[C++20] Add consteval-specific semantic for functions

[Tyker]

Summary

Changes:

• Calls to consteval function and constructors are not evaluated as soon as they are reached.
• Add diagnostic for taking address of a consteval function in non-constexpr context.
• Add diagnostic for address of consteval function accessible at runtime.
• Add tests

Serialization and importing depends on https://reviews.llvm.org/D63640

[Tyker]

ping @rsmith

[Tyker]

@rsmith ping

[Tyker]

ping @rsmith

[Tyker]

Rebased

@rsmith Ping

[rsmith]

clang/include/clang/Basic/DiagnosticSemaKinds.td

2361

Let's use the proper terminology here so people can search for it: "outside of an immediate invocation"

2362–2364

This diagnostic should be moved to DiagnosticASTKinds (and produced by CheckConstantExpression; see my other comment on this). Please rephrase this to follow the same pattern as note_constexpr_non_global.

clang/lib/Sema/SemaExpr.cpp

5636–5640

This should be checked when determining whether the core constant expression is a constant expression, which happens in CheckConstantExpression in ExprConstant.cpp, not here.

[rsmith]

clang/include/clang/Basic/DiagnosticSemaKinds.td

2368

"consteval specified" -> "declared consteval"

3869–3870

consteval should not affect overload resolution. I don't think this diagnostic makes sense. (We should first pick the function and *then* check whether it's usable.)

clang/include/clang/Sema/Sema.h

2279

This should be called CheckImmediateInvocation or similar. ActOn* functions should only be called by the parser in response to source constructs, not internally by Sema.

clang/lib/Sema/SemaDeclCXX.cpp

2302

Please keep lines to 80 columns or fewer.

clang/lib/Sema/SemaExpr.cpp

5584

This is the wrong place to call ActOnConstevalCall from: that needs to happen when we actually build the resolved call expression (which isn't always done by this function).

5605–5606

!isConstantEvaluated() should be checked in the CXXConstructExpr case too; consider moving that check into ActOnConstevalCall.

5623–5628

I don't think this approach can work as-is. You need to know the complete bounds of the immediate invocation before you can check it. For example:

enum E {};
consteval int operator+(int (*f)(), E) { return f(); }
consteval int fn() { return 42; }
consteval auto get() -> int(*)() { return fn; }

int k = get() + E();

This is (or at least should be) valid code: the immediate invocation here is the entire get() + E() expression, and evaluating that produces 42. But this approach will reject the code, because get() in isolation is a constant expression. (The current wording suggests that this code is invalid, but I'm trying to get that fixed; I'm confident that was not the intent.)

I think we need a different approach. Perhaps:

• when we see a candidate immediate invocation, immediately create a ConstantExpr wrapping it, but don't evaluate it yet
• add that ConstantExpr to a list on the expression evaluation context record
• if there already were any expressions listed there, check to see if any of them is a subexpression of the current expression, and if so, perform a TreeTransform to remove the ConstantExpr wrappers from those subexpressions and remove them from the list on the evaluation context record
• when we pop an expression evaluation context record, *then* evaluate all remaining ConstantExprs for immediate invocations and check they're valid

(Or some other approach that handles this wrinkle; the above is just a suggestion.)

5722–5723

What's the purpose of this change?

clang/lib/Sema/SemaOverload.cpp

9802–9813

This isn't the right way to deal with this, in part because we don't know whether we're in an immediate invocation or not at this point. Instead, we should track that we saw a use of a consteval function from Sema::DiagnoseUseOfDecl, and issue a diagnostic if that use is not within an immediate invocation / within a consteval function.

[Tyker]

sorry i didn't realize the full complexity of immediate invocations.
i am working on a patch fixing issues.

clang/lib/Sema/SemaExpr.cpp

5722–5723

this change was to prevent a double error message for code like:

consteval f() { return 0; }
auto* p = __builtin_addressof(f);

it is going to disappear because the error reporting locations are going to move.

clang/lib/Sema/SemaOverload.cpp

9802–9813

from what i understood Sema::DiagnoseUseOfDecl is called as soon as the Decl is used. but there is some cases similar to the one you gave in an other comment where we can't know yet if the use is during an immediate invokation. like the following

enum E {};
consteval int operator+(int (*f)(), E) { return f(); }
consteval int fn() { return 42; }

int k = &fn + E();

this should be valid i think. but at the point of call of Sema::DiagnoseUseOfDecl for the use of fn we don't know yet that we are in an immediate invokation. so i think this check should be delayed until we know the bounds of the immediate invokation.

[Tyker]

I narrowed the patch because it was getting quite big. this patch only handle consteval function, not constructors.

Changes:

• keep track of DeclRefExpr on consteval decaltions.
• keep track of candidates for immediate invocation.
• detect and remove nested immediate invocation candidates
• detect and remove DeclRefExpr on consteval declarations inside immediate invocations.
• diagnose non removed DeclRefExpr on consteval declarations and candidates for immediate invocation.
• fixe existing bug where defaulted special members declared consteval where marked constexpr instead.
• improve tests.

[Tyker]

Changes:

• Rebased
• Fixed typos

[Tyker]

@rsmith Ping

[Tyker]

ping @rsmith

[rsmith]

Very cool, this is an elegant approach.

clang/include/clang/Basic/DiagnosticASTKinds.td

58

on -> to

clang/include/clang/Basic/DiagnosticSemaKinds.td

2366

Please add 's around the function name. Instead of encoding the name as an integer, you can stream the declaration itself into the diagnostic and just use "%0 cannot be declared consteval" here.

clang/include/clang/Sema/Sema.h

811–813

The comment should first describe what this field means, before saying how to use it (if necessary). So:

/// Whether immediate invocation candidates and references to consteval functions should be tracked.

or something like that.

That said, consider renaming this to RebuildingImmediateInvocation to avoid suggesting it's a general mechanism to turn off immediate invocation tracking.

1072

ImmediateInvocationsCandidates -> ImmediateInvocationCandidates

1076

ReferenceOnConsteval -> ReferencesToConsteval or similar.

clang/lib/AST/ExprConstant.cpp

2015

It would be useful for the diagnostic to say what consteval function we have a pointer to, and maybe include a note pointing at its declaration.

clang/lib/Sema/SemaDeclCXX.cpp

11459–11460

I don't think this is necessary: implicit special members are never consteval. (We use DeclareImplicitDefaultConstructor when there is no user-declared default constructor, in which case there can't possibly be a defaulted consteval default constructor.) I don't think you need any of the DetermineSpecialMemberConstexprKind machinery, nor the tracking of DefaultedSpecialMemberIsConsteval in CXXRecordDecl.

clang/lib/Sema/SemaExpr.cpp

15138

Consider using E.get()->IgnoreImplicit() here; we want to step over (at least) any CXXBindTemporaryExprs inserted by MaybeBindToTemporary.

15138–15141

Please add a comment to this to point out that it's just an optimization, for example:

"Opportunistically remove the callee from ReferencesToConsteval if we can. It's OK if this fails; we'll also remove this in HandleImmediateInvocations, but catching it here allows us to avoid walking the AST looking for it in simple cases."

15142–15145

We will need an ExprWithCleanups wrapping E.get() if there might be any temporaries within it.

We don't actually know whether there definitely are any such temporaries, but we can conservatively assume we might need an ExprWithCleanups if Cleanup.exprNeedsCleanups() returns true.

You can test this with something like:

struct A { int *p = new int(42); consteval int get() { return *p; } constexpr ~A() { delete p; } };
int k = A().get();

... which should be valid, but will be rejected (or might assert) if we don't destroy the A() temporary as part of evaluating the consteval call to get.

15177

< on SourceLocation doesn't really mean anything. It's not correct to use it here. Also, using source locations will not give you outer expressions before inner ones. Consider:

enum E {};
consteval E f();
consteval int operator+(E, int);
void g() {
  f() + 1;
}

Here, the begin location of both the call to f() and the call to operator+ are in the same place: at the f token.

I don't think you need this sort at all: instead, you can process immediate invocations in ImmediateInvocationCandidates in reverse order: subexpressions must have been built before their enclosing expressions, so walking in reverse order will guarantee that you process an outer candidate before any inner ones.

15208

We should look for an exact match here, not merely for something nearby. (Maybe: form a pointer set of immediate invocation pointer expressions, and remove things from the set when you encounter them in this walk; then when iterating over the immediate invocations, skip ones that have already been erased from the set.)

15224–15225

I don't imagine this will ever actually happen, because immutability of the AST is so important to other parts of Clang. This should be a rare case, so it likely doesn't matter; I'd just remove this FIXME.

[Tyker]

almost all comments were addressed.

clang/lib/Sema/SemaDeclCXX.cpp

11459–11460

all the code you mention is to fixe an issue i saw while working on consteval.
the AST of

struct A {
    consteval A() = default;
    consteval A(int) { }
};

is

TranslationUnitDecl
`-CXXRecordDecl <line:1:1, line:4:1> line:1:8 struct A definition
  |-DefinitionData pass_in_registers empty standard_layout trivially_copyable trivial literal has_user_declared_ctor has_constexpr_non_copy_move_ctor can_const_default_init
  | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr
  | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
  | |-MoveConstructor exists simple trivial needs_implicit
  | |-CopyAssignment trivial has_const_param needs_implicit implicit_has_const_param
  | |-MoveAssignment exists simple trivial needs_implicit
  | `-Destructor simple irrelevant trivial needs_implicit
  |-CXXRecordDecl <col:1, col:8> col:8 implicit referenced struct A
  |-CXXConstructorDecl <line:2:5, col:27> col:15 constexpr A 'void ()' default trivial noexcept-unevaluated 0x55585ae25160
  `-CXXConstructorDecl <line:3:5, col:24> col:15 consteval A 'void (int)'
    |-ParmVarDecl <col:17> col:20 'int'
    `-CompoundStmt <col:22, col:24>

godbolt
notice that A::A() is marked as constexpr instead of consteval.
and A::A(int) is marked correctly.

this is because consteval defaulted special members are not marked as consteval.
those code changes are intended to fix that. with this patch both constructor are marked as consteval

clang/lib/Sema/SemaExpr.cpp

15177

the sort is used in combination with a lower_bounds in ComplexRemove::TransformConstantExpr to lookup in ImmediateInvocationsCandidates using binary search.
but i think your proposition is more efficient except in some very rare cases.

15208

the issue i had with a set is that we can't remove element during iteration as removing element invalidate iterators.
but to get an exact match we can use the pointer as "key", they are unique.

[Tyker]

improve performance in a bad case

[Tyker]

The now that constexpr destructors are legal. The code in this patch need to be adapted, I have question about the following code.

struct A {
  constexpr ~A() {}
};

consteval A f() {
    return A{};
}

void test() {
    A a;
    a = f(); // <-- here
}

At the point i marked.
The invocation of f causes an immediate invocation (http://eel.is/c++draft/expr.const#12).
Immediate invocation are full expression (http://eel.is/c++draft/intro.execution#5).
Full expression resolve all there side-effects before evaluating the next full expression (http://eel.is/c++draft/intro.execution#9).
The return value of f() is created inside the immediate invocation.
So the destructor of the value returned by f() should be destroyed within the immediate evaluation.
But that value is needed for the assignment operator.
This seem contradictory. What have i misunderstood ? What should happen here ?

[Tyker]

ping @rsmith

[Tyker]

ping @rsmith

[rsmith]

In D63960#1712398, @Tyker wrote:

The now that constexpr destructors are legal. The code in this patch need to be adapted, I have question about the following code.

struct A {
  constexpr ~A() {}
};

consteval A f() {
    return A{};
}

void test() {
    A a;
    a = f(); // <-- here
}

At the point i marked.
The invocation of f causes an immediate invocation (http://eel.is/c++draft/expr.const#12).
Immediate invocation are full expression (http://eel.is/c++draft/intro.execution#5).
Full expression resolve all there side-effects before evaluating the next full expression (http://eel.is/c++draft/intro.execution#9).
The return value of f() is created inside the immediate invocation.
So the destructor of the value returned by f() should be destroyed within the immediate evaluation.
But that value is needed for the assignment operator.
This seem contradictory. What have i misunderstood ? What should happen here ?

After desugaring, the above example looks like this:

a.operator=( <temporary materialization conversion>( <consteval full-expression>( f() ) ) );

That is: the temporary is materialized outside the immediate evaluation, so no A destructor should be run as part of evaluating that full-expression (the destructor is run in the enclosing full-expression). If the example were instead:

constexpr A g() {
  return A {};
}
consteval A &id (A &&a) { return a; }

void test() {
  A a;
  a = id(g());
}

then we would have

a.operator=( <consteval full-expression>( id( <temporary materialization conversion>( g() ) ) ) );

... and now the A temporary should be destroyed inside the full-expression. This example is ill-formed because the id call is not a constant expression (because its evaluated value refers to an object with automatic storage duration).

[Tyker]

@rsmith

Changes:

• Rebased on recent master.
• Adapted this patch to constexpr destructors.
• Fixed issues with handling of temporaries.
• Improve Tests.

[Tyker]

minor fixes
improved tests

[Tyker]

@rsmith ping

[Tyker]

@rsmith ping

[rsmith]

clang/include/clang/AST/Expr.h

1033–1035

I'd remove the "CausedBy" here -- the ConstantExpr is our representation of the immediate invocation.

Also, please start this function name with a lowercase i for local consistency.

clang/include/clang/Sema/Sema.h

1027

II doesn't mean anything to a reader here; please expand the abbreviation.

5438–5439

potention -> potential
Missing period at end of sentence.

clang/lib/AST/ExprConstant.cpp

2012–2013

Please add braces to these if statements (their controlled statement is too long to omit the braces). As a general rule: if an inner construct needs braces, put braces on the outer constructs too.

13618

This isn't sufficient: the evaluation process can refer back to the object under construction (eg, via this), and we need an lvalue that actually names the result in order for this to work.

I think you'll need to do something like creating a suitable object (perhaps a LifetimeExtendedTemporaryDecl) in the caller to represent the result of the immediate invocation, and passing that into here.

clang/lib/Sema/Sema.cpp

164

Consider using a default member initializer instead.

clang/lib/Sema/SemaDecl.cpp

8835–8836

Please add braces here.

8836

This rule also applies to non-member allocator functions. (Don't check for a CXXMethodDecl here.)

clang/lib/Sema/SemaDeclCXX.cpp

11459–11460

This change is incorrect: an implicit special member is never consteval. (Note that implicit special members are the ones the compiler declares for you, not the ones you get from = default.) We don't need this, and as a consequence we don't need DefaultedSpecialMemberIsConsteval on DefinitionData.

clang/lib/Sema/SemaExpr.cpp

15192

This is unreachable; a CXXMemberCallExpr is a CallExpr so this was already handled above. Can you just delete this case?

15197

Typo unhandeld -> unhandled

15248

Transfromer -> Transformer

clang/lib/Sema/TreeTransform.h

3451–3452

I'm surprised that we can remove this; I'd expect this to be necessary to properly handle transforming cases such as

struct X { X(int); ~X(); int n; };
struct Y { explicit Y(int); };
template<typename T> void f() {
  Y y(X(sizeof(sizeof(T))).n);
}
template void f<int>();

... where we need to strip off an ExprWithCleanups to find the syntactic initializer inside it.

If you want to special-case this from ComplexRemove, to make sure you see all ConstantExprs, can you override TransformInitializer there instead of changing it globally?

[Tyker]

comments were fixed.

clang/lib/AST/ExprConstant.cpp

13618

i believe this solution should work. without LifetimeExtendedTemporaryDecl because reference/pointer on temporaries are not valid results of constant evaluation. so the AST should never store an APValue whose LValue is a ConstantExpr.

[Tyker]

ping @rsmith

[Tyker]

ping @rsmith

[Tyker]

@rsmith friendly reminder

[Tyker]

@aaron.ballman do you think you can review this ?

[rsmith]

Some nits and very minor things, but I think this is generally good to go with those fixed. Sorry for the long review delay, and thanks for working on it!

clang/include/clang/Basic/DiagnosticSemaKinds.td

2364

could not be evaluated -> is not a constant expression

clang/include/clang/Sema/Sema.h

811

rebuild -> rebuilt

1075

candidate -> candidates

1078

DeclRefExpr -> DeclRefExprs

1079

know -> known

clang/lib/AST/ExprConstant.cpp

13618

OK, I think that should be fine. Please update the comment above class LValueExprEvaluator to describe this additional case.

clang/lib/Sema/SemaDecl.cpp

8830

In C++20, we should probably reset to constexpr rather than unspecified. Before C++20, it's better for error recovery to leave the function as constexpr. (So unconditionally using CSK_constexpr here would be OK.)

clang/lib/Sema/SemaExpr.cpp

15199

You can use `"%q0" to get a qualified name in a diagnostic; you don't need to turn it into a string yourself.

[Tyker]

thank you for the review.
i am sorry there were so many back and forth.

[ldionne]

@Tyker @rsmith You might be interested in reading https://llvm.org/PR45912, which describes what I believe to be an issue with this patch (or another related patch for consteval -- not sure).

clang/include/clang/AST/CXXRecordDeclDefinitionBits.def

 FIELD(DeclaredNonTrivialSpecialMembers, 6, MERGE_OR)
 
 /// These bits keep track of the declared special members that are
 /// non-trivial for the purpose of calls.
 /// Only the bits corresponding to SMF_CopyConstructor,
 /// SMF_MoveConstructor, and SMF_Destructor are meaningful here.
 FIELD(DeclaredNonTrivialSpecialMembersForCall, 6, MERGE_OR)
 
+/// The special member which have been defaulted with the consteval keyword.
+FIELD(DefaultedSpecialMemberIsConsteval, 6, MERGE_OR)
+
 /// True when this class has a destructor with no semantic effect.
 FIELD(HasIrrelevantDestructor, 1, NO_MERGE)
 
 /// True when this class has at least one user-declared constexpr
 /// constructor which is neither the copy nor move constructor.
 FIELD(HasConstexprNonCopyMoveConstructor, 1, MERGE_OR)
 
 /// True if this class has a (possibly implicit) defaulted default

clang/include/clang/AST/DeclCXX.h

public:
   }
 
   /// Determine whether this class has a move constructor.
   bool hasMoveConstructor() const {
     return (data().DeclaredSpecialMembers & SMF_MoveConstructor) ||
            needsImplicitMoveConstructor();
   }
 
+  /// Determine whether this class has a defaulted consteval default
+  /// constructor.
+  bool hasDefaultedConstevalDefaultCtor() const {
+    return data().DefaultedSpecialMemberIsConsteval & SMF_DefaultConstructor;
+  }
+
+  /// Determine whether this class has a defaulted consteval copy
+  /// constructor.
+  bool hasDefaultedConstevalCopyCtor() const {
+    return data().DefaultedSpecialMemberIsConsteval & SMF_CopyConstructor;
+  }
+
+  /// Determine whether this class has a defaulted consteval move
+  /// constructor.
+  bool hasDefaultedConstevalMoveCtor() const {
+    return data().DefaultedSpecialMemberIsConsteval & SMF_MoveConstructor;
+  }
+
+  /// Determine whether this class has a defaulted consteval copy assignment
+  /// operator.
+  bool hasDefaultedConstevalCopyAssignment() const {
+    return data().DefaultedSpecialMemberIsConsteval & SMF_CopyAssignment;
+  }
+
+  /// Determine whether this class has a defaulted consteval move assignment
+  /// operator.
+  bool hasDefaultedConstevalMoveAssignment() const {
+    return data().DefaultedSpecialMemberIsConsteval & SMF_MoveAssignment;
+  }
+
+  /// Determine whether this class has a defaulted consteval destructor.
+  bool hasDefaultedConstevalDestructor() const {
+    return data().DefaultedSpecialMemberIsConsteval & SMF_Destructor;
+  }
+
   /// Set that we attempted to declare an implicit copy
   /// constructor, but overload resolution failed so we deleted it.
   void setImplicitCopyConstructorIsDeleted() {
     assert((data().DefaultedCopyConstructorIsDeleted ||
             needsOverloadResolutionForCopyConