a permitted flavor.
 bool Sema::ICEConvertDiagnoser::match(QualType T) {
   return AllowScopedEnumerations ? T->isIntegralOrEnumerationType()
                                  : T->isIntegralOrUnscopedEnumerationType();
 }
 
 static ExprResult
static bool recordConversion(Sema &SemaRef, SourceLocation Loc, Expr *&From,
   QualType ToType = Conversion->getConversionType().getNonReferenceType();
   if (!Converter.SuppressConversion) {
     if (SemaRef.isSFINAEContext())
       return true;
 
     Converter.diagnoseConversion(SemaRef, Loc, T, ToType)
         << From->getSourceRange();
   }
 
   ExprResult Result = SemaRef.BuildCXXMemberCallExpr(From, Found, Conversion,
                                                      HadMultipleCandidates);
   if (Result.isInvalid())
     return true;
   // Record usage of conversion in an implicit cast.
   From = ImplicitCastExpr::Create(SemaRef.Context, Result.get()->getType(),
                                   CK_UserDefinedConversion, Result.get(),
                                   nullptr, Result.get()->getValueKind(),
if (ConvTemplate) {
       Conversion = cast<CXXConversionDecl>(D);
 
     assert((!ConvTemplate || getLangOpts().CPlusPlus14) &&
            "Conversion operator templates are considered potentially "
            "viable in C++1y");
 
     QualType CurToType = Conversion->getConversionType().getNonReferenceType();
     if (Converter.match(CurToType) || ConvTemplate) {
 
       if (Conversion->isExplicit()) {
         // FIXME: For C++1y, do we need this restriction?
         // cf. diagnoseNoViableConversion()
         if (!ConvTemplate)
           ExplicitConversions.addDecl(I.getDecl(), I.getAccess());
       } else {
         if (!ConvTemplate && getLangOpts().CPlusPlus14) {
           if (ToType.isNull())
static bool convertArgsForAvailabilityChecks(
     Sema &S, FunctionDecl *Function, Expr *ThisArg, SourceLocation CallLoc,
     ArrayRef<Expr *> Args, Sema::SFINAETrap &Trap, bool MissingImplicitThis,
     Expr *&ConvertedThis, SmallVectorImpl<Expr *> &ConvertedArgs) {
   if (ThisArg) {
     CXXMethodDecl *Method = cast<CXXMethodDecl>(Function);
     assert(!isa<CXXConstructorDecl>(Method) &&
            "Shouldn't have `this` for ctors!");
     assert(!Method->isStatic() && "Shouldn't have `this` for static methods!");
-    ExprResult R = S.PerformObjectArgumentInitialization(
+    ExprResult R = S.PerformImplicitObjectArgumentInitialization(
         ThisArg, /*Qualifier=*/nullptr, Method, Method);
     if (R.isInvalid())
       return false;
     ConvertedThis = R.get();
   } else {
     if (auto *MD = dyn_cast<CXXMethodDecl>(Function)) {
       (void)MD;
       assert((MissingImplicitThis || MD->isStatic() ||
Sema::AddMethodCandidate(CXXMethodDecl *Method, DeclAccessPair FoundDecl,
   Candidate.FoundDecl = FoundDecl;
   Candidate.Function = Method;
   Candidate.RewriteKind =
       CandidateSet.getRewriteInfo().getRewriteKind(Method, PO);
   Candidate.IsSurrogate = false;
   Candidate.IgnoreObjectArgument = false;
   Candidate.ExplicitCallArguments = Args.size();
 
-  unsigned NumParams = Proto->getNumParams();
+  unsigned NumParams = Method->getNumExplicitParams();
+  unsigned ExplicitOffset = Method->isExplicitObjectMemberFunction() ? 1 : 0;
 
   // (C++ 13.3.2p2): A candidate function having fewer than m
   // parameters is viable only if it has an ellipsis in its parameter
   // list (8.3.5).
   if (TooManyArguments(NumParams, Args.size(), PartialOverloading) &&
       !Proto->isVariadic() &&
       shouldEnforceArgLimit(PartialOverloading, Method)) {
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_too_many_arguments;
     return;
   }
 
   // (C++ 13.3.2p2): A candidate function having more than m parameters
   // is viable only if the (m+1)st parameter has a default argument
   // (8.3.6). For the purposes of overload resolution, the
   // parameter list is truncated on the right, so that there are
   // exactly m parameters.
-  unsigned MinRequiredArgs = Method->getMinRequiredArguments();
+  unsigned MinRequiredArgs = Method->getMinRequiredExplicitArguments();
   if (Args.size() < MinRequiredArgs && !PartialOverloading) {
     // Not enough arguments.
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_too_few_arguments;
     return;
   }
 
   Candidate.Viable = true;
else if (Method->isStatic()) {
     // apply it retroactively because we want to support static lambdas as an
     // extension and it doesn't hurt previous code.
     Candidate.Conversions[FirstConvIdx].setStaticObjectArgument();
   } else {
     // Determine the implicit conversion sequence for the object
     // parameter.
     Candidate.Conversions[FirstConvIdx] = TryObjectArgumentInitialization(
         *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
-        Method, ActingContext);
+        Method, ActingContext, /*InOverloadResolution=*/true);
     if (Candidate.Conversions[FirstConvIdx].isBad()) {
       Candidate.Viable = false;
       Candidate.FailureKind = ovl_fail_bad_conversion;
       return;
     }
   }
 
   // (CUDA B.1): Check for invalid calls between targets.
for (unsigned ArgIdx = 0; ArgIdx < Args.size(); ++ArgIdx) {
     if (Candidate.Conversions[ConvIdx].isInitialized()) {
       // We already formed a conversion sequence for this parameter during
       // template argument deduction.
     } else if (ArgIdx < NumParams) {
       // (C++ 13.3.2p3): for F to be a viable function, there shall
       // exist for each argument an implicit conversion sequence
       // (13.3.3.1) that converts that argument to the corresponding
       // parameter of F.
-      QualType ParamType = Proto->getParamType(ArgIdx);
+      QualType ParamType = Proto->getParamType(ArgIdx + ExplicitOffset);
       Candidate.Conversions[ConvIdx]
         = TryCopyInitialization(*this, Args[ArgIdx], ParamType,
                                 SuppressUserConversions,
                                 /*InOverloadResolution=*/true,
                                 /*AllowObjCWritebackConversion=*/
                                   getLangOpts().ObjCAutoRefCount);
       if (Candidate.Conversions[ConvIdx].isBad()) {
         Candidate.Viable = false;
void Sema::AddMethodTemplateCandidate(
   //   functions. In such a case, the candidate functions generated from each
   //   function template are combined with the set of non-template candidate
   //   functions.
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   FunctionDecl *Specialization = nullptr;
   ConversionSequenceList Conversions;
   if (TemplateDeductionResult Result = DeduceTemplateArguments(
           MethodTmpl, ExplicitTemplateArgs, Args, Specialization, Info,
-          PartialOverloading, /*AggregateDeductionCandidate=*/false,
-          [&](ArrayRef<QualType> ParamTypes) {
+          PartialOverloading, /*AggregateDeductionCandidate=*/false, ObjectType,
+          ObjectClassification, [&](ArrayRef<QualType> ParamTypes) {
             return CheckNonDependentConversions(
                 MethodTmpl, ParamTypes, Args, CandidateSet, Conversions,
                 SuppressUserConversions, ActingContext, ObjectType,
                 ObjectClassification, PO);
           })) {
     OverloadCandidate &Candidate =
         CandidateSet.addCandidate(Conversions.size(), Conversions);
     Candidate.FoundDecl = FoundDecl;
void Sema::AddTemplateOverloadCandidate(
   //   function template are combined with the set of non-template candidate
   //   functions.
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   FunctionDecl *Specialization = nullptr;
   ConversionSequenceList Conversions;
   if (TemplateDeductionResult Result = DeduceTemplateArguments(
           FunctionTemplate, ExplicitTemplateArgs, Args, Specialization, Info,
           PartialOverloading, AggregateCandidateDeduction,
+          /*ObjectType=*/QualType(),
+          /*ObjectClassification=*/Expr::Classification(),
           [&](ArrayRef<QualType> ParamTypes) {
             return CheckNonDependentConversions(
                 FunctionTemplate, ParamTypes, Args, CandidateSet, Conversions,
                 SuppressUserConversions, nullptr, QualType(), {}, PO);
           })) {
     OverloadCandidate &Candidate =
         CandidateSet.addCandidate(Conversions.size(), Conversions);
     Candidate.FoundDecl = FoundDecl;
EnterExpressionEvaluationContext Unevaluated(
       *this, Sema::ExpressionEvaluationContext::Unevaluated);
 
   // For a method call, check the 'this' conversion here too. DR1391 doesn't
   // require that, but this check should never result in a hard error, and
   // overload resolution is permitted to sidestep instantiations.
   if (HasThisConversion && !cast<CXXMethodDecl>(FD)->isStatic() &&
       !ObjectType.isNull()) {
     unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed ? 1 : 0;
+    if (!FD->hasCXXExplicitFunctionObjectParameter() ||
+        !ParamTypes[0]->isDependentType()) {
       Conversions[ConvIdx] = TryObjectArgumentInitialization(
           *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
-        Method, ActingContext);
+          Method, ActingContext, /*InOverloadResolution=*/true,
+          FD->hasCXXExplicitFunctionObjectParameter() ? ParamTypes[0]
+                                                      : QualType());
       if (Conversions[ConvIdx].isBad())
         return true;
     }
+  }
+
+  unsigned Offset =
+      Method && Method->hasCXXExplicitFunctionObjectParameter() ? 1 : 0;
 
   for (unsigned I = 0, N = std::min(ParamTypes.size(), Args.size()); I != N;
        ++I) {
-    QualType ParamType = ParamTypes[I];
+    QualType ParamType = ParamTypes[I + Offset];
     if (!ParamType->isDependentType()) {
       unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed
                              ? 0
                              : (ThisConversions + I);
       Conversions[ConvIdx]
         = TryCopyInitialization(*this, Args[I], ParamType,
                                 SuppressUserConversions,
                                 /*InOverloadResolution=*/true,
void Sema::AddConversionCandidate(
 
   // C++ [over.match.funcs]p4:
   //   For conversion functions, the function is considered to be a member of
   //   the class of the implicit implied object argument for the purpose of
   //   defining the type of the implicit object parameter.
   //
   // Determine the implicit conversion sequence for the implicit
   // object parameter.
-  QualType ImplicitParamType = From->getType();
-  if (const PointerType *FromPtrType = ImplicitParamType->getAs<PointerType>())
-    ImplicitParamType = FromPtrType->getPointeeType();
-  CXXRecordDecl *ConversionContext
-    = cast<CXXRecordDecl>(ImplicitParamType->castAs<RecordType>()->getDecl());
-
+  QualType ObjectType = From->getType();
+  if (const auto *FromPtrType = ObjectType->getAs<PointerType>())
+    ObjectType = FromPtrType->getPointeeType();
+  const auto *ConversionContext =
+      cast<CXXRecordDecl>(ObjectType->castAs<RecordType>()->getDecl());
+
+  // C++23 [over.best.ics.general]
+  // However, if the target is [...]
+  // - the object parameter of a user-defined conversion function
+  // [...] user-defined conversion sequences are not considered.
   Candidate.Conversions[0] = TryObjectArgumentInitialization(
       *this, CandidateSet.getLocation(), From->getType(),
-      From->Classify(Context), Conversion, ConversionContext);
+      From->Classify(Context), Conversion, ConversionContext,
+      /*InOverloadResolution*/ false, /*ExplicitParameterType=*/QualType(),
+      /*SuppressUserConversion*/ true);
 
   if (Candidate.Conversions[0].isBad()) {
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_bad_conversion;
     return;
   }
 
   if (Conversion->getTrailingRequiresClause()) {
if (!AllowExplicit && isNonDependentlyExplicit(FunctionTemplate)) {
     OverloadCandidate &Candidate = CandidateSet.addCandidate();
     Candidate.FoundDecl = FoundDecl;
     Candidate.Function = FunctionTemplate->getTemplatedDecl();
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_explicit;
     return;
   }
 
+  QualType ObjectType = From->getType();
+  Expr::Classification ObjectClassification = From->Classify(getASTContext());
+
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   CXXConversionDecl *Specialization = nullptr;
-  if (TemplateDeductionResult Result
-        = DeduceTemplateArguments(FunctionTemplate, ToType,
-                                  Specialization, Info)) {
+  if (TemplateDeductionResult Result = DeduceTemplateArguments(
+          FunctionTemplate, ObjectType, ObjectClassification, ToType,
     OverloadCandidate &Candidate = CandidateSet.addCandidate();
     Candidate.FoundDecl = FoundDecl;
     Candidate.Function = FunctionTemplate->getTemplatedDecl();
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_bad_deduction;
     Candidate.IsSurrogate = false;
     Candidate.IgnoreObjectArgument = false;
     Candidate.ExplicitCallArguments = 1;
void Sema::AddSurrogateCandidate(CXXConversionDecl *Conversion,
   Candidate.Surrogate = Conversion;
   Candidate.Viable = true;
   Candidate.IsSurrogate = true;
   Candidate.IgnoreObjectArgument = false;
   Candidate.ExplicitCallArguments = Args.size();
 
   // Determine the implicit conversion sequence for the implicit
   // object parameter.
-  ImplicitConversionSequence ObjectInit = TryObjectArgumentInitialization(
+  ImplicitConversionSequence ObjectInit;
+  if (Conversion->hasCXXExplicitFunctionObjectParameter()) {
+    ObjectInit = TryCopyInitialization(*this, Object,
+                                       Conversion->getParamDecl(0)->getType(),
+                                       /*SuppressUserConversions=*/false,
+                                       /*InOverloadResolution=*/true, false);
+  } else {
+    ObjectInit = TryObjectArgumentInitialization(
         *this, CandidateSet.getLocation(), Object->getType(),
         Object->Classify(Context), Conversion, ActingContext);
+  }
+
   if (ObjectInit.isBad()) {
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_bad_conversion;
     Candidate.Conversions[0] = ObjectInit;
     return;
   }
 
   // The first conversion is actually a user-defined conversion whose

 getImplicitObjectParamType(ASTContext &Context, const FunctionDecl *F) {
   if (!isa<CXXMethodDecl>(F) || isa<CXXConstructorDecl>(F))
     return std::nullopt;
 
   auto *M = cast<CXXMethodDecl>(F);
   // Static member functions' object parameters match all types.
   if (M->isStatic())
     return QualType();
-
-  QualType T = M->getThisObjectType();
-  if (M->getRefQualifier() == RQ_RValue)
-    return Context.getRValueReferenceType(T);
-  return Context.getLValueReferenceType(T);
+  return M->getFunctionObjectParameterReferenceType();
 }
 
 static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
                                    const FunctionDecl *F2, unsigned NumParams) {
   if (declaresSameEntity(F1, F2))
     return true;
 
   auto NextParam = [&](const FunctionDecl *F, unsigned &I, bool First) {
assert(isa<FunctionDecl>(D) &&
       "The templated declaration should at least be a function"
       " when diagnosing bad template argument deduction due to too many"
       " or too few arguments");
 
   FunctionDecl *Fn = cast<FunctionDecl>(D);
 
   // TODO: treat calls to a missing default constructor as a special case
   const auto *FnTy = Fn->getType()->castAs<FunctionProtoType>();
-  unsigned MinParams = Fn->getMinRequiredArguments();
+  unsigned MinParams = Fn->getMinRequiredExplicitArguments();
 
   // at least / at most / exactly
+  bool HasExplicitObjectParam = Fn->hasCXXExplicitFunctionObjectParameter();
+  unsigned ParamCount = FnTy->getNumParams() - (HasExplicitObjectParam ? 1 : 0);
   unsigned mode, modeCount;
   if (NumFormalArgs < MinParams) {
-    if (MinParams != FnTy->getNumParams() || FnTy->isVariadic() ||
+    if (MinParams != ParamCount || FnTy->isVariadic() ||
         FnTy->isTemplateVariadic())
       mode = 0; // "at least"
     else
       mode = 2; // "exactly"
     modeCount = MinParams;
   } else {
-    if (MinParams != FnTy->getNumParams())
+    if (MinParams != ParamCount)
       mode = 1; // "at most"
     else
       mode = 2; // "exactly"
-    modeCount = FnTy->getNumParams();
+    modeCount = ParamCount;
   }
 
   std::string Description;
   std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
       ClassifyOverloadCandidate(S, Found, Fn, CRK_None, Description);
 
-  if (modeCount == 1 && Fn->getParamDecl(0)->getDeclName())
+  if (modeCount == 1 &&
+      Fn->getParamDecl(HasExplicitObjectParam ? 1 : 0)->getDeclName())
     S.Diag(Fn->getLocation(), diag::note_ovl_candidate_arity_one)
         << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
-        << Description << mode << Fn->getParamDecl(0) << NumFormalArgs
-        << Fn->getParametersSourceRange();
+        << Description << mode
+        << Fn->getParamDecl(HasExplicitObjectParam ? 1 : 0) << NumFormalArgs
+        << HasExplicitObjectParam << Fn->getParametersSourceRange();
   else
     S.Diag(Fn->getLocation(), diag::note_ovl_candidate_arity)
         << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
         << Description << mode << modeCount << NumFormalArgs
-        << Fn->getParametersSourceRange();
+        << HasExplicitObjectParam << Fn->getParametersSourceRange();
 
   MaybeEmitInheritedConstructorNote(S, Found);
 }
 
 /// Arity mismatch diagnosis specific to a function overload candidate.
 static void DiagnoseArityMismatch(Sema &S, OverloadCandidate *Cand,
                                   unsigned NumFormalArgs) {
   if (!CheckArityMismatch(S, Cand, NumFormalArgs))
private:
 
   // return true if any matching specializations were found
   bool AddMatchingTemplateFunction(FunctionTemplateDecl* FunctionTemplate,
                                    const DeclAccessPair& CurAccessFunPair) {
     if (CXXMethodDecl *Method
               = dyn_cast<CXXMethodDecl>(FunctionTemplate->getTemplatedDecl())) {
       // Skip non-static function templates when converting to pointer, and
       // static when converting to member pointer.
-      if (Method->isStatic() == TargetTypeIsNonStaticMemberFunction)
+      bool CanConvertToFunctionPointer =
+          Method->isStatic() || Method->isExplicitObjectMemberFunction();
+      if (CanConvertToFunctionPointer == TargetTypeIsNonStaticMemberFunction)
         return false;
     }
     else if (TargetTypeIsNonStaticMemberFunction)
       return false;
 
     // C++ [over.over]p2:
     //   If the name is a function template, template argument deduction is
     //   done (14.8.2.2), and if the argument deduction succeeds, the
bool AddMatchingTemplateFunction(FunctionTemplateDecl* FunctionTemplate,
     return true;
   }
 
   bool AddMatchingNonTemplateFunction(NamedDecl* Fn,
                                       const DeclAccessPair& CurAccessFunPair) {
     if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn)) {
       // Skip non-static functions when converting to pointer, and static
       // when converting to member pointer.
-      if (Method->isStatic() == TargetTypeIsNonStaticMemberFunction)
+      bool CanConvertToFunctionPointer =
+          Method->isStatic() || Method->isExplicitObjectMemberFunction();
+      if (CanConvertToFunctionPointer == TargetTypeIsNonStaticMemberFunction)
         return false;
     }
     else if (TargetTypeIsNonStaticMemberFunction)
       return false;
 
     if (FunctionDecl *FunDecl = dyn_cast<FunctionDecl>(Fn)) {
       if (S.getLangOpts().CUDA)
         if (FunctionDecl *Caller = S.getCurFunctionDecl(/*AllowLambda=*/true))
if (!Found || Found->isCPUDispatchMultiVersion() ||
       Found->isCPUSpecificMultiVersion())
     return false;
 
   // Emitting multiple diagnostics for a function that is both inaccessible and
   // unavailable is consistent with our behavior elsewhere. So, always check
   // for both.
   DiagnoseUseOfDecl(Found, E->getExprLoc());
   CheckAddressOfMemberAccess(E, DAP);
-  Expr *Fixed = FixOverloadedFunctionReference(E, DAP, Found);
+  ExprResult Res = FixOverloadedFunctionReference(E, DAP, Found);
+  if (Res.isInvalid())
+    return false;
+  Expr *Fixed = Res.get();
   if (DoFunctionPointerConversion && Fixed->getType()->isFunctionType())
     SrcExpr = DefaultFunctionArrayConversion(Fixed, /*Diagnose=*/false);
   else
     SrcExpr = Fixed;
   return true;
 }
 
 /// Given an expression that refers to an overloaded function, try to
static ExprResult FinishOverloadedCallExpr(Sema &SemaRef, Scope *S, Expr *Fn,
                                            OverloadingResult OverloadResult,
                                            bool AllowTypoCorrection) {
   switch (OverloadResult) {
   case OR_Success: {
     FunctionDecl *FDecl = (*Best)->Function;
     SemaRef.CheckUnresolvedLookupAccess(ULE, (*Best)->FoundDecl);
     if (SemaRef.DiagnoseUseOfDecl(FDecl, ULE->getNameLoc()))
       return ExprError();
-    Fn = SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
-    return SemaRef.BuildResolvedCallExpr(Fn, FDecl, LParenLoc, Args, RParenLoc,
-                                         ExecConfig, /*IsExecConfig=*/false,
-                                         (*Best)->IsADLCandidate);
+    ExprResult Res =
+        SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
+    if (Res.isInvalid())
+      return ExprError();
+    return SemaRef.BuildResolvedCallExpr(
+        Res.get(), FDecl, LParenLoc, Args, RParenLoc, ExecConfig,
+        /*IsExecConfig=*/false, (*Best)->IsADLCandidate);
   }
 
   case OR_No_Viable_Function: {
     // Try to recover by looking for viable functions which the user might
     // have meant to call.
     ExprResult Recovery = BuildRecoveryCallExpr(SemaRef, S, Fn, ULE, LParenLoc,
                                                 Args, RParenLoc,
                                                 CandidateSet->empty(),
CandidateSet->NoteCandidates(
         PartialDiagnosticAt(Fn->getBeginLoc(),
                             SemaRef.PDiag(diag::err_ovl_deleted_call)
                                 << ULE->getName() << Fn->getSourceRange()),
         SemaRef, OCD_AllCandidates, Args);
 
     // We emitted an error for the unavailable/deleted function call but keep
     // the call in the AST.
     FunctionDecl *FDecl = (*Best)->Function;
-    Fn = SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
-    return SemaRef.BuildResolvedCallExpr(Fn, FDecl, LParenLoc, Args, RParenLoc,
-                                         ExecConfig, /*IsExecConfig=*/false,
-                                         (*Best)->IsADLCandidate);
+    ExprResult Res =
+        SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
+    if (Res.isInvalid())
+      return ExprError();
+    return SemaRef.BuildResolvedCallExpr(
+        Res.get(), FDecl, LParenLoc, Args, RParenLoc, ExecConfig,
+        /*IsExecConfig=*/false, (*Best)->IsADLCandidate);
   }
   }
 
   // Overload resolution failed, try to recover.
   SmallVector<Expr *, 8> SubExprs = {Fn};
   SubExprs.append(Args.begin(), Args.end());
   return SemaRef.CreateRecoveryExpr(Fn->getBeginLoc(), RParenLoc, SubExprs,
                                     chooseRecoveryType(*CandidateSet, Best));
ExprResult Sema::CreateUnresolvedLookupExpr(CXXRecordDecl *NamingClass,
                                             DeclarationNameInfo DNI,
                                             const UnresolvedSetImpl &Fns,
                                             bool PerformADL) {
   return UnresolvedLookupExpr::Create(Context, NamingClass, NNSLoc, DNI,
                                       PerformADL, IsOverloaded(Fns),
                                       Fns.begin(), Fns.end());
 }
 
+ExprResult Sema::BuildCXXMemberCallExpr(Expr *E, NamedDecl *FoundDecl,
+                                        CXXConversionDecl *Method,
+                                        bool HadMultipleCandidates) {
+  // Convert the expression to match the conversion function's implicit object
+  // parameter.
+  ExprResult Exp;
+  if (Method->isExplicitObjectMemberFunction())
+    Exp = InitializeExplicitObjectArgument(*this, E, Method);
+  else
+    Exp = PerformImplicitObjectArgumentInitialization(E, /*Qualifier=*/nullptr,
+                                                      FoundDecl, Method);
+  if (Exp.isInvalid())
+    return true;
+
+  if (Method->getParent()->isLambda() &&
+      Method->getConversionType()->isBlockPointerType()) {
+    // This is a lambda conversion to block pointer; check if the argument
+    // was a LambdaExpr.
+    Expr *SubE = E;
+    auto *CE = dyn_cast<CastExpr>(SubE);
+    if (CE && CE->getCastKind() == CK_NoOp)
+      SubE = CE->getSubExpr();
+    SubE = SubE->IgnoreParens();
+    if (auto *BE = dyn_cast<CXXBindTemporaryExpr>(SubE))
+      SubE = BE->getSubExpr();
+    if (isa<LambdaExpr>(SubE)) {
+      // For the conversion to block pointer on a lambda expression, we
+      // construct a special BlockLiteral instead; this doesn't really make
+      // a difference in ARC, but outside of ARC the resulting block literal
+      // follows the normal lifetime rules for block literals instead of being
+      // autoreleased.
+      PushExpressionEvaluationContext(
+          ExpressionEvaluationContext::PotentiallyEvaluated);
+      ExprResult BlockExp = BuildBlockForLambdaConversion(
+          Exp.get()->getExprLoc(), Exp.get()->getExprLoc(), Method, Exp.get());
+      PopExpressionEvaluationContext();
+
+      // FIXME: This note should be produced by a CodeSynthesisContext.
+      if (BlockExp.isInvalid())
+        Diag(Exp.get()->getExprLoc(), diag::note_lambda_to_block_conv);
+      return BlockExp;
+    }
+  }
+  CallExpr *CE;
+  QualType ResultType = Method->getReturnType();
+  ExprValueKind VK = Expr::getValueKindForType(ResultType);
+  ResultType = ResultType.getNonLValueExprType(Context);
+  if (Method->isExplicitObjectMemberFunction()) {
+    ExprResult FnExpr =
+        CreateFunctionRefExpr(*this, Method, FoundDecl, Exp.get(),
+                              HadMultipleCandidates, E->getBeginLoc());
+    if (FnExpr.isInvalid())
+      return ExprError();
+    Expr *ObjectParam = Exp.get();
+    CE = CallExpr::Create(Context, FnExpr.get(), MultiExprArg(&ObjectParam, 1),
+                          ResultType, VK, Exp.get()->getEndLoc(),
+                          CurFPFeatureOverrides());
+  } else {
+    MemberExpr *ME =
+        BuildMemberExpr(Exp.get(), /*IsArrow=*/false, SourceLocation(),
+                        NestedNameSpecifierLoc(), SourceLocation(), Method,
+                        DeclAccessPair::make(FoundDecl, FoundDecl->getAccess()),
+                        HadMultipleCandidates, DeclarationNameInfo(),
+                        Context.BoundMemberTy, VK_PRValue, OK_Ordinary);
+
+    CE = CXXMemberCallExpr::Create(Context, ME, /*Args=*/{}, ResultType, VK,
+                                   Exp.get()->getEndLoc(),
+                                   CurFPFeatureOverrides());
+  }
+
+  if (CheckFunctionCall(Method, CE,
+                        Method->getType()->castAs<FunctionProtoType>()))
+    return ExprError();
+
+  return CheckForImmediateInvocation(CE, CE->getDirectCallee());
+}
+
 /// Create a unary operation that may resolve to an overloaded
 /// operator.
 ///
 /// \param OpLoc The location of the operator itself (e.g., '*').
 ///
 /// \param Opc The UnaryOperatorKind that describes this operator.
 ///
 /// \param Fns The set of non-member functions that will be
case OR_Success: {
 
     if (FnDecl) {
       Expr *Base = nullptr;
       // We matched an overloaded operator. Build a call to that
       // operator.
 
       // Convert the arguments.
       if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FnDecl)) {
-        CheckMemberOperatorAccess(OpLoc, Args[0], nullptr, Best->FoundDecl);
+        CheckMemberOperatorAccess(OpLoc, Input, nullptr, Best->FoundDecl);
 
-        ExprResult InputRes =
-          PerformObjectArgumentInitialization(Input, /*Qualifier=*/nullptr,
-                                              Best->FoundDecl, Method);
-        if (InputRes.isInvalid())
+        ExprResult InputInit;
+        if (Method->isExplicitObjectMemberFunction())
+          InputInit = InitializeExplicitObjectArgument(*this, Input, Method);
+        else
+          InputInit = PerformImplicitObjectArgumentInitialization(
+              Input, /*Qualifier=*/nullptr, Best->FoundDecl, Method);
+        if (InputInit.isInvalid())
           return ExprError();
-        Base = Input = InputRes.get();
+        Base = Input = InputInit.get();
       } else {
         // Convert the arguments.
         ExprResult InputInit
           = PerformCopyInitialization(InitializedEntity::InitializeParameter(
                                                       Context,
                                                       FnDecl->getParamDecl(0)),
                                       SourceLocation(),
                                       Input);
if (FnDecl) {
         return ExprError();
 
       // Determine the result type.
       QualType ResultTy = FnDecl->getReturnType();
       ExprValueKind VK = Expr::getValueKindForType(ResultTy);
       ResultTy = ResultTy.getNonLValueExprType(Context);
 
       Args[0] = Input;
       CallExpr *TheCall = CXXOperatorCallExpr::Create(
           Context, Op, FnExpr.get(), ArgsArray, ResultTy, VK, OpLoc,
           CurFPFeatureOverrides(), Best->IsADLCandidate);
 
       if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall, FnDecl))
         return ExprError();
 
       if (CheckFunctionCall(FnDecl, TheCall,
                             FnDecl->getType()->castAs<FunctionProtoType>()))
case OR_Success: {
                    diag::note_ovl_ambiguous_oper_binary_reversed_self);
               // Mark member== const or provide matching != to disallow reversed
               // args. Eg.
               // struct S { bool operator==(const S&); };
               // S()==S();
               if (auto *MD = dyn_cast<CXXMethodDecl>(FnDecl))
                 if (Op == OverloadedOperatorKind::OO_EqualEqual &&
                     !MD->isConst() &&
+                    !MD->hasCXXExplicitFunctionObjectParameter() &&
                     Context.hasSameUnqualifiedType(
-                        MD->getThisObjectType(),
+                        MD->getFunctionObjectParameterType(),
                         MD->getParamDecl(0)->getType().getNonReferenceType()) &&
-                    Context.hasSameUnqualifiedType(MD->getThisObjectType(),
-                                                   Args[0]->getType()) &&
-                    Context.hasSameUnqualifiedType(MD->getThisObjectType(),
-                                                   Args[1]->getType()))
+                    Context.hasSameUnqualifiedType(
+                        MD->getFunctionObjectParameterType(),
+                    Context.hasSameUnqualifiedType(
+                        MD->getFunctionObjectParameterType(),
                   Diag(FnDecl->getLocation(),
                        diag::note_ovl_ambiguous_eqeq_reversed_self_non_const);
             } else {
               Diag(FnDecl->getLocation(),
                    diag::note_ovl_ambiguous_oper_binary_selected_candidate);
               for (auto *F : AmbiguousWith)
                 Diag(F->getLocation(),
                      diag::note_ovl_ambiguous_oper_binary_reversed_candidate);
             }
           }
         }
 
         // Convert the arguments.
         if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FnDecl)) {
           // Best->Access is only meaningful for class members.
           CheckMemberOperatorAccess(OpLoc, Args[0], Args[1], Best->FoundDecl);
 
-          ExprResult Arg1 =
-            PerformCopyInitialization(
-              InitializedEntity::InitializeParameter(Context,
-                                                     FnDecl->getParamDecl(0)),
+          ExprResult Arg0, Arg1;
+          unsigned ParamIdx = 0;
+          if (Method->isExplicitObjectMemberFunction()) {
+            Arg0 = InitializeExplicitObjectArgument(*this, Args[0], FnDecl);
+            ParamIdx = 1;
+          } else {
+            Arg0 = PerformImplicitObjectArgumentInitialization(
+                Args[0], /*Qualifier=*/nullptr, Best->FoundDecl, Method);
+          }
+          Arg1 = PerformCopyInitialization(
+              InitializedEntity::InitializeParameter(
+                  Context, FnDecl->getParamDecl(ParamIdx)),
               SourceLocation(), Args[1]);
-          if (Arg1.isInvalid())
+          if (Arg0.isInvalid() || Arg1.isInvalid())
             return ExprError();
 
-          ExprResult Arg0 =
-            PerformObjectArgumentInitialization(Args[0], /*Qualifier=*/nullptr,
-                                                Best->FoundDecl, Method);
-          if (Arg0.isInvalid())
-            return ExprError();
           Base = Args[0] = Arg0.getAs<Expr>();
           Args[1] = RHS = Arg1.getAs<Expr>();
         } else {
           // Convert the arguments.
           ExprResult Arg0 = PerformCopyInitialization(
             InitializedEntity::InitializeParameter(Context,
                                                    FnDecl->getParamDecl(0)),
             SourceLocation(), Args[0]);
case OR_Success: {
         if (FnExpr.isInvalid())
           return ExprError();
 
         // Determine the result type.
         QualType ResultTy = FnDecl->getReturnType();
         ExprValueKind VK = Expr::getValueKindForType(ResultTy);
         ResultTy = ResultTy.getNonLValueExprType(Context);
 
-        CXXOperatorCallExpr *TheCall = CXXOperatorCallExpr::Create(
+        CallExpr *TheCall;
+        ArrayRef<const Expr *> ArgsArray(Args, 2);
+        const Expr *ImplicitThis = nullptr;
+
+        // We always create a CXXOperatorCallExpr, even for explicit object
+        // members; CodeGen should take care not to emit the this pointer.
+        TheCall = CXXOperatorCallExpr::Create(
             Context, ChosenOp, FnExpr.get(), Args, ResultTy, VK, OpLoc,
             CurFPFeatureOverrides(), Best->IsADLCandidate);
 
-        if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall,
-                                FnDecl))
-          return ExprError();
-
-        ArrayRef<const Expr *> ArgsArray(Args, 2);
-        const Expr *ImplicitThis = nullptr;
+        if (const auto *Method = dyn_cast<CXXMethodDecl>(FnDecl);
+            Method && Method->isImplicitObjectMemberFunction()) {
           // Cut off the implicit 'this'.
-        if (isa<CXXMethodDecl>(FnDecl)) {
           ImplicitThis = ArgsArray[0];
           ArgsArray = ArgsArray.slice(1);
         }
 
+        if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall,
+                                FnDecl))
+          return ExprError();
+
         // Check for a self move.
         if (Op == OO_Equal)
           DiagnoseSelfMove(Args[0], Args[1], OpLoc);
 
         if (ImplicitThis) {
           QualType ThisType = Context.getPointerType(ImplicitThis->getType());
           QualType ThisTypeFromDecl = Context.getPointerType(
-              cast<CXXMethodDecl>(FnDecl)->getThisObjectType());
+              cast<CXXMethodDecl>(FnDecl)->getFunctionObjectParameterType());
 
           CheckArgAlignment(OpLoc, FnDecl, "'this'", ThisType,
                             ThisTypeFromDecl);
         }
 
         checkCall(FnDecl, nullptr, ImplicitThis, ArgsArray,
                   isa<CXXMethodDecl>(FnDecl), OpLoc, TheCall->getSourceRange(),
                   VariadicDoesNotApply);
case OR_Success: {
 
         CheckMemberOperatorAccess(LLoc, Args[0], ArgExpr, Best->FoundDecl);
 
         // Convert the arguments.
         CXXMethodDecl *Method = cast<CXXMethodDecl>(FnDecl);
         SmallVector<Expr *, 2> MethodArgs;
 
         // Handle 'this' parameter if the selected function is not static.
-        if (Method->isInstance()) {
-          ExprResult Arg0 = PerformObjectArgumentInitialization(
+        if (Method->isExplicitObjectMemberFunction()) {
+          ExprResult Res =
+              InitializeExplicitObjectArgument(*this, Args[0], Method);
+          if (Res.isInvalid())
+            return ExprError();
+          Args[0] = Res.get();
+          ArgExpr = Args;
+        } else if (Method->isInstance()) {
+          ExprResult Arg0 = PerformImplicitObjectArgumentInitialization(
               Args[0], /*Qualifier=*/nullptr, Best->FoundDecl, Method);
           if (Arg0.isInvalid())
             return ExprError();
 
           MethodArgs.push_back(Arg0.get());
         }
 
         bool IsError = PrepareArgumentsForCallToObjectOfClassType(
return CallExpr::Create(Context, MemExprE, Args, Context.VoidTy, VK_PRValue,
                             RParenLoc, CurFPFeatureOverrides());
 
   UnbridgedCastsSet UnbridgedCasts;
   if (checkArgPlaceholdersForOverload(*this, Args, UnbridgedCasts))
     return ExprError();
 
   MemberExpr *MemExpr;
   CXXMethodDecl *Method = nullptr;
+  bool HadMultipleCandidates = false;
   DeclAccessPair FoundDecl = DeclAccessPair::make(nullptr, AS_public);
   NestedNameSpecifier *Qualifier = nullptr;
   if (isa<MemberExpr>(NakedMemExpr)) {
     MemExpr = cast<MemberExpr>(NakedMemExpr);
     Method = cast<CXXMethodDecl>(MemExpr->getMemberDecl());
     FoundDecl = MemExpr->getFoundDecl();
     Qualifier = MemExpr->getQualifier();
     UnbridgedCasts.restore();
if (isa<MemberExpr>(NakedMemExpr)) {
     if (UnresExpr->hasExplicitTemplateArgs()) {
       UnresExpr->copyTemplateArgumentsInto(TemplateArgsBuffer);
       TemplateArgs = &TemplateArgsBuffer;
     }
 
     for (UnresolvedMemberExpr::decls_iterator I = UnresExpr->decls_begin(),
            E = UnresExpr->decls_end(); I != E; ++I) {
 
+      QualType ExplicitObjectType = ObjectType;
+
       NamedDecl *Func = *I;
       CXXRecordDecl *ActingDC = cast<CXXRecordDecl>(Func->getDeclContext());
       if (isa<UsingShadowDecl>(Func))
         Func = cast<UsingShadowDecl>(Func)->getTargetDecl();
 
+      bool HasExplicitParameter = false;
+      if (const auto *M = dyn_cast<FunctionDecl>(Func);
+          M && M->hasCXXExplicitFunctionObjectParameter())
+        HasExplicitParameter = true;
+      else if (const auto *M = dyn_cast<FunctionTemplateDecl>(Func);
+               M &&
+               M->getTemplatedDecl()->hasCXXExplicitFunctionObjectParameter())
+        HasExplicitParameter = true;
+
+      if (HasExplicitParameter)
+        ExplicitObjectType = GetExplicitObjectType(*this, UnresExpr);
 
       // Microsoft supports direct constructor calls.
       if (getLangOpts().MicrosoftExt && isa<CXXConstructorDecl>(Func)) {
         AddOverloadCandidate(cast<CXXConstructorDecl>(Func), I.getPair(), Args,
                              CandidateSet,
                              /*SuppressUserConversions*/ false);
       } else if ((Method = dyn_cast<CXXMethodDecl>(Func))) {
         // If explicit template arguments were provided, we can't call a
         // non-template member function.
         if (TemplateArgs)
           continue;
 
-        AddMethodCandidate(Method, I.getPair(), ActingDC, ObjectType,
+        AddMethodCandidate(Method, I.getPair(), ActingDC, ExplicitObjectType,
                            ObjectClassification, Args, CandidateSet,
                            /*SuppressUserConversions=*/false);
       } else {
-        AddMethodTemplateCandidate(
-            cast<FunctionTemplateDecl>(Func), I.getPair(), ActingDC,
-            TemplateArgs, ObjectType, ObjectClassification, Args, CandidateSet,
+        AddMethodTemplateCandidate(cast<FunctionTemplateDecl>(Func),
+                                   I.getPair(), ActingDC, TemplateArgs,
+                                   ExplicitObjectType, ObjectClassification,
+                                   Args, CandidateSet,
                                    /*SuppressUserConversions=*/false);
       }
     }
 
+    HadMultipleCandidates = (CandidateSet.size() > 1);
+
     DeclarationName DeclName = UnresExpr->getMemberName();
 
     UnbridgedCasts.restore();
 
     OverloadCandidateSet::iterator Best;
     bool Succeeded = false;
     switch (CandidateSet.BestViableFunction(*this, UnresExpr->getBeginLoc(),
                                             Best)) {
case OR_Deleted:
                                   << DeclName << MemExprE->getSourceRange()),
           *this, OCD_AllCandidates, Args);
       break;
     }
     // Overload resolution fails, try to recover.
     if (!Succeeded)
       return BuildRecoveryExpr(chooseRecoveryType(CandidateSet, &Best));
 
-    MemExprE = FixOverloadedFunctionReference(MemExprE, FoundDecl, Method);
+    ExprResult Res =
+        FixOverloadedFunctionReference(MemExprE, FoundDecl, Method);
+    if (Res.isInvalid())
+      return ExprError();
+    MemExprE = Res.get();
 
-    // If overload resolution picked a static member, build a
-    // non-member call based on that function.
+    // If overload resolution picked a static member
+    // build a non-member call based on that function.
     if (Method->isStatic()) {
       return BuildResolvedCallExpr(MemExprE, Method, LParenLoc, Args, RParenLoc,
                                    ExecConfig, IsExecConfig);
     }
 
     MemExpr = cast<MemberExpr>(MemExprE->IgnoreParens());
   }
 
   QualType ResultType = Method->getReturnType();
   ExprValueKind VK = Expr::getValueKindForType(ResultType);
   ResultType = ResultType.getNonLValueExprType(Context);
 
   assert(Method && "Member call to something that isn't a method?");
   const auto *Proto = Method->getType()->castAs<FunctionProtoType>();
-  CXXMemberCallExpr *TheCall = CXXMemberCallExpr::Create(
-      Context, MemExprE, Args, ResultType, VK, RParenLoc,
-      CurFPFeatureOverrides(), Proto->getNumParams());
 
-  // Check for a valid return type.
-  if (CheckCallReturnType(Method->getReturnType(), MemExpr->getMemberLoc(),
-                          TheCall, Method))
-    return BuildRecoveryExpr(ResultType);
+  CallExpr *TheCall = nullptr;
+  llvm::SmallVector<Expr *, 8> NewArgs;
+  if (Method->isExplicitObjectMemberFunction()) {
+    PrepareExplicitObjectArgument(*this, Method, MemExpr->getBase(), Args,
+                                  NewArgs);
+    // Build the actual expression node.
+    ExprResult FnExpr =
+        CreateFunctionRefExpr(*this, Method, FoundDecl, MemExpr,
+                              HadMultipleCandidates, MemExpr->getExprLoc());
+    if (FnExpr.isInvalid())
+      return ExprError();
 
+    TheCall =
+        CallExpr::Create(Context, FnExpr.get(), Args, ResultType, VK, RParenLoc,
+                         CurFPFeatureOverrides(), Proto->getNumParams());
+  } else {
     // Convert the object argument (for a non-static member function call).
     // We only need to do this if there was actually an overload; otherwise
     // it was done at lookup.
-  if (!Method->isStatic()) {
-    ExprResult ObjectArg =
-      PerformObjectArgumentInitialization(MemExpr->getBase(), Qualifier,
-                                          FoundDecl, Method);
+    ExprResult ObjectArg = PerformImplicitObjectArgumentInitialization(
+        MemExpr->getBase(), Qualifier, FoundDecl, Method);
     if (ObjectArg.isInvalid())
       return ExprError();
     MemExpr->setBase(ObjectArg.get());
+    TheCall = CXXMemberCallExpr::Create(Context, MemExprE, Args, ResultType, VK,
+                                        RParenLoc, CurFPFeatureOverrides(),
+                                        Proto->getNumParams());
   }
 
+  // Check for a valid return type.
+  if (CheckCallReturnType(Method->getReturnType(), MemExpr->getMemberLoc(),
+                          TheCall, Method))
+    return BuildRecoveryExpr(ResultType);
+
   // Convert the rest of the arguments
   if (ConvertArgumentsForCall(TheCall, MemExpr, Method, Proto, Args,
                               RParenLoc))
     return BuildRecoveryExpr(ResultType);
 
   DiagnoseSentinelCalls(Method, LParenLoc, Args);
 
   if (CheckFunctionCall(Method, TheCall, Proto))
if (const EnableIfAttr *Attr =
            diag::err_ovl_no_viable_member_function_in_call)
           << Method << Method->getSourceRange();
       Diag(Method->getLocation(),
            diag::note_ovl_candidate_disabled_by_function_cond_attr)
           << Attr->getCond()->getSourceRange() << Attr->getMessage();
       return ExprError();
     }
   }
 
-  if ((isa<CXXConstructorDecl>(CurContext) ||
-       isa<CXXDestructorDecl>(CurContext)) &&
-      TheCall->getMethodDecl()->isPure()) {
-    const CXXMethodDecl *MD = TheCall->getMethodDecl();
+  if (isa<CXXConstructorDecl, CXXDestructorDecl>(CurContext) &&
+      TheCall->getDirectCallee()->isPure()) {
+    const FunctionDecl *MD = TheCall->getDirectCallee();
 
     if (isa<CXXThisExpr>(MemExpr->getBase()->IgnoreParenCasts()) &&
         MemExpr->performsVirtualDispatch(getLangOpts())) {
       Diag(MemExpr->getBeginLoc(),
            diag::warn_call_to_pure_virtual_member_function_from_ctor_dtor)
           << MD->getDeclName() << isa<CXXDestructorDecl>(CurContext)
           << MD->getParent();
 
       Diag(MD->getBeginLoc(), diag::note_previous_decl) << MD->getDeclName();
       if (getLangOpts().AppleKext)
         Diag(MemExpr->getBeginLoc(), diag::note_pure_qualified_call_kext)
             << MD->getParent() << MD->getDeclName();
     }
   }
 
-  if (CXXDestructorDecl *DD =
-          dyn_cast<CXXDestructorDecl>(TheCall->getMethodDecl())) {
+  if (auto *DD = dyn_cast<CXXDestructorDecl>(TheCall->getDirectCallee())) {
     // a->A::f() doesn't go through the vtable, except in AppleKext mode.
     bool CallCanBeVirtual = !MemExpr->hasQualifier() || getLangOpts().AppleKext;
     CheckVirtualDtorCall(DD, MemExpr->getBeginLoc(), /*IsDelete=*/false,
                          CallCanBeVirtual, /*WarnOnNonAbstractTypes=*/true,
                          MemExpr->getMemberLoc());
   }
 
   return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall),
-                                     TheCall->getMethodDecl());
+                                     TheCall->getDirectCallee());
 }
 
 /// BuildCallToObjectOfClassType - Build a call to an object of class
 /// type (C++ [over.call.object]), which can end up invoking an
 /// overloaded function call operator (@c operator()) or performing a
 /// user-defined conversion on the object argument.
 ExprResult
 Sema::BuildCallToObjectOfClassType(Scope *S, Expr *Obj,
Sema::BuildCallToObjectOfClassType(Scope *S, Expr *Obj,
   SmallVector<Expr *, 8> MethodArgs;
   MethodArgs.reserve(NumParams + 1);
 
   bool IsError = false;
 
   // Initialize the implicit object parameter if needed.
   // Since C++23, this could also be a call to a static call operator
   // which we emit as a regular CallExpr.
-  if (Method->isInstance()) {
-    ExprResult ObjRes = PerformObjectArgumentInitialization(
+  llvm::SmallVector<Expr *, 8> NewArgs;
+  if (Method->isExplicitObjectMemberFunction()) {
+    // FIXME: we should do that during the definition of the lambda when we can.
+    DiagnoseInvalidExplicitObjectParameterInLambda(Method);
+    PrepareExplicitObjectArgument(*this, Method, Obj, Args, NewArgs);
+  } else if (Method->isInstance()) {
+    ExprResult ObjRes = PerformImplicitObjectArgumentInitialization(
         Object.get(), /*Qualifier=*/nullptr, Best->FoundDecl, Method);
     if (ObjRes.isInvalid())
       IsError = true;
     else
       Object = ObjRes;
     MethodArgs.push_back(Object.get());
   }
 
CandidateSet.NoteCandidates(
         *this, OCD_AllCandidates, Base);
     return ExprError();
   }
 
   CheckMemberOperatorAccess(OpLoc, Base, nullptr, Best->FoundDecl);
 
   // Convert the object parameter.
   CXXMethodDecl *Method = cast<CXXMethodDecl>(Best->Function);
-  ExprResult BaseResult =
-    PerformObjectArgumentInitialization(Base, /*Qualifier=*/nullptr,
-                                        Best->FoundDecl, Method);
+
+  if (Method->isExplicitObjectMemberFunction()) {
+    ExprResult R = InitializeExplicitObjectArgument(*this, Base, Method);
+    if (R.isInvalid())
+      return ExprError();
+    Base = R.get();
+  } else {
+    ExprResult BaseResult = PerformImplicitObjectArgumentInitialization(
+        Base, /*Qualifier=*/nullptr, Best->FoundDecl, Method);
     if (BaseResult.isInvalid())
       return ExprError();
     Base = BaseResult.get();
+  }
 
   // Build the operator call.
   ExprResult FnExpr = CreateFunctionRefExpr(*this, Method, Best->FoundDecl,
                                             Base, HadMultipleCandidates, OpLoc);
   if (FnExpr.isInvalid())
     return ExprError();
 
   QualType ResultTy = Method->getReturnType();
   ExprValueKind VK = Expr::getValueKindForType(ResultTy);
   ResultTy = ResultTy.getNonLValueExprType(Context);
-  CXXOperatorCallExpr *TheCall =
+
+  CallExpr *TheCall =
       CXXOperatorCallExpr::Create(Context, OO_Arrow, FnExpr.get(), Base,
                                   ResultTy, VK, OpLoc, CurFPFeatureOverrides());
 
   if (CheckCallReturnType(Method->getReturnType(), OpLoc, TheCall, Method))
     return ExprError();
 
   if (CheckFunctionCall(Method, TheCall,
                         Method->getType()->castAs<FunctionProtoType>()))

 }
 
 
 /// FixOverloadedFunctionReference - E is an expression that refers to
 /// a C++ overloaded function (possibly with some parentheses and
 /// perhaps a '&' around it). We have resolved the overloaded function
 /// to the function declaration Fn, so patch up the expression E to
 /// refer (possibly indirectly) to Fn. Returns the new expr.
-Expr *Sema::FixOverloadedFunctionReference(Expr *E, DeclAccessPair Found,
+ExprResult Sema::FixOverloadedFunctionReference(Expr *E, DeclAccessPair Found,
                                                 FunctionDecl *Fn) {
   if (ParenExpr *PE = dyn_cast<ParenExpr>(E)) {
-    Expr *SubExpr = FixOverloadedFunctionReference(PE->getSubExpr(),
-                                                   Found, Fn);
-    if (SubExpr == PE->getSubExpr())
+    ExprResult SubExpr =
+        FixOverloadedFunctionReference(PE->getSubExpr(), Found, Fn);
+    if (SubExpr.isInvalid())
+      return ExprError();
+    if (SubExpr.get() == PE->getSubExpr())
       return PE;
 
-    return new (Context) ParenExpr(PE->getLParen(), PE->getRParen(), SubExpr);
+    return new (Context)
+        ParenExpr(PE->getLParen(), PE->getRParen(), SubExpr.get());
   }
 
   if (ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(E)) {
-    Expr *SubExpr = FixOverloadedFunctionReference(ICE->getSubExpr(),
-                                                   Found, Fn);
+    ExprResult SubExpr =
+        FixOverloadedFunctionReference(ICE->getSubExpr(), Found, Fn);
+    if (SubExpr.isInvalid())
+      return ExprError();
     assert(Context.hasSameType(ICE->getSubExpr()->getType(),
-                               SubExpr->getType()) &&
+                               SubExpr.get()->getType()) &&
            "Implicit cast type cannot be determined from overload");
     assert(ICE->path_empty() && "fixing up hierarchy conversion?");
-    if (SubExpr == ICE->getSubExpr())
+    if (SubExpr.get() == ICE->getSubExpr())
       return ICE;
 
     return ImplicitCastExpr::Create(Context, ICE->getType(), ICE->getCastKind(),
-                                    SubExpr, nullptr, ICE->getValueKind(),
+                                    SubExpr.get(), nullptr, ICE->getValueKind(),
                                     CurFPFeatureOverrides());
   }
 
   if (auto *GSE = dyn_cast<GenericSelectionExpr>(E)) {
     if (!GSE->isResultDependent()) {
-      Expr *SubExpr =
+      ExprResult SubExpr =
           FixOverloadedFunctionReference(GSE->getResultExpr(), Found, Fn);
-      if (SubExpr == GSE->getResultExpr())
+      if (SubExpr.isInvalid())
+        return ExprError();
+      if (SubExpr.get() == GSE->getResultExpr())
         return GSE;
 
       // Replace the resulting type information before rebuilding the generic
       // selection expression.
       ArrayRef<Expr *> A = GSE->getAssocExprs();
       SmallVector<Expr *, 4> AssocExprs(A.begin(), A.end());
       unsigned ResultIdx = GSE->getResultIndex();
-      AssocExprs[ResultIdx] = SubExpr;
+      AssocExprs[ResultIdx] = SubExpr.get();
 
       if (GSE->isExprPredicate())
         return GenericSelectionExpr::Create(
             Context, GSE->getGenericLoc(), GSE->getControllingExpr(),
             GSE->getAssocTypeSourceInfos(), AssocExprs, GSE->getDefaultLoc(),
             GSE->getRParenLoc(), GSE->containsUnexpandedParameterPack(),
             ResultIdx);
       return GenericSelectionExpr::Create(
if (UnaryOperator *UnOp = dyn_cast<UnaryOperator>(E)) {
     if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn)) {
       if (Method->isStatic()) {
         // Do nothing: static member functions aren't any different
         // from non-member functions.
       } else {
         // Fix the subexpression, which really has to be an
         // UnresolvedLookupExpr holding an overloaded member function
         // or template.
-        Expr *SubExpr = FixOverloadedFunctionReference(UnOp->getSubExpr(),
-                                                       Found, Fn);
-        if (SubExpr == UnOp->getSubExpr())
+        ExprResult SubExpr =
+            FixOverloadedFunctionReference(UnOp->getSubExpr(), Found, Fn);
+        if (SubExpr.isInvalid())
+          return ExprError();
+        if (SubExpr.get() == UnOp->getSubExpr())
           return UnOp;
 
-        assert(isa<DeclRefExpr>(SubExpr)
-               && "fixed to something other than a decl ref");
-        assert(cast<DeclRefExpr>(SubExpr)->getQualifier()
-               && "fixed to a member ref with no nested name qualifier");
+        if (DiagnoseMissingQualifiersInAddressOfOperand(UnOp->getBeginLoc(),
+                                                        SubExpr.get(), Method))
+          return ExprError();
+
+        assert(isa<DeclRefExpr>(SubExpr.get()) &&
+               "fixed to something other than a decl ref");
+        assert(cast<DeclRefExpr>(SubExpr.get())->getQualifier() &&
+               "fixed to a member ref with no nested name qualifier");
 
         // We have taken the address of a pointer to member
         // function. Perform the computation here so that we get the
         // appropriate pointer to member type.
         QualType ClassType
           = Context.getTypeDeclType(cast<RecordDecl>(Method->getDeclContext()));
         QualType MemPtrType
           = Context.getMemberPointerType(Fn->getType(), ClassType.getTypePtr());
         // Under the MS ABI, lock down the inheritance model now.
         if (Context.getTargetInfo().getCXXABI().isMicrosoft())
           (void)isCompleteType(UnOp->getOperatorLoc(), MemPtrType);
 
-        return UnaryOperator::Create(
-            Context, SubExpr, UO_AddrOf, MemPtrType, VK_PRValue, OK_Ordinary,
-            UnOp->getOperatorLoc(), false, CurFPFeatureOverrides());
+        return UnaryOperator::Create(Context, SubExpr.get(), UO_AddrOf,
+                                     MemPtrType, VK_PRValue, OK_Ordinary,
+                                     UnOp->getOperatorLoc(), false,
+                                     CurFPFeatureOverrides());
       }
     }
-    Expr *SubExpr = FixOverloadedFunctionReference(UnOp->getSubExpr(),
-                                                   Found, Fn);
-    if (SubExpr == UnOp->getSubExpr())
+    ExprResult SubExpr =
+        FixOverloadedFunctionReference(UnOp->getSubExpr(), Found, Fn);
+    if (SubExpr.isInvalid())
+      return ExprError();
+    if (SubExpr.get() == UnOp->getSubExpr())
       return UnOp;
 
-    // FIXME: This can't currently fail, but in principle it could.
-    return CreateBuiltinUnaryOp(UnOp->getOperatorLoc(), UO_AddrOf, SubExpr)
-        .get();
+    return CreateBuiltinUnaryOp(UnOp->getOperatorLoc(), UO_AddrOf,
+                                SubExpr.get());
   }
 
   if (UnresolvedLookupExpr *ULE = dyn_cast<UnresolvedLookupExpr>(E)) {
     // FIXME: avoid copy.
     TemplateArgumentListInfo TemplateArgsBuffer, *TemplateArgs = nullptr;
     if (ULE->hasExplicitTemplateArgs()) {
       ULE->copyTemplateArgumentsInto(TemplateArgsBuffer);
       TemplateArgs = &TemplateArgsBuffer;

clang/lib/Sema/SemaTemplate.cpp

bool MightBeCxx11UnevalField =
       getLangOpts().CPlusPlus11 && isUnevaluatedContext();
 
   // Check if the nested name specifier is an enum type.
   bool IsEnum = false;
   if (NestedNameSpecifier *NNS = SS.getScopeRep())
     IsEnum = isa_and_nonnull<EnumType>(NNS->getAsType());
 
   if (!MightBeCxx11UnevalField && !isAddressOfOperand && !IsEnum &&
-      isa<CXXMethodDecl>(DC) && cast<CXXMethodDecl>(DC)->isInstance()) {
+      isa<CXXMethodDecl>(DC) &&
+      cast<CXXMethodDecl>(DC)->isImplicitObjectMemberFunction()) {
     QualType ThisType = cast<CXXMethodDecl>(DC)->getThisType();
 
     // Since the 'this' expression is synthesized, we don't need to
     // perform the double-lookup check.
     NamedDecl *FirstQualifierInScope = nullptr;
 
     return CXXDependentScopeMemberExpr::Create(
         Context, /*This*/ nullptr, ThisType, /*IsArrow*/ true,
return S.Diag(Arg->getBeginLoc(),
                   diag::err_template_arg_not_pointer_to_member_form)
            << Arg->getSourceRange();
 
   if (isa<FieldDecl>(DRE->getDecl()) ||
       isa<IndirectFieldDecl>(DRE->getDecl()) ||
       isa<CXXMethodDecl>(DRE->getDecl())) {
     assert((isa<FieldDecl>(DRE->getDecl()) ||
             isa<IndirectFieldDecl>(DRE->getDecl()) ||
-            !cast<CXXMethodDecl>(DRE->getDecl())->isStatic()) &&
+            cast<CXXMethodDecl>(DRE->getDecl())
+                ->isImplicitObjectMemberFunction()) &&
            "Only non-static member pointers can make it here");
 
     // Okay: this is the address of a non-static member, and therefore
     // a member pointer constant.
     if (Arg->isTypeDependent() || Arg->isValueDependent()) {
       SugaredConverted = TemplateArgument(Arg);
       CanonicalConverted =
           S.Context.getCanonicalTemplateArgument(SugaredConverted);
if (// -- For a non-type template-parameter of type pointer to
 
     if (Arg->getType() == Context.OverloadTy) {
       if (FunctionDecl *Fn = ResolveAddressOfOverloadedFunction(Arg, ParamType,
                                                                 true,
                                                                 FoundResult)) {
         if (DiagnoseUseOfDecl(Fn, Arg->getBeginLoc()))
           return ExprError();
 
-        Arg = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        ExprResult Res = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        if (Res.isInvalid())
+          return ExprError();
+        Arg = Res.get();
         ArgType = Arg->getType();
       } else
         return ExprError();
     }
 
     if (!ParamType->isMemberPointerType()) {
       if (CheckTemplateArgumentAddressOfObjectOrFunction(
               *this, Param, ParamType, Arg, SugaredConverted,
if (const ReferenceType *ParamRefType = ParamType->getAs<ReferenceType>()) {
 
     if (Arg->getType() == Context.OverloadTy) {
       if (FunctionDecl *Fn = ResolveAddressOfOverloadedFunction(Arg,
                                                  ParamRefType->getPointeeType(),
                                                                 true,
                                                                 FoundResult)) {
         if (DiagnoseUseOfDecl(Fn, Arg->getBeginLoc()))
           return ExprError();
-
-        Arg = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        ExprResult Res = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        if (Res.isInvalid())
+          return ExprError();
+        Arg = Res.get();
         ArgType = Arg->getType();
       } else
         return ExprError();
     }
 
     if (CheckTemplateArgumentAddressOfObjectOrFunction(
             *this, Param, ParamType, Arg, SugaredConverted, CanonicalConverted))
       return ExprError();

clang/lib/Sema/SemaTemplateDeduction.cpp

if (OriginalCallArgs) {
     //   In general, the deduction process attempts to find template argument
     //   values that will make the deduced A identical to A (after the type A
     //   is transformed as described above). [...]
     llvm::SmallDenseMap<std::pair<unsigned, QualType>, QualType> DeducedATypes;
     for (unsigned I = 0, N = OriginalCallArgs->size(); I != N; ++I) {
       OriginalCallArg OriginalArg = (*OriginalCallArgs)[I];
 
       auto ParamIdx = OriginalArg.ArgIdx;
-      if (ParamIdx >= Specialization->getNumParams())
+      unsigned ExplicitOffset =
+          Specialization->hasCXXExplicitFunctionObjectParameter() ? 1 : 0;
+      if (ParamIdx >= Specialization->getNumParams() - ExplicitOffset)
         // FIXME: This presumably means a pack ended up smaller than we
         // expected while deducing. Should this not result in deduction
         // failure? Can it even happen?
         continue;
 
       QualType DeducedA;
       if (!OriginalArg.DecomposedParam) {
         // P is one of the function parameters, just look up its substituted
         // type.
-        DeducedA = Specialization->getParamDecl(ParamIdx)->getType();
+        DeducedA =
+            Specialization->getParamDecl(ParamIdx + ExplicitOffset)->getType();
       } else {
         // P is a decomposed element of a parameter corresponding to a
         // braced-init-list argument. Substitute back into P to find the
         // deduced A.
         QualType &CacheEntry =
             DeducedATypes[{ParamIdx, OriginalArg.OriginalParamType}];
         if (CacheEntry.isNull()) {
           ArgumentPackSubstitutionIndexRAII PackIndex(

 static QualType GetTypeOfFunction(Sema &S, const OverloadExpr::FindResult &R,
                                   FunctionDecl *Fn) {
   // We may need to deduce the return type of the function now.
   if (S.getLangOpts().CPlusPlus14 && Fn->getReturnType()->isUndeducedType() &&
       S.DeduceReturnType(Fn, R.Expression->getExprLoc(), /*Diagnose*/ false))
     return {};
 
   if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn))
-    if (Method->isInstance()) {
+    if (Method->isImplicitObjectMemberFunction()) {
       // An instance method that's referenced in a form that doesn't
       // look like a member pointer is just invalid.
       if (!R.HasFormOfMemberPointer)
         return {};
 
       return S.Context.getMemberPointerType(Fn->getType(),
                S.Context.getTypeDeclType(Method->getParent()).getTypePtr());
     }

 /// Perform the adjustments to the parameter and argument types
 /// described in C++ [temp.deduct.call].
 ///
 /// \returns true if the caller should not attempt to perform any template
 /// argument deduction based on this P/A pair because the argument is an
 /// overloaded function set that could not be resolved.
 static bool AdjustFunctionParmAndArgTypesForDeduction(
     Sema &S, TemplateParameterList *TemplateParams, unsigned FirstInnerIndex,
-    QualType &ParamType, QualType &ArgType, Expr *Arg, unsigned &TDF,
+    QualType &ParamType, QualType &ArgType,
+    Expr::Classification ArgClassification, Expr *Arg, unsigned &TDF,
     TemplateSpecCandidateSet *FailedTSC = nullptr) {
   // C++0x [temp.deduct.call]p3:
   //   If P is a cv-qualified type, the top level cv-qualifiers of P's type
   //   are ignored for type deduction.
   if (ParamType.hasQualifiers())
     ParamType = ParamType.getUnqualifiedType();
 
   //   [...] If P is a reference type, the type referred to by P is
   //   used for type deduction.
   const ReferenceType *ParamRefType = ParamType->getAs<ReferenceType>();
   if (ParamRefType)
     ParamType = ParamRefType->getPointeeType();
 
   // Overload sets usually make this parameter an undeduced context,
   // but there are sometimes special circumstances.  Typically
   // involving a template-id-expr.
   if (ArgType == S.Context.OverloadTy) {
+    assert(Arg && "expected a non-null arg expression");
     ArgType = ResolveOverloadForDeduction(S, TemplateParams, Arg, ParamType,
                                           ParamRefType != nullptr, FailedTSC);
     if (ArgType.isNull())
       return true;
   }
 
   if (ParamRefType) {
     // If the argument has incomplete array type, try to complete its type.
-    if (ArgType->isIncompleteArrayType())
+    if (ArgType->isIncompleteArrayType()) {
+      assert(Arg && "expected a non-null arg expression");
       ArgType = S.getCompletedType(Arg);
+    }
 
     // C++1z [temp.deduct.call]p3:
     //   If P is a forwarding reference and the argument is an lvalue, the type
     //   "lvalue reference to A" is used in place of A for type deduction.
     if (isForwardingReference(QualType(ParamRefType, 0), FirstInnerIndex) &&
-        Arg->isLValue()) {
+        ArgClassification.isLValue()) {
       if (S.getLangOpts().OpenCL && !ArgType.hasAddressSpace())
         ArgType = S.Context.getAddrSpaceQualType(
             ArgType, S.Context.getDefaultOpenCLPointeeAddrSpace());
       ArgType = S.Context.getLValueReferenceType(ArgType);
     }
   } else {
     // C++ [temp.deduct.call]p2:
     //   If P is not a reference type:

 }
 
 static bool
 hasDeducibleTemplateParameters(Sema &S, FunctionTemplateDecl *FunctionTemplate,
                                QualType T);
 
 static Sema::TemplateDeductionResult DeduceTemplateArgumentsFromCallArgument(
     Sema &S, TemplateParameterList *TemplateParams, unsigned FirstInnerIndex,
-    QualType ParamType, Expr *Arg, TemplateDeductionInfo &Info,
+    QualType ParamType, QualType ArgType,
+    Expr::Classification ArgClassification, Expr *Arg,
+    TemplateDeductionInfo &Info,
     SmallVectorImpl<DeducedTemplateArgument> &Deduced,
     SmallVectorImpl<Sema::OriginalCallArg> &OriginalCallArgs,
     bool DecomposedParam, unsigned ArgIdx, unsigned TDF,
     TemplateSpecCandidateSet *FailedTSC = nullptr);
 
 /// Attempt template argument deduction from an initializer list
 ///        deemed to be an argument in a function call.
 static Sema::TemplateDeductionResult DeduceFromInitializerList(
static Sema::TemplateDeductionResult DeduceFromInitializerList(
   for (Expr *E : ILE->inits())
     if (isa<DesignatedInitExpr>(E))
       return Sema::TDK_Success;
 
   // Deduction only needs to be done for dependent types.
   if (ElTy->isDependentType()) {
     for (Expr *E : ILE->inits()) {
       if (auto Result = DeduceTemplateArgumentsFromCallArgument(
-              S, TemplateParams, 0, ElTy, E, Info, Deduced, OriginalCallArgs, true,
-              ArgIdx, TDF))
+              S, TemplateParams, 0, ElTy, E->getType(),
+              E->Classify(S.getASTContext()), E, Info, Deduced,
+              OriginalCallArgs, true, ArgIdx, TDF))
         return Result;
     }
   }
 
   //   in the P0[N] case, if N is a non-type template parameter, N is deduced
   //   from the length of the initializer list.
   if (auto *DependentArrTy = dyn_cast_or_null<DependentSizedArrayType>(ArrTy)) {
     // Determine the array bound is something we can deduce.
static Sema::TemplateDeductionResult DeduceFromInitializerList(
 
   return Sema::TDK_Success;
 }
 
 /// Perform template argument deduction per [temp.deduct.call] for a
 ///        single parameter / argument pair.
 static Sema::TemplateDeductionResult DeduceTemplateArgumentsFromCallArgument(
     Sema &S, TemplateParameterList *TemplateParams, unsigned FirstInnerIndex,
-    QualType ParamType, Expr *Arg, TemplateDeductionInfo &Info,
+    QualType ParamType, QualType ArgType,
+    Expr::Classification ArgClassification, Expr *Arg,
+    TemplateDeductionInfo &Info,
     SmallVectorImpl<DeducedTemplateArgument> &Deduced,
     SmallVectorImpl<Sema::OriginalCallArg> &OriginalCallArgs,
     bool DecomposedParam, unsigned ArgIdx, unsigned TDF,
     TemplateSpecCandidateSet *FailedTSC) {
-  QualType ArgType = Arg->getType();
+
   QualType OrigParamType = ParamType;
 
   //   If P is a reference type [...]
   //   If P is a cv-qualified type [...]
-  if (AdjustFunctionParmAndArgTypesForDeduction(S, TemplateParams,
-                                                FirstInnerIndex, ParamType,
-                                                ArgType, Arg, TDF, FailedTSC))
+  if (AdjustFunctionParmAndArgTypesForDeduction(
+          S, TemplateParams, FirstInnerIndex, ParamType, ArgType,
+          ArgClassification, Arg, TDF, FailedTSC))
     return Sema::TDK_Success;
 
   //   If [...] the argument is a non-empty initializer list [...]
-  if (InitListExpr *ILE = dyn_cast<InitListExpr>(Arg))
+  if (InitListExpr *ILE = dyn_cast_if_present<InitListExpr>(Arg))
     return DeduceFromInitializerList(S, TemplateParams, ParamType, ILE, Info,
                                      Deduced, OriginalCallArgs, ArgIdx, TDF);
 
   //   [...] the deduction process attempts to find template argument values
   //   that will make the deduced A identical to A
   //
   // Keep track of the argument type and corresponding parameter index,
   // so we can check for compatibility between the deduced A and A.
+  if (Arg)
     OriginalCallArgs.push_back(
         Sema::OriginalCallArg(OrigParamType, DecomposedParam, ArgIdx, ArgType));
   return DeduceTemplateArgumentsByTypeMatch(S, TemplateParams, ParamType,
                                             ArgType, Info, Deduced, TDF);
 }
 
 /// Perform template argument deduction from a function call
 /// (C++ [temp.deduct.call]).
 ///
 /// \param FunctionTemplate the function template for which we are performing

 /// types (after substituting explicit template arguments).
 ///
 /// \returns the result of template argument deduction.
 Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
     FunctionTemplateDecl *FunctionTemplate,
     TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
     FunctionDecl *&Specialization, TemplateDeductionInfo &Info,
     bool PartialOverloading, bool AggregateDeductionCandidate,
+    QualType ObjectType, Expr::Classification ObjectClassification,
     llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent) {
   if (FunctionTemplate->isInvalidDecl())
     return TDK_Invalid;
 
   FunctionDecl *Function = FunctionTemplate->getTemplatedDecl();
   unsigned NumParams = Function->getNumParams();
+  bool HasExplicitObject = false;
+  int ExplicitObjectOffset = 0;
+  if (Function->hasCXXExplicitFunctionObjectParameter()) {
+    HasExplicitObject = true;
+    ExplicitObjectOffset = 1;
+  }
 
   unsigned FirstInnerIndex = getFirstInnerIndex(FunctionTemplate);
 
   // C++ [temp.deduct.call]p1:
   //   Template argument deduction is done by comparing each function template
   //   parameter type (call it P) with the type of the corresponding argument
   //   of the call (call it A) as described below.
-  if (Args.size() < Function->getMinRequiredArguments() && !PartialOverloading)
+  if (Args.size() < Function->getMinRequiredExplicitArguments() &&
+      !PartialOverloading)
     return TDK_TooFewArguments;
-  else if (TooManyArguments(NumParams, Args.size(), PartialOverloading)) {
+  else if (TooManyArguments(NumParams, Args.size() + ExplicitObjectOffset,
+                            PartialOverloading)) {
     const auto *Proto = Function->getType()->castAs<FunctionProtoType>();
     if (Proto->isTemplateVariadic())
       /* Do nothing */;
     else if (!Proto->isVariadic())
       return TDK_TooManyArguments;
   }
 
   // The types of the parameters from which we will perform template argument
if (ExplicitTemplateArgs) {
     // Just fill in the parameter types from the function declaration.
     for (unsigned I = 0; I != NumParams; ++I)
       ParamTypes.push_back(Function->getParamDecl(I)->getType());
   }
 
   SmallVector<OriginalCallArg, 8> OriginalCallArgs;
 
   // Deduce an argument of type ParamType from an expression with index ArgIdx.
-  auto DeduceCallArgument = [&](QualType ParamType, unsigned ArgIdx) {
+  auto DeduceCallArgument = [&](QualType ParamType, unsigned ArgIdx,
+                                bool ExplicitObjetArgument) {
     // C++ [demp.deduct.call]p1: (DR1391)
     //   Template argument deduction is done by comparing each function template
     //   parameter that contains template-parameters that participate in
     //   template argument deduction ...
     if (!hasDeducibleTemplateParameters(*this, FunctionTemplate, ParamType))
       return Sema::TDK_Success;
 
+    if (ExplicitObjetArgument) {
       //   ... with the type of the corresponding argument
       return DeduceTemplateArgumentsFromCallArgument(
-        *this, TemplateParams, FirstInnerIndex, ParamType, Args[ArgIdx], Info, Deduced,
-        OriginalCallArgs, /*Decomposed*/false, ArgIdx, /*TDF*/ 0);
+          *this, TemplateParams, FirstInnerIndex, ParamType, ObjectType,
+          ObjectClassification,
+          /*Arg=*/nullptr, Info, Deduced, OriginalCallArgs,
+          /*Decomposed*/ false, ArgIdx, /*TDF*/ 0);
+    }
+
+    //   ... with the type of the corresponding argument
+    return DeduceTemplateArgumentsFromCallArgument(
+        *this, TemplateParams, FirstInnerIndex, ParamType,
+        Args[ArgIdx]->getType(), Args[ArgIdx]->Classify(getASTContext()),
+        Args[ArgIdx], Info, Deduced, OriginalCallArgs, /*Decomposed*/ false,
+        ArgIdx, /*TDF*/ 0);
   };
 
   // Deduce template arguments from the function parameters.
   Deduced.resize(TemplateParams->size());
   SmallVector<QualType, 8> ParamTypesForArgChecking;
   for (unsigned ParamIdx = 0, NumParamTypes = ParamTypes.size(), ArgIdx = 0;
        ParamIdx != NumParamTypes; ++ParamIdx) {
     QualType ParamType = ParamTypes[ParamIdx];
 
     const PackExpansionType *ParamExpansion =
         dyn_cast<PackExpansionType>(ParamType);
     if (!ParamExpansion) {
       // Simple case: matching a function parameter to a function argument.
-      if (ArgIdx >= Args.size())
+      if (ArgIdx >= Args.size() && !(HasExplicitObject && ParamIdx == 0))
         break;
 
       ParamTypesForArgChecking.push_back(ParamType);
-      if (auto Result = DeduceCallArgument(ParamType, ArgIdx++))
+
+      if (ParamIdx == 0 && HasExplicitObject) {
+        if (auto Result = DeduceCallArgument(ParamType, 0,
+                                             /*ExplicitObjetArgument=*/true))
+          return Result;
+        continue;
+      }
+
+      if (auto Result = DeduceCallArgument(ParamType, ArgIdx++,
+                                           /*ExplicitObjetArgument=*/false))
         return Result;
 
       continue;
     }
 
     bool IsTrailingPack = ParamIdx + 1 == NumParamTypes;
 
     QualType ParamPattern = ParamExpansion->getPattern();
for (unsigned ParamIdx = 0, NumParamTypes = ParamTypes.size(), ArgIdx = 0;
     // we instead notionally deduce the pack against N arguments, where N is
     // the length of the explicitly-specified pack if it's expanded by the
     // parameter pack and 0 otherwise, and we treat each deduction as a
     // non-deduced context.
     if (IsTrailingPack || PackScope.hasFixedArity()) {
       for (; ArgIdx < Args.size() && PackScope.hasNextElement();
            PackScope.nextPackElement(), ++ArgIdx) {
         ParamTypesForArgChecking.push_back(ParamPattern);
-        if (auto Result = DeduceCallArgument(ParamPattern, ArgIdx))
+        if (auto Result = DeduceCallArgument(ParamPattern, ArgIdx,
+                                             /*ExplicitObjetArgument=*/false))
           return Result;
       }
     } else {
       // If the parameter type contains an explicitly-specified pack that we
       // could not expand, skip the number of parameters notionally created
       // by the expansion.
       std::optional<unsigned> NumExpansions =
           ParamExpansion->getNumExpansions();
Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
   }
 
   return TDK_Success;
 }
 
 /// Deduce template arguments for a templated conversion
 /// function (C++ [temp.deduct.conv]) and, if successful, produce a
 /// conversion function template specialization.
-Sema::TemplateDeductionResult
-Sema::DeduceTemplateArguments(FunctionTemplateDecl *ConversionTemplate,
-                              QualType ToType,
-                              CXXConversionDecl *&Specialization,
-                              TemplateDeductionInfo &Info) {
+Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
+    FunctionTemplateDecl *ConversionTemplate, QualType ObjectType,
+    Expr::Classification ObjectClassification, QualType ToType,
+    CXXConversionDecl *&Specialization, TemplateDeductionInfo &Info) {
   if (ConversionTemplate->isInvalidDecl())
     return TDK_Invalid;
 
   CXXConversionDecl *ConversionGeneric
     = cast<CXXConversionDecl>(ConversionTemplate->getTemplatedDecl());
 
   QualType FromType = ConversionGeneric->getConversionType();
 
Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
   //       conversion.
   //
   // (C++0x [temp.deduct.conv]p6 clarifies that this only happens when
   // both P and A are pointers or member pointers. In this case, we
   // just ignore cv-qualifiers completely).
   if ((P->isPointerType() && A->isPointerType()) ||
       (P->isMemberPointerType() && A->isMemberPointerType()))
     TDF |= TDF_IgnoreQualifiers;
+
+  SmallVector<Sema::OriginalCallArg, 1> OriginalCallArgs;
+  if (ConversionGeneric->isExplicitObjectMemberFunction()) {
+    QualType ParamType = ConversionGeneric->getParamDecl(0)->getType();
+    if (TemplateDeductionResult Result =
+            DeduceTemplateArgumentsFromCallArgument(
+                *this, TemplateParams, getFirstInnerIndex(ConversionTemplate),
+                ParamType, ObjectType, ObjectClassification,
+                /*Arg=*/nullptr, Info, Deduced, OriginalCallArgs,
+                /*Decomposed*/ false, 0, /*TDF*/ 0))
+      return Result;
+  }
+
   if (TemplateDeductionResult Result
         = DeduceTemplateArgumentsByTypeMatch(*this, TemplateParams,
                                              P, A, Info, Deduced, TDF))
     return Result;
 
   // Create an Instantiation Scope for finalizing the operator.
   LocalInstantiationScope InstScope(*this);
   // Finish template argument deduction.
   FunctionDecl *ConversionSpecialized = nullptr;
   TemplateDeductionResult Result;
   runWithSufficientStackSpace(Info.getLocation(), [&] {
     Result = FinishTemplateArgumentDeduction(ConversionTemplate, Deduced, 0,
-                                             ConversionSpecialized, Info);
+                                             ConversionSpecialized, Info,
+                                             &OriginalCallArgs);
   });
   Specialization = cast_or_null<CXXConversionDecl>(ConversionSpecialized);
   return Result;
 }
 
 /// Deduce template arguments for a function template when there is
 /// nothing to deduce against (C++0x [temp.arg.explicit]p3).
 ///
if (InitList) {
 
       SourceRange DeducedFromInitRange;
       for (Expr *Init : InitList->inits()) {
         // Resolving a core issue: a braced-init-list containing any designators
         // is a non-deduced context.
         if (isa<DesignatedInitExpr>(Init))
           return TDK_Invalid;
         if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-                *this, TemplateParamsSt.get(), 0, TemplArg, Init, Info, Deduced,
+                *this, TemplateParamsSt.get(), 0, TemplArg, Init->getType(),
+                Init->Classify(getASTContext()), Init, Info, Deduced,
                 OriginalCallArgs, /*Decomposed=*/true,
                 /*ArgIdx=*/0, /*TDF=*/0)) {
           if (TDK == TDK_Inconsistent) {
             Diag(Info.getLocation(), diag::err_auto_inconsistent_deduction)
                 << Info.FirstArg << Info.SecondArg << DeducedFromInitRange
                 << Init->getSourceRange();
             return DeductionFailed(TDK_AlreadyDiagnosed);
           }
if (InitList) {
         Diag(Loc, diag::err_auto_bitfield);
         return TDK_AlreadyDiagnosed;
       }
       QualType FuncParam =
           SubstituteDeducedTypeTransform(*this, TemplArg).Apply(Type);
       assert(!FuncParam.isNull() &&
              "substituting template parameter for 'auto' failed");
       if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-              *this, TemplateParamsSt.get(), 0, FuncParam, Init, Info, Deduced,
+              *this, TemplateParamsSt.get(), 0, FuncParam, Init->getType(),
+              Init->Classify(getASTContext()), Init, Info, Deduced,
               OriginalCallArgs, /*Decomposed=*/false, /*ArgIdx=*/0, /*TDF=*/0,
               FailedTSC))
         return DeductionFailed(TDK);
     }
 
     // Could be null if somehow 'auto' appears in a non-deduced context.
     if (Deduced[0].getKind() != TemplateArgument::Type)
       return DeductionFailed(TDK_Incomplete);
AddImplicitObjectParameterType(ASTContext &Context,
                                SmallVectorImpl<QualType> &ArgTypes) {
   // C++11 [temp.func.order]p3:
   //   [...] The new parameter is of type "reference to cv A," where cv are
   //   the cv-qualifiers of the function template (if any) and A is
   //   the class of which the function template is a member.
   //
   // The standard doesn't say explicitly, but we pick the appropriate kind of
   // reference type based on [over.match.funcs]p4.
+  assert(Method && Method->isImplicitObjectMemberFunction() &&
+         "expected an implicit objet function");
   QualType ArgTy = Context.getTypeDeclType(Method->getParent());
   ArgTy = Context.getQualifiedType(ArgTy, Method->getMethodQualifiers());
   if (Method->getRefQualifier() == RQ_RValue)
     ArgTy = Context.getRValueReferenceType(ArgTy);
   else
     ArgTy = Context.getLValueReferenceType(ArgTy);
   ArgTypes.push_back(ArgTy);
 }
case TPOC_Call: {
     // functions don't make any sense.
     //
     // C++98/03 doesn't have this provision but we've extended DR532 to cover
     // it as wording was broken prior to it.
     SmallVector<QualType, 4> Args1;
 
     unsigned NumComparedArguments = NumCallArguments1;
 
-    if (!Method2 && Method1 && !Method1->isStatic()) {
+    if (!Method2 && Method1 && Method1->isImplicitObjectMemberFunction()) {
       // Compare 'this' from Method1 against first parameter from Method2.
       AddImplicitObjectParameterType(S.Context, Method1, Args1);
       ++NumComparedArguments;
-    } else if (!Method1 && Method2 && !Method2->isStatic()) {
+    } else if (!Method1 && Method2 &&
+               Method2->isImplicitObjectMemberFunction()) {
       // Compare 'this' from Method2 against first parameter from Method1.
       AddImplicitObjectParameterType(S.Context, Method2, Args2);
-    } else if (Method1 && Method2 && Reversed) {
+    } else if (Method1 && Method2 && Reversed &&
+               Method1->isImplicitObjectMemberFunction() &&
+               Method2->isImplicitObjectMemberFunction()) {
       // Compare 'this' from Method1 against second parameter from Method2
       // and 'this' from Method2 against second parameter from Method1.
       AddImplicitObjectParameterType(S.Context, Method1, Args1);
       AddImplicitObjectParameterType(S.Context, Method2, Args2);
       ++NumComparedArguments;
     }
 
     Args1.insert(Args1.end(), Proto1->param_type_begin(),

clang/lib/Sema/SemaTemplateInstantiate.cpp

if (OldParm->hasUninstantiatedDefaultArg()) {
     // outer lambda's declaration context.
     //   template <typename T>
     //   auto f() {
     //     return [](T = []{ return T{}; }()) { return 0; };
     //   }
     NewParm->setUninstantiatedDefaultArg(Arg);
   }
 
+  NewParm->setExplicitObjectParameterLoc(
+      OldParm->getExplicitObjectParamThisLoc());
   NewParm->setHasInheritedDefaultArg(OldParm->hasInheritedDefaultArg());
 
   if (OldParm->isParameterPack() && !NewParm->isParameterPack()) {
     // Add the new parameter to the instantiated parameter pack.
     CurrentInstantiationScope->InstantiatedLocalPackArg(OldParm, NewParm);
   } else {
     // Introduce an Old -> New mapping
     CurrentInstantiationScope->InstantiatedLocal(OldParm, NewParm);

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

TemplateDeclInstantiator::SubstFunctionType(FunctionDecl *D,
   TypeSourceInfo *OldTInfo = D->getTypeSourceInfo();
   assert(OldTInfo && "substituting function without type source info");
   assert(Params.empty() && "parameter vector is non-empty at start");
 
   CXXRecordDecl *ThisContext = nullptr;
   Qualifiers ThisTypeQuals;
   if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
     ThisContext = cast<CXXRecordDecl>(Owner);
-    ThisTypeQuals = Method->getMethodQualifiers();
+    ThisTypeQuals = Method->getFunctionObjectParameterType().getQualifiers();
   }
 
   TypeSourceInfo *NewTInfo = SemaRef.SubstFunctionDeclType(
       OldTInfo, TemplateArgs, D->getTypeSpecStartLoc(), D->getDeclName(),
       ThisContext, ThisTypeQuals, EvaluateConstraints);
   if (!NewTInfo)
     return nullptr;
 

clang/lib/Serialization/ASTReaderDecl.cpp

if (isObjCMethodParam) {
     PD->ParmVarDeclBits.ScopeDepthOrObjCQuals = declQualifier;
   } else {
     PD->setScopeInfo(scopeDepth, scopeIndex);
   }
   PD->ParmVarDeclBits.IsKNRPromoted = Record.readInt();
   PD->ParmVarDeclBits.HasInheritedDefaultArg = Record.readInt();
   if (Record.readInt()) // hasUninstantiatedDefaultArg.
     PD->setUninstantiatedDefaultArg(Record.readExpr());
+  PD->ExplicitObjectParameterIntroducerLoc = Record.readSourceLocation();
 
   // FIXME: If this is a redeclaration of a function from another module, handle
   // inheritance of default arguments.
 }
 
 void ASTDeclReader::VisitDecompositionDecl(DecompositionDecl *DD) {
   VisitVarDecl(DD);
   auto **BDs = DD->getTrailingObjects<BindingDecl *>();

clang/lib/Serialization/ASTReaderStmt.cpp

void ASTStmtReader::VisitDeclRefExpr(DeclRefExpr *E) {
 
   E->DeclRefExprBits.HasQualifier = Record.readInt();
   E->DeclRefExprBits.HasFoundDecl = Record.readInt();
   E->DeclRefExprBits.HasTemplateKWAndArgsInfo = Record.readInt();
   E->DeclRefExprBits.HadMultipleCandidates = Record.readInt();
   E->DeclRefExprBits.RefersToEnclosingVariableOrCapture = Record.readInt();
   E->DeclRefExprBits.NonOdrUseReason = Record.readInt();
   E->DeclRefExprBits.IsImmediateEscalating = Record.readInt();
+  E->DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = false;
   unsigned NumTemplateArgs = 0;
   if (E->hasTemplateKWAndArgsInfo())
     NumTemplateArgs = Record.readInt();
 
   if (E->hasQualifier())
     new (E->getTrailingObjects<NestedNameSpecifierLoc>())
         NestedNameSpecifierLoc(Record.readNestedNameSpecifierLoc());
 

clang/lib/Serialization/ASTWriterDecl.cpp

void ASTDeclWriter::VisitParmVarDecl(ParmVarDecl *D) {
   Record.push_back(D->getFunctionScopeDepth());
   Record.push_back(D->getFunctionScopeIndex());
   Record.push_back(D->getObjCDeclQualifier()); // FIXME: stable encoding
   Record.push_back(D->isKNRPromoted());
   Record.push_back(D->hasInheritedDefaultArg());
   Record.push_back(D->hasUninstantiatedDefaultArg());
   if (D->hasUninstantiatedDefaultArg())
     Record.AddStmt(D->getUninstantiatedDefaultArg());
+  Record.AddSourceLocation(D->getExplicitObjectParamThisLoc());
   Code = serialization::DECL_PARM_VAR;
 
   // If the assumptions about the DECL_PARM_VAR abbrev are true, use it.  Here
   // we dynamically check for the properties that we optimize for, but don't
   // know are true of all PARM_VAR_DECLs.
-  if (D->getDeclContext() == D->getLexicalDeclContext() &&
-      !D->hasAttrs() &&
-      !D->hasExtInfo() &&
-      !D->isImplicit() &&
-      !D->isUsed(false) &&
-      !D->isInvalidDecl() &&
-      !D->isReferenced() &&
-      D->getAccess() == AS_none &&
-      !D->isModulePrivate() &&
-      D->getStorageClass() == 0 &&
+  if (D->getDeclContext() == D->getLexicalDeclContext() && !D->hasAttrs() &&
+      !D->hasExtInfo() && !D->isImplicit() && !D->isUsed(false) &&
+      !D->isInvalidDecl() && !D->isReferenced() && D->getAccess() == AS_none &&
+      !D->isModulePrivate() && D->getStorageClass() == 0 &&
       D->getInitStyle() == VarDecl::CInit && // Can params have anything else?
-      D->getFunctionScopeDepth() == 0 &&
-      D->getObjCDeclQualifier() == 0 &&
-      !D->isKNRPromoted() &&
-      !D->hasInheritedDefaultArg() &&
-      D->getInit() == nullptr &&
-      !D->hasUninstantiatedDefaultArg())  // No default expr.
+      D->getFunctionScopeDepth() == 0 && D->getObjCDeclQualifier() == 0 &&
+      !D->isKNRPromoted() && !D->isExplicitObjectParameter() &&
+      !D->hasInheritedDefaultArg() && D->getInit() == nullptr &&
+      !D->hasUninstantiatedDefaultArg()) // No default expr.
     AbbrevToUse = Writer.getDeclParmVarAbbrev();
 
   // Check things we know are true of *every* PARM_VAR_DECL, which is more than
   // just us assuming it.
   assert(!D->getTSCSpec() && "PARM_VAR_DECL can't use TLS");
   assert(!D->isThisDeclarationADemotedDefinition()
          && "PARM_VAR_DECL can't be demoted definition.");
   assert(D->getAccess() == AS_none && "PARM_VAR_DECL can't be public/private");

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

if (const auto *MD = dyn_cast<ObjCMethodDecl>(D)) {
     if (std::optional<Loc> LV = V.getAs<Loc>()) {
       // Assume that the pointer value in 'self' is non-null.
       state = state->assume(*LV, true);
       assert(state && "'self' cannot be null");
     }
   }
 
   if (const auto *MD = dyn_cast<CXXMethodDecl>(D)) {
-    if (!MD->isStatic()) {
+    if (MD->isImplicitObjectMemberFunction()) {
       // Precondition: 'this' is always non-null upon entry to the
       // top-level function.  This is our starting assumption for
       // analyzing an "open" program.
       const StackFrameContext *SFC = InitLoc->getStackFrame();
       if (SFC->getParent() == nullptr) {
         loc::MemRegionVal L = svalBuilder.getCXXThis(MD, SFC);
         SVal V = state->getSVal(L);
         if (std::optional<Loc> LV = V.getAs<Loc>()) {
switch (S->getStmtClass()) {
 
     case Stmt::CXXOperatorCallExprClass: {
       const auto *OCE = cast<CXXOperatorCallExpr>(S);
 
       // For instance method operators, make sure the 'this' argument has a
       // valid region.
       const Decl *Callee = OCE->getCalleeDecl();
       if (const auto *MD = dyn_cast_or_null<CXXMethodDecl>(Callee)) {
-        if (MD->isInstance()) {
+        if (MD->isImplicitObjectMemberFunction()) {
           ProgramStateRef State = Pred->getState();
           const LocationContext *LCtx = Pred->getLocationContext();
           ProgramStateRef NewState =
             createTemporaryRegionIfNeeded(State, LCtx, OCE->getArg(0));
           if (NewState != State) {
             Pred = Bldr.generateNode(OCE, Pred, NewState, /*tag=*/nullptr,
                                      ProgramPoint::PreStmtKind);
             // Did we cache out?
if (isa<VarDecl, EnumConstantDecl>(Member)) {
 
     for (const auto I : CheckedSet) {
       ProgramStateRef state = I->getState();
       const LocationContext *LCtx = I->getLocationContext();
       Expr *BaseExpr = M->getBase();
 
       // Handle C++ method calls.
       if (const auto *MD = dyn_cast<CXXMethodDecl>(Member)) {
-        if (MD->isInstance())
+        if (MD->isImplicitObjectMemberFunction())
           state = createTemporaryRegionIfNeeded(state, LCtx, BaseExpr);
 
         SVal MDVal = svalBuilder.getFunctionPointer(MD);
         state = state->BindExpr(M, LCtx, MDVal);
 
         Bldr.generateNode(M, I, state);
         continue;
       }

clang/lib/StaticAnalyzer/Core/LoopWidening.cpp

ProgramStateRef getWidenedLoopState(ProgramStateRef PrevState,
   }
 
 
   // 'this' pointer is not an lvalue, we should not invalidate it. If the loop
   // is located in a method, constructor or destructor, the value of 'this'
   // pointer should remain unchanged.  Ignore static methods, since they do not
   // have 'this' pointers.
   const CXXMethodDecl *CXXMD = dyn_cast<CXXMethodDecl>(STC->getDecl());
-  if (CXXMD && !CXXMD->isStatic()) {
+  if (CXXMD && CXXMD->isImplicitObjectMemberFunction()) {
     const CXXThisRegion *ThisR =
         MRMgr.getCXXThisRegion(CXXMD->getThisType(), STC);
     ITraits.setTrait(ThisR,
                      RegionAndSymbolInvalidationTraits::TK_PreserveContents);
   }
 
   return PrevState->invalidateRegions(Regions, getLoopCondition(LoopStmt),
                                       BlockCount, LCtx, true, nullptr, nullptr,
                                       &ITraits);
 }
 
 } // end namespace ento
 } // end namespace clang

clang/lib/StaticAnalyzer/Core/SValBuilder.cpp

DefinedSVal SValBuilder::getMemberPointer(const NamedDecl *ND) {
   assert(!ND || (isa<CXXMethodDecl, FieldDecl, IndirectFieldDecl>(ND)));
 
   if (const auto *MD = dyn_cast_or_null<CXXMethodDecl>(ND)) {
     // Sema treats pointers to static member functions as have function pointer
     // type, so return a function pointer for the method.
     // We don't need to play a similar trick for static member fields
     // because these are represented as plain VarDecls and not FieldDecls
     // in the AST.
-    if (MD->isStatic())
+    if (!MD->isImplicitObjectMemberFunction())
       return getFunctionPointer(MD);
   }
 
   return nonloc::PointerToMember(ND);
 }
 
 DefinedSVal SValBuilder::getFunctionPointer(const FunctionDecl *func) {
   return loc::MemRegionVal(MemMgr.getFunctionCodeRegion(func));

clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/dcl.fct.def.default/p1.cpp

 // RUN: %clang_cc1 -std=c++11 -verify %s
 
 // A function that is explicitly defaulted shall
 // [...]
 //   -- not have default arguments
 struct DefArg {
   static DefArg &&make();
-  DefArg(int n = 5) = default; // expected-error {{an explicitly-defaulted constructor cannot have default arguments}}
-  DefArg(const DefArg &DA = make()) = default; // expected-error {{an explicitly-defaulted constructor cannot have default arguments}}
+  DefArg(int n = 5) = default; // expected-error {{an explicitly-defaulted default constructor cannot have default arguments}}
+  DefArg(const DefArg &DA = make()) = default; // expected-error {{an explicitly-defaulted default constructor cannot have default arguments}}
   DefArg(const DefArg &DA, int k = 3) = default; // expected-error {{an explicitly-defaulted copy constructor cannot have default arguments}}
-  DefArg(DefArg &&DA = make()) = default; // expected-error {{an explicitly-defaulted constructor cannot have default arguments}}
+  DefArg(DefArg &&DA = make()) = default; // expected-error {{an explicitly-defaulted default constructor cannot have default arguments}}
   DefArg(DefArg &&DA, int k = 3) = default; // expected-error {{an explicitly-defaulted move constructor cannot have default arguments}}
   DefArg &operator=(const DefArg&, int k = 4) = default; // expected-error {{parameter of overloaded 'operator=' cannot have a default argument}}
   DefArg &operator=(DefArg&&, int k = 4) = default; // expected-error {{parameter of overloaded 'operator=' cannot have a default argument}}
   ~DefArg(int k = 5) = default; // expected-error {{destructor cannot have any parameters}}
 };

clang/test/CXX/drs/dr25xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus < 201103L
 // expected-no-diagnostics
 #endif
 
 namespace dr2516 { // dr2516: yes
                    // NB: reusing 1482 test
 #if __cplusplus >= 201103L
 template <typename T> struct S {
   typedef char I;
 };
 enum E2 : S<E2>::I { e };
 // expected-error@-1 {{use of undeclared identifier 'E2'}}
 #endif
 } // namespace dr2516
 
 namespace dr2518 { // dr2518: 17
 
 #if __cplusplus >= 201103L
 template <class T>
 void f(T t) {
   if constexpr (sizeof(T) != sizeof(int)) {
 #if __cplusplus < 201703L
 // expected-error@-2 {{constexpr if is a C++17 extension}}
 #endif
     static_assert(false, "must be int-sized"); // expected-error {{must be int-size}}
   }
 }
 
 void g(char c) {
   f(0);
   f(c); // expected-note {{requested here}}
 }
 
 template <typename Ty>
 struct S {
   static_assert(false); // expected-error {{static assertion failed}}
 #if __cplusplus < 201703L
 // expected-error@-2 {{'static_assert' with no message is a C++17 extension}}
 #endif
 };
 
 template <>
 struct S<int> {};
 
 template <>
 struct S<float> {};
 
 int test_specialization() {
   S<int> s1;
   S<float> s2;
   S<double> s3; // expected-note {{in instantiation of template class 'dr2518::S<double>' requested here}}
 }
 #endif
 
 }
 
 namespace dr2521 { // dr2521: 17
 #if __cplusplus >= 201103L
 #pragma clang diagnostic push
 #pragma clang diagnostic warning "-Wdeprecated-literal-operator"
 long double operator""      _\u03C0___(long double);
 // expected-warning@-1 {{identifier '_π___' preceded by whitespace in a literal operator declaration is deprecated}}
 // expected-warning@-2 {{user-defined literal suffixes containing '__' are reserved}}
 
 template <char... Chars> decltype(sizeof 0)
 operator""  _div();
 // expected-warning@-1 {{identifier '_div' preceded by whitespace in a literal operator declaration is deprecated}}
 
 using ::dr2521::operator"" _\u03C0___;
 using ::dr2521::operator""_div;
 // expected-warning@-2 {{identifier '_π___' preceded by whitespace in a literal operator declaration is deprecated}}
 #pragma clang diagnostic pop
 #endif
 } // namespace dr2521
 
+
+#if __cplusplus >= 202302L
+namespace dr2553 { // dr2553: 16 open
+struct B {
+  virtual void f(this B&);   // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+  static void f(this B&);   // expected-error {{an explicit object parameter cannot appear in a static function}}
+  virtual void g(); // expected-note {{here}}
+};
+struct D : B {
+  void g(this D&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+};
+}
+#endif
+
+#if __cplusplus >= 202302L
+namespace dr2554 { // dr2554: 16 open
+struct B {
+  virtual void f(); // expected-note {{here}}
+};
+
+struct D : B {
+  void f(this D&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+};
+}
+#endif
+
+#if __cplusplus >= 202302L
+namespace dr2561 { // dr2561: 16 open
+struct C {
+    constexpr C(auto) { }
+};
+void foo() {
+    constexpr auto b = [](this C) { return 1; };
+    constexpr int (*fp)(C) = b;
+    static_assert(fp(1) == 1);
+    static_assert((&decltype(b)::operator())(1) == 1);
+}
+
+}
+#endif
+
+
 namespace dr2565 { // dr2565: 16 open
 #if __cplusplus >= 202002L
   template<typename T>
     concept C = requires (typename T::type x) {
       x + 1;
     };
   static_assert(!C<int>);
 

clang/test/CXX/drs/dr26xx.cpp

-// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify
+// RUN: %clang_cc1 -std=c++20 -Wno-c++2b-extensions -triple x86_64-unknown-unknown %s -verify
+// RUN: %clang_cc1 -std=c++2b -triple x86_64-unknown-unknown %s -verify
+
 
 namespace dr2621 { // dr2621: yes
 enum class E { a };
 namespace One {
 using E_t = E;
 using enum E_t; // typedef ok
 auto v = a;
 }

 
 auto z = [a = 42](int a) { // expected-error {{a lambda parameter cannot shadow an explicitly captured entity}} \
                            // expected-note {{variable 'a' is explicitly captured here}}
      return 1;
 };
 
 }
 
+#if __cplusplus >= 202302L
 namespace dr2650 { // dr2650: yes
 template <class T, T> struct S {};
 template <class T> int f(S<T, T{}>*); // expected-note {{type 'X' of non-type template parameter is not a structural type}}
 class X {
   int m;
 };
 int i0 = f<X>(0);   //expected-error {{no matching function for call to 'f'}}
 }
+#endif
+
+#if __cplusplus >= 202302L
+namespace dr2653 { // dr2653: 16
+  struct Test { void f(this const auto& = Test{}); };
+  // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+  auto L =[](this const auto& = Test{}){};
+  // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+}
+#endif
 
 namespace dr2654 { // dr2654: 16
 void f() {
     int neck, tail;
     volatile int brachiosaur;
     brachiosaur += neck;                // OK
     brachiosaur -= neck;                // OK
     brachiosaur |= neck;                // OK

 
 H h = { "abc" };
 I i = { "def" };
 static_assert(__is_same(decltype(h), H<char, 4>));  // Not H<const char, 4>
 static_assert(__is_same(decltype(i), I<char, 4>));
 
 J j = { "ghi" };  // expected-error {{no viable constructor or deduction guide}}
 }
+
+#if __cplusplus >= 202302L
+namespace dr2687 { // dr2687: 18
+struct S{
+    void f(int);
+    static void g(int);
+    void h(this const S&, int);
+};
+
+void test() {
+    (&S::f)(1); //expected-error {{called object type 'void (dr2687::S::*)(int)' is not a function or function pointer}}
+    (&S::g)(1);
+    (&S::h)(S(), 1);
+}
+}
+#endif

clang/test/CXX/over/over.load/p2-0x.cpp

 // RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
 
 // Member function declarations with the same name and the same
 // parameter-type-list as well as mem- ber function template
 // declarations with the same name, the same parameter-type-list, and
 // the same template parameter lists cannot be overloaded if any of
 // them, but not all, have a ref-qualifier (8.3.5).
 
-class Y { 
-  void h() &; 
-  void h() const &; 
-  void h() &&; 
+class Y {
+  void h() &;
+  void h() const &;
+  void h() &&;
   void i() &; // expected-note{{previous declaration}}
   void i() const; // expected-error{{cannot overload a member function without a ref-qualifier with a member function with ref-qualifier '&'}}
 
   template<typename T> void f(T*) &;
   template<typename T> void f(T*) &&;
 
   template<typename T> void g(T*) &; // expected-note{{previous declaration}}
   template<typename T> void g(T*); // expected-error{{cannot overload a member function without a ref-qualifier with a member function with ref-qualifier '&'}}

clang/test/CXX/special/class.copy/p20.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
 
 struct ConstCopy {
   ConstCopy();
   ConstCopy &operator=(const ConstCopy&);
 };
 
 struct NonConstCopy {
   NonConstCopy();
   NonConstCopy &operator=(NonConstCopy&);
 };
 
-struct VirtualInheritsNonConstCopy : virtual NonConstCopy { 
+struct VirtualInheritsNonConstCopy : virtual NonConstCopy {
   VirtualInheritsNonConstCopy();
   VirtualInheritsNonConstCopy &operator=(const VirtualInheritsNonConstCopy&);
 };
 
 struct ImplicitNonConstCopy1 : NonConstCopy {  // expected-note{{the implicit copy assignment operator}}
   ImplicitNonConstCopy1();
 };
 

clang/test/CXX/special/class.copy/p25-0x.cpp

 // RUN: %clang_cc1 -std=c++11 -verify %s -Wno-deprecated-builtins
 // RUN: %clang_cc1 -std=c++11 -verify %s -Wno-deprecated-builtins -fclang-abi-compat=14 -DCLANG_ABI_COMPAT=14
+// RUN: %clang_cc1 -fsyntax-only -std=c++2b -DDEDUCING_THIS -Wno-deprecated-builtins  %s -verify
 
 // expected-no-diagnostics
 
+#if DEDUCING_THIS
+#define EXPLICIT_PARAMETER(...) this __VA_ARGS__,
+#else
+#define EXPLICIT_PARAMETER(Param)
+#endif
+
+
 template<typename T, bool B> struct trivially_assignable_check {
   static_assert(B == __has_trivial_assign(T), "");
   static_assert(B == __is_trivially_assignable(T&, T), "");
   static_assert(B == __is_trivially_assignable(T&, const T &), "");
   static_assert(B == __is_trivially_assignable(T&, T &&), "");
   static_assert(B == __is_trivially_assignable(T&&, T), "");
   static_assert(B == __is_trivially_assignable(T&&, const T &), "");
   static_assert(B == __is_trivially_assignable(T&&, T &&), "");
   typedef void type;
 };
 template<typename T> using trivially_assignable =
   typename trivially_assignable_check<T, true>::type;
 template<typename T> using not_trivially_assignable =
   typename trivially_assignable_check<T, false>::type;
 
 struct Trivial {};
 using _ = trivially_assignable<Trivial>;
 
 // A copy/move assignment operator for class X is trivial if it is not user-provided,
 struct UserProvided {
-  UserProvided &operator=(const UserProvided &);
+  UserProvided &operator=(EXPLICIT_PARAMETER(UserProvided&)
+                          const UserProvided &);
 };
 using _ = not_trivially_assignable<UserProvided>;
 
 // its declared parameter type is the same as if it had been implicitly
 // declared,
 struct NonConstCopy {
-  NonConstCopy &operator=(NonConstCopy &) = default;
+  NonConstCopy &operator=(EXPLICIT_PARAMETER(NonConstCopy&) NonConstCopy &) = default;
+#if DEDUCING_THIS
+  NonConstCopy &operator=(EXPLICIT_PARAMETER(NonConstCopy&&) NonConstCopy &) = default;
+#endif
 };
 #if defined(CLANG_ABI_COMPAT) && CLANG_ABI_COMPAT <= 14
 // Up until (and including) Clang 14, non-const copy assignment operators were not trivial because
 // of dr2171
 using _ = not_trivially_assignable<NonConstCopy>;
 #else
 // In the latest Clang version, all defaulted assignment operators are trivial, even if non-const,
 // because dr2171 is fixed
 static_assert(__has_trivial_assign(NonConstCopy), "");
 static_assert(__is_trivially_assignable(NonConstCopy &, NonConstCopy &), "");
 static_assert(!__is_trivially_assignable(NonConstCopy &, const NonConstCopy &), "");
 static_assert(!__is_trivially_assignable(NonConstCopy &, NonConstCopy), "");
 static_assert(!__is_trivially_assignable(NonConstCopy &, NonConstCopy &&), "");
 static_assert(__is_trivially_assignable(NonConstCopy &&, NonConstCopy &), "");
 static_assert(!__is_trivially_assignable(NonConstCopy &&, const NonConstCopy &), "");
 static_assert(!__is_trivially_assignable(NonConstCopy &&, NonConstCopy), "");
 static_assert(!__is_trivially_assignable(NonConstCopy &&, NonConstCopy &&), "");
 
 struct DefaultedSpecialMembers {
-  DefaultedSpecialMembers &operator=(const DefaultedSpecialMembers &) = default;
-  DefaultedSpecialMembers &operator=(DefaultedSpecialMembers &) = default;
-  DefaultedSpecialMembers &operator=(DefaultedSpecialMembers &&) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&)
+                                     const DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&)
+                                     DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&)
+                                     DefaultedSpecialMembers &&) = default;
+#if DEDUCING_THIS
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&&)
+                                     const DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&&)
+                                     DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&&)
+                                     DefaultedSpecialMembers &&) = default;
+#endif
 };
 using _ = trivially_assignable<DefaultedSpecialMembers>;
 #endif
 
 // class X has no virtual functions
 struct VFn {
   virtual void f();
 };

 struct MutableTemplateCtorMember {
   mutable TemplateCtor mtc;
 };
 static_assert(!__is_trivially_assignable(MutableTemplateCtorMember, const MutableTemplateCtorMember &), "");
 static_assert(__is_trivially_assignable(MutableTemplateCtorMember, MutableTemplateCtorMember &&), "");
 
 // Both trivial and non-trivial special members.
 struct TNT {
-  TNT &operator=(const TNT &) = default; // trivial
-  TNT &operator=(TNT &); // non-trivial
-
-  TNT &operator=(TNT &&) = default; // trivial
-  TNT &operator=(const TNT &&); // non-trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) const TNT &) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) TNT &); // non-trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) TNT &&) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) const TNT &&); // non-trivial
+#if DEDUCING_THIS
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) const TNT &) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) TNT &); // non-trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) TNT &&) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) const TNT &&); // non-trivial
+#endif
 };
 
 static_assert(!__has_trivial_assign(TNT), "lie deliberately for gcc compatibility");
 static_assert(__is_trivially_assignable(TNT, TNT), "");
 static_assert(!__is_trivially_assignable(TNT, TNT &), "");
 static_assert(__is_trivially_assignable(TNT, const TNT &), "");
 static_assert(!__is_trivially_assignable(TNT, volatile TNT &), "");
 static_assert(__is_trivially_assignable(TNT, TNT &&), "");

clang/test/CodeGenCXX/cxx2b-deducing-this.cpp

+// RUN: %clang_cc1 -std=c++2b %s -emit-llvm -triple x86_64-linux -o - | FileCheck %s
+// RUN: %clang_cc1 -std=c++2b %s -emit-llvm -triple x86_64-windows-msvc -o - | FileCheck %s
+
+struct TrivialStruct {
+    void explicit_object_function(this TrivialStruct) {}
+};
+void test() {
+    TrivialStruct s;
+    s.explicit_object_function();
+}
+// CHECK:      define {{.*}}test{{.*}}
+// CHECK-NEXT: entry:
+// CHECK:      {{.*}} = alloca %struct.TrivialStruct, align 1
+// CHECK:      {{.*}} = alloca %struct.TrivialStruct, align 1
+// CHECK:      call void {{.*}}explicit_object_function{{.*}}
+// CHECK-NEXT: ret void
+// CHECK-NEXT: }
+
+// CHECK:      define {{.*}}explicit_object_function{{.*}}
+// CHECK-NEXT: entry:
+// CHECK:        {{.*}} = alloca %struct.TrivialStruct, align 1
+// CHECK:        ret void
+// CHECK-NEXT: }

clang/test/CodeGenCXX/cxx2b-mangle-deducing-this.cpp

+// RUN: %clang_cc1 -std=c++2b -fno-rtti -emit-llvm -triple x86_64-linux -o - %s  2>/dev/null | FileCheck %s
+
+struct S {
+friend void test();
+public:
+    void a(this auto){}
+    void b(this auto&){}
+    void c(this S){}
+    void c(this S, int){}
+private:
+    void d(this auto){}
+    void e(this auto&){}
+    void f(this S){}
+    void f(this S, int){}
+protected:
+    void g(this auto){}
+    void h(this auto&){}
+    void i(this S){}
+    void i(this S, int){}
+};
+
+
+void test() {
+    S s;
+    s.a();
+    // CHECK: call void @_ZNH1S1aIS_EEvT_
+    s.b();
+    // CHECK: call void @_ZNH1S1bIS_EEvRT_
+    s.c();
+    // CHECK: call void @_ZNH1S1cES_
+    s.c(0);
+    // CHECK: call void @_ZNH1S1cES_i
+    s.d();
+    // CHECK: call void @_ZNH1S1dIS_EEvT_
+    s.e();
+    // CHECK: call void @_ZNH1S1eIS_EEvRT_
+    s.f();
+    // CHECK: call void @_ZNH1S1fES_
+    s.f(0);
+    // CHECK: call void @_ZNH1S1fES_i
+    s.g();
+    // CHECK: call void @_ZNH1S1gIS_EEvT_
+    s.h();
+    // CHECK: call void @_ZNH1S1hIS_EEvRT_
+    s.i();
+    // CHECK: call void @_ZNH1S1iES_
+    s.i(0);
+    // CHECK: call void @_ZNH1S1iES_i
+}
+
+struct StaticAndExplicit {
+  static void f(StaticAndExplicit);
+  void f(this StaticAndExplicit);
+};
+
+void test2() {
+    StaticAndExplicit s;
+
+    StaticAndExplicit::f(s);
+    // CHECK: call void @_ZN17StaticAndExplicit1fES_
+
+    s.f();
+    // CHECK: call void @_ZNH17StaticAndExplicit1fES_
+}

clang/test/CodeGenCXX/microsoft-abi-explicit-object-parameters.cpp

+// RUN: %clang_cc1 -std=c++2b -fno-rtti -emit-llvm -triple=x86_64-pc-win32 -o - %s  2>/dev/null | FileCheck %s
+struct S {
+friend void test();
+public:
+    void a(this auto){}
+    void b(this auto&){}
+    void c(this S){}
+    void c(this S, int){}
+private:
+    void d(this auto){}
+    void e(this auto&){}
+    void f(this S){}
+    void f(this S, int){}
+protected:
+    void g(this auto){}
+    void h(this auto&){}
+    void i(this S){}
+    void i(this S, int){}
+};
+
+void test() {
+    S s;
+    s.a();
+    // CHECK: call void @"??$a@US@@@S@@SAX_VU0@@Z"
+    s.b();
+    // CHECK: call void @"??$b@US@@@S@@SAX_VAEAU0@@Z"
+    s.c();
+    // CHECK: call void @"?c@S@@SAX_VU1@@Z"
+    s.c(0);
+    // CHECK: call void @"?c@S@@SAX_VU1@H@Z"
+    s.d();
+    // CHECK: call void @"??$d@US@@@S@@CAX_VU0@@Z"
+    s.e();
+    // CHECK: call void @"??$e@US@@@S@@CAX_VAEAU0@@Z"
+    s.f();
+    // CHECK: call void @"?f@S@@CAX_VU1@@Z"
+    s.f(0);
+    // CHECK: call void @"?f@S@@CAX_VU1@H@Z"
+    s.g();
+    // CHECK: call void @"??$g@US@@@S@@KAX_VU0@@Z"
+    s.h();
+    // CHECK: call void @"??$h@US@@@S@@KAX_VAEAU0@@Z"
+    s.i();
+    // CHECK: call void @"?i@S@@KAX_VU1@@Z"
+    s.i(0);
+    // CHECK: call void @"?i@S@@KAX_VU1@H@Z"
+}

clang/test/FixIt/fixit-deducing-this.cpp

+// RUN: %clang_cc1 -verify -std=c++2c %s
+// RUN: cp %s %t
+// RUN: not %clang_cc1 -x c++ -std=c++2c -fixit %t
+// RUN: %clang_cc1 -x c++ -std=c++2c %t
+// RUN: not %clang_cc1 -std=c++2c -x c++ -fsyntax-only -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
+struct S {
+    void f(this S);
+    void g() {
+        (void)&f;  // expected-error {{must explicitly qualify name of member function when taking its address}}
+// CHECK: fix-it:{{.*}}:{9:16-9:16}:"S::"
+    }
+};
+
+struct S2 {
+    void f(this S2 foo) {
+        g(); // expected-error {{call to non-static member function without an object argument}}
+// CHECK: fix-it:{{.*}}:{16:9-16:9}:"foo."
+
+        h(); // expected-error {{call to explicit member function without an object argument}}
+// CHECK: fix-it:{{.*}}:{19:9-19:9}:"foo."
+
+        i();
+
+        var; // expected-error {{invalid use of member 'var' in explicit object member function}}
+// CHECK: fix-it:{{.*}}:{24:9-24:9}:"foo."
+
+    }
+    void g();
+    void h(this S2 s);
+    static void i();
+    int var;
+};

clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++2b %s -verify
+// expected-no-diagnostics
+
+template <typename Base>
+struct Wrap : Base {
+
+};
+
+struct S {
+    constexpr int f(this const S&) {
+        return 42;
+    }
+    constexpr int f(this const S&, auto&&... args) {
+        return (args + ... + 0);
+    }
+    constexpr int operator[](this const S&) {
+        return 42;
+    }
+    constexpr int operator[](this const S& self, int i) {
+        return i + self.base;
+    }
+    constexpr int operator()(this const S&) {
+        return 42;
+    }
+    constexpr int operator()(this const S& self, int i) {
+        return self.base + i;
+    }
+    constexpr bool operator==(this const S& self, auto && test) {
+        return self.base == test;
+    };
+    constexpr int operator*(this const S& self) {
+        return self.base + 22;
+    };
+    constexpr operator Wrap<S> (this const S& self) {
+        return Wrap<S>{self};
+    };
+    constexpr int operator <<(this Wrap<S> self, int i) {
+        return self.base+i;
+    }
+
+    int base = 20;
+};
+
+consteval void test() {
+    constexpr S s;
+    static_assert(s.f() == 42);
+    static_assert(s[] == 42);
+    static_assert(s[22] == 42);
+    static_assert(s.f() == 42);
+    static_assert(s() == 42);
+    static_assert(s(22) == 42);
+    static_assert(s == 20);
+    static_assert(s != 0);
+    static_assert(*s == 42);
+    static_assert((s << 11) == 31);
+}

clang/test/SemaCXX/cxx2b-deducing-this-coro.cpp

+// RUN: %clang_cc1 -std=c++2b %s -verify
+
+
+#include "Inputs/std-coroutine.h"
+
+struct S;
+template <typename T>
+class coro_test {
+public:
+    struct promise_type;
+    using handle = std::coroutine_handle<promise_type>;
+  struct promise_type {
+        promise_type(const promise_type&) = delete; // #copy-ctr
+        promise_type(T);  // #candidate
+        coro_test get_return_object();
+        std::suspend_never initial_suspend();
+      std::suspend_never final_suspend() noexcept;
+      void return_void();
+        void unhandled_exception();
+
+
+        template<typename Arg, typename... Args>
+        void* operator new(decltype(0UL) sz, Arg&&, Args&... args) {
+            static_assert(!__is_same(__decay(Arg), S), "Ok"); // expected-error 2{{Ok}}
+        }
+
+    };
+private:
+  handle h;
+};
+
+
+template <typename Ret, typename... P>
+struct std::coroutine_traits<coro_test<S&>, Ret, P...> {
+  using promise_type = coro_test<S&>::promise_type;
+  static_assert(!__is_same(Ret, S&), "Ok"); // expected-error{{static assertion failed due to requirement '!__is_same(S &, S &)': Ok}}
+};
+
+
+struct S {
+
+    coro_test<S&> ok(this S&, int) {
+        co_return; // expected-note {{in instantiation}}
+    }
+
+    coro_test<const S&> ok2(this const S&) { // expected-note {{in instantiation}}
+        co_return;
+    }
+
+    coro_test<int> ko(this const S&) {  // expected-error {{no matching constructor for initialization of 'std::coroutine_traits<coro_test<int>, const S &>::promise_type'}} \
+                                        // expected-note {{in instantiation}} \
+                                       // FIXME: the message below is unhelpful but this is pre-existing
+                                       // expected-note@#candidate {{candidate constructor not viable: requires 1 argument, but 0 were provided}} \
+                                       // expected-note@#copy-ctr  {{candidate constructor not viable}}
+        co_return;
+    }
+
+};

clang/test/SemaCXX/cxx2b-deducing-this.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++2b -Woverloaded-virtual %s -verify
+
+
+// FIXME: can we improve these diagnostics?
+void f(this); // expected-error{{variable has incomplete type 'void'}} \
+              // expected-error{{invalid use of 'this' outside of a non-static member function}}
+
+void g(this auto); // expected-error{{an explicit object parameter cannot appear in a non-member function}}
+
+auto l1 = [] (this auto) static {}; // expected-error{{an explicit object parameter cannot appear in a static lambda}}
+auto l2 = [] (this auto) mutable {}; // expected-error{{a lambda with an explicit object parameter cannot be mutable}}
+auto l3 = [](this auto...){}; // expected-error {{an explicit object parameter cannot be a function parameter pack}}
+auto l4 = [](int, this auto){}; // expected-error {{an explicit object parameter can only appear as the first parameter of the lambda}}
+
+struct S {
+    static void f(this auto); // expected-error{{an explicit object parameter cannot appear in a static function}}
+    virtual void f(this S); // expected-error{{an explicit object parameter cannot appear in a virtual function}}
+
+    void g(this auto) const; // expected-error{{a function with an explicit object parameter cannot be const}}
+    void h(this auto) &; // expected-error{{a function with an explicit object parameter cannot be reference-qualified}}
+    void i(this auto) &&; // expected-error{{a function with an explicit object parameter cannot be reference-qualified}}
+    void j(this auto) volatile; // expected-error{{a function with an explicit object parameter cannot have qualifiers}}
+    void k(this auto) __restrict; // expected-error{{a function with an explicit object parameter cannot have qualifiers}}
+    void l(this auto) _Nonnull; // expected-error{{a function with an explicit object parameter cannot have qualifiers}}
+
+
+    void variadic(this auto...); // expected-error{{an explicit object parameter cannot be a function parameter pack}}
+    void not_first(int, this auto); // expected-error {{an explicit object parameter can only appear as the first parameter of the function}}
+
+    S(this auto); // expected-error {{an explicit object parameter cannot appear in a constructor}}
+    ~S(this S) {} // expected-error {{an explicit object parameter cannot appear in a destructor}} \
+                  // expected-error {{destructor cannot have any parameters}}
+};
+
+namespace Override {
+struct A {
+    virtual void f(); // expected-note 2{{here}}
+    virtual void g(int); // expected-note {{here}}
+    virtual void h() const; // expected-note 5{{here}}
+};
+
+// CWG2553
+struct B : A {
+    int f(this B&, int); // expected-warning {{hides overloaded virtual function}}
+    int f(this B&);  // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int g(this B&); // expected-warning {{hides overloaded virtual function}}
+    int h(this B&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this B&&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this const B&&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this A&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this int); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+};
+}
+
+namespace DefaultArgs {
+     struct Test { void f(this const auto& = Test{}); };
+    // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+    auto L =[](this const auto& = Test{}){};
+    // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+}
+
+struct CannotUseThis {
+    int fun();
+    int m;
+    void f(this auto) {
+        this->fun(); // expected-error{{invalid use of 'this' in a function with an explicit object parameter}}
+        fun(); // expected-error {{call to non-static member function without an object argument}}
+        m = 0; // expected-error {{invalid use of member 'm' in explicit object member function}}
+    }
+};
+
+namespace ThisInLambdaWithCaptures {
+
+struct Test {
+    Test(auto&&);
+};
+
+void test() {
+
+    [i = 0](this Test) { }();
+    // expected-error@-1 {{invalid explicit object parameter type 'ThisInLambdaWithCaptures::Test' in lambda with capture; the type must be the same as, or derived from, the lambda}}
+
+    struct Derived;
+    auto ok = [i = 0](this const Derived&) {};
+    auto ko = [i = 0](this const Test&) {};
+    // expected-error@-1 {{invalid explicit object parameter type 'ThisInLambdaWithCaptures::Test' in lambda with capture; the type must be the same as, or derived from, the lambda}}
+
+    struct Derived : decltype(ok){};
+    Derived dok{ok};
+    dok();
+
+    struct DerivedErr : decltype(ko){};
+    DerivedErr dko{ko};
+    dko();
+}
+
+}
+
+struct Corresponding {
+    void a(this Corresponding&); // expected-note 2{{here}}
+    void a(); // expected-error{{cannot be redeclared}}
+    void a() &; // expected-error{{cannot be redeclared}}
+    void a(this Corresponding&, int);
+    void a(this Corresponding&, double);
+
+    void b(this const Corresponding&); // expected-note 2{{here}}
+    void b() const; // expected-error{{cannot be redeclared}}
+    void b() const &; // expected-error{{cannot be redeclared}}
+
+    void c(this Corresponding&&); // expected-note {{here}}
+    void c() &&; // expected-error{{cannot be redeclared}}
+
+    void d(this Corresponding&);
+    void d(this Corresponding&&);
+    void d(this const Corresponding&);
+    void d(this const int&);
+    void d(this const int);
+    void d(this int);
+
+    void e(this const Corresponding&&); // expected-note {{here}}
+    void e() const &&; // expected-error{{cannot be redeclared}}
+
+};
+
+template <typename T>
+struct CorrespondingTpl {
+    void a(this CorrespondingTpl&); // expected-note 2{{here}}
+    void a(); // expected-error{{cannot be redeclared}}
+    void a() &; // expected-error{{cannot be redeclared}}
+    void a(this Corresponding&, int);
+    void a(this Corresponding&, double);
+    void a(long);
+
+
+    void b(this const CorrespondingTpl&); // expected-note 2{{here}}
+    void b() const; // expected-error{{cannot be redeclared}}
+    void b() const &; // expected-error{{cannot be redeclared}}
+
+    void c(this CorrespondingTpl&&); // expected-note {{here}}
+    void c() &&; // expected-error{{cannot be redeclared}}
+
+    void d(this Corresponding&);
+    void d(this Corresponding&&);
+    void d(this const Corresponding&);
+    void d(this const int&);
+    void d(this const int);
+    void d(this int);
+
+    void e(this const CorrespondingTpl&&); // expected-note {{here}}
+    void e() const &&; // expected-error{{cannot be redeclared}}
+};
+
+struct C {
+    template <typename T>
+    C(T){}
+};
+
+void func(int i) {
+    (void)[=](this auto&&) { return i; }();
+    (void)[=](this const auto&) { return i; }();
+    (void)[i](this C) { return i; }(); // expected-error{{invalid explicit object parameter type 'C'}}
+    (void)[=](this C) { return i; }(); // expected-error{{invalid explicit object parameter type 'C'}}
+    (void)[](this C) { return 42; }();
+    auto l = [=](this auto&) {};
+    struct D : decltype(l) {};
+    D d{l};
+    d();
+}
+
+void TestMutationInLambda() {
+    [i = 0](this auto &&){ i++; }();
+    [i = 0](this auto){ i++; }();
+    [i = 0](this const auto&){ i++; }();
+    // expected-error@-1 {{cannot assign to a variable captured by copy in a non-mutable lambda}}
+}
+
+struct Over_Call_Func_Example {
+    void a();
+    void b() {
+        a(); // ok, (*this).a()
+    }
+
+    void f(this const Over_Call_Func_Example&); //expected-note {{here}}
+    void g() const {
+        f();       // ok: (*this).f()
+        f(*this);  // expected-error{{too many non-object arguments to function call}}
+        this->f(); // ok
+    }
+
+    static void h() {
+        f();       // expected-error{{call to non-static member function without an object argument}}
+        f(Over_Call_Func_Example{});   // expected-error{{call to non-static member function without an object argument}}
+        Over_Call_Func_Example{}.f();   // ok
+    }
+
+    void k(this int);
+    operator int() const;
+    void m(this const Over_Call_Func_Example& c) {
+        c.k();     // ok
+    }
+};
+
+namespace arity_diagnostics {
+struct S {
+    void f(this auto &&, auto, auto); // expected-note {{requires 2 non-object arguments, but 0 were provided}}
+    void g(this auto &&, auto, auto); // expected-note {{requires 2 non-object arguments, but 3 were provided}}
+    void h(this auto &&, int, int i = 0); // expected-note {{requires at least 1 non-object argument, but 0 were provided}}
+    void i(this S&&, int); // expected-note 2{{declared here}}
+};
+
+int test() {
+    void(*f)(S&&, int, int) = &S::f;
+    f(S{}, 1, 2);
+    f(S{}, 1); // expected-error {{too few arguments to function call, expected 3, have 2}}
+    f(S{}); // expected-error {{too few arguments to function call, expected 3, have 1}}
+    f(S{}, 1, 2, 3); //expected-error {{too many arguments to function call, expected 3, have 4}}
+
+    S{}.f(1, 2);
+    S{}.f(); //  expected-error{{no matching member function for call to 'f'}}
+    S{}.g(1,2,3); // expected-error {{no matching member function for call to 'g'}}
+    S{}.h(); // expected-error {{no matching member function for call to 'h'}}
+    S{}.i(); // expected-error {{too few non-object arguments to function call, expected 1, have 0}}
+    S{}.i(1, 2, 3); // expected-error {{too many non-object arguments to function call, expected 1, have 3}}
+}
+
+}
+
+namespace AddressOf {
+
+struct s {
+    static void f(int);
+    void f(this auto &&) {}
+    void g(this s &&) {};
+
+    void test_qual() {
+        using F = void(s&&);
+        F* a = &f; // expected-error {{must explicitly qualify name of member function when taking its address}}
+        F* b = &g; // expected-error {{must explicitly qualify name of member function when taking its address}}
+        F* c = &s::f;
+        F* d = &s::g;
+    }
+};
+
+void test() {
+    using F = void(s&&);
+    F* a = &s::f;
+    F* b = &s::g;
+    a(s{});
+    b(s{});
+}
+
+}
+
+namespace std {
+  struct strong_ordering {
+    int n;
+    constexpr operator int() const { return n; }
+    static const strong_ordering equal, greater, less;
+  };
+  constexpr strong_ordering strong_ordering::equal = {0};
+  constexpr strong_ordering strong_ordering::greater = {1};
+  constexpr strong_ordering strong_ordering::less = {-1};
+}
+
+namespace operators_deduction {
+
+template <typename T, typename U>
+constexpr bool is_same = false;
+
+template <typename T>
+constexpr bool is_same<T, T> = true;
+
+template <template <typename> typename T>
+struct Wrap {
+void f();
+struct S {
+    operator int(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    Wrap* operator->(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return new Wrap();
+    }
+    int operator[](this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator()(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator++(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator++(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator--(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator--(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator*(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    bool operator==(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return false;
+    }
+    bool operator<=>(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return false;
+    }
+    bool operator<<(this auto&& self, int b) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return false;
+    }
+};
+};
+
+template <typename T>
+struct lvalue_reference {
+    using type = T&;
+};
+template <typename T>
+struct const_lvalue_reference {
+    using type = const T&;
+};
+template <typename T>
+struct volatile_lvalue_reference {
+    using type = volatile T&;
+};
+template <typename T>
+struct rvalue_reference {
+    using type = T&&;
+};
+template <typename T>
+struct const_rvalue_reference {
+    using type = const T&&;
+};
+
+
+void test() {
+    {
+        Wrap<lvalue_reference>::S s;
+        s++;
+        s.operator++(0);
+        ++s;
+        s.operator++();
+        s--;
+        s.operator--(0);
+        --s;
+        s.operator--();
+        s[0];
+        s.operator[](0);
+        s(0);
+        s.operator()(0);
+        *s;
+        s.operator*();
+        s->f();
+        s.operator->();
+        int i = s;
+        (void)(s << 0);
+        s.operator<<(0);
+        (void)(s == 0);
+        s.operator==(0);
+        (void)(s <=> 0);
+        s.operator<=>(0);
+    }
+    {
+        const Wrap<const_lvalue_reference>::S s;
+        s++;
+        s.operator++(0);
+        ++s;
+        s.operator++();
+        s--;
+        s.operator--(0);
+        --s;
+        s.operator--();
+        s[0];
+        s.operator[](0);
+        s(0);
+        s.operator()(0);
+        *s;
+        s.operator*();
+        s->f();
+        s.operator->();
+        int i = s;
+        (void)(s << 0);
+        s.operator<<(0);
+        (void)(s == 0);
+        s.operator==(0);
+        (void)(s <=> 0);
+        s.operator<=>(0);
+    }
+    {
+        volatile Wrap<volatile_lvalue_reference>::S s;
+        s++;
+        s.operator++(0);
+        ++s;
+        s.operator++();
+        s--;
+        s.operator--(0);
+        --s;
+        s.operator--();
+        s[0];
+        s.operator[](0);
+        s(0);
+        s.operator()(0);
+        *s;
+        s.operator*();
+        s->f();
+        s.operator->();
+        int i = s;
+        (void)(s << 0);
+        s.operator<<(0);
+        (void)(s == 0);
+        s.operator==(0);
+        (void)(s <=> 0);
+        s.operator<=>(0);
+    }
+    {
+        Wrap<rvalue_reference>::S s;
+        using M = Wrap<rvalue_reference>::S&&;
+        ((M)s)++;
+        ((M)s).operator++(0);
+        ++((M)s);
+        ((M)s).operator++();
+        ((M)s)--;
+        ((M)s).operator--(0);
+        --((M)s);
+        ((M)s).operator--();
+        ((M)s)[0];
+        ((M)s).operator[](0);
+        ((M)s)(0);
+        ((M)s).operator()(0);
+        *((M)s);
+        ((M)s).operator*();
+        ((M)s)->f();
+        ((M)s).operator->();
+        int i = ((M)s);
+        (void)(((M)s) << 0);
+        ((M)s).operator<<(0);
+        (void)(((M)s) == 0);
+        ((M)s).operator==(0);
+        (void)(((M)s) <=> 0);
+        ((M)s).operator<=>(0);
+    }
+}
+}
+
+namespace conversions {
+//[over.best.ics]
+struct Y { Y(int); }; //expected-note 3{{candidate}}
+struct A { operator int(this auto&&); };  //expected-note {{candidate}}
+Y y1 = A();   // expected-error{{no viable conversion from 'A' to 'Y'}}
+
+struct X { X(); }; //expected-note 3{{candidate}}
+struct B { operator X(this auto&&); };
+B b;
+X x{{b}}; // expected-error{{no matching constructor for initialization of 'X'}}
+
+struct T{}; // expected-note 2{{candidate constructor}}
+struct C {
+    operator T (this int); // expected-note {{candidate function not viable: no known conversion from 'C' to 'int' for object argument}}
+    operator int() const; // expected-note {{candidate function}}
+};
+
+void foo(C c) {
+   T d = c; // expected-error {{no viable conversion from 'C' to 'T'}}
+}
+
+}
+
+namespace surrogate {
+using fn_t = void();
+struct C {
+    operator fn_t * (this C const &);
+};
+
+void foo(C c) {
+   c();
+}
+
+}

clang/www/cxx_dr_status.html

<tr class="open" id="2552">
     <td>open</td>
     <td>Constant evaluation of non-defining variable declarations</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr class="open" id="2553">
     <td><a href="https://cplusplus.github.io/CWG/issues/2553.html">2553</a></td>
     <td>open</td>
     <td>Restrictions on explicit object member functions</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2554">
     <td><a href="https://cplusplus.github.io/CWG/issues/2554.html">2554</a></td>
     <td>open</td>
     <td>Overriding virtual functions, also with explicit object parameters</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2555">
     <td><a href="https://cplusplus.github.io/CWG/issues/2555.html">2555</a></td>
     <td>open</td>
     <td>Ineffective redeclaration prevention for <I>using-declarator</I>s</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr class="open" id="2556">
<tr class="open" id="2560">
     <td>open</td>
     <td>Parameter type determination in a <I>requirement-parameter-list</I></td>
     <td align="center">Not resolved</td>
   </tr>
   <tr class="open" id="2561">
     <td><a href="https://cplusplus.github.io/CWG/issues/2561.html">2561</a></td>
     <td>open</td>
     <td>Conversion to function pointer for lambda with explicit object parameter</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2562">
     <td><a href="https://cplusplus.github.io/CWG/issues/2562.html">2562</a></td>
     <td>open</td>
     <td>Exceptions thrown during coroutine startup</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr class="open" id="2563">
<tr id="2652">
     <td>WP</td>
     <td>Overbroad definition of <TT>__STDCPP_BFLOAT16_T__</TT></td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2653">
     <td><a href="https://cplusplus.github.io/CWG/issues/2653.html">2653</a></td>
     <td>WP</td>
     <td>Can an explicit object parameter have a default argument?</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr id="2654">
     <td><a href="https://cplusplus.github.io/CWG/issues/2654.html">2654</a></td>
     <td>DRWP</td>
     <td>Un-deprecation of compound volatile assignments</td>
     <td class="full" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2655">

clang/www/cxx_status.html

<tr>
       <td>Mixed string literal concatenation</td>
       <td><a href="https://wg21.link/p2201r1">P2201R1</a></td>
       <td class="full" align="center">Yes</td>
     </tr>
     <!-- October 2021 papers -->
     <tr>
       <td rowspan=2>Deducing this</td>
       <td><a href="https://wg21.link/P0847R7">P0847R7</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 18</td>
     </tr>
     <tr>
       <td><a href="https://wg21.link/P2797R0">P2797R0</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 18</td>
     </tr>
     <tr>
       <td rowspan=2>Change scope of lambda trailing-return-type</td>
       <td><a href="https://wg21.link/P2036R3">P2036R3</a></td>
       <td rowspan=2 class="unreleased" align="center">Clang 17</td>
     </tr>
     <tr>
       <td><a href="https://wg21.link/P2579R0">P2579R0</a></td>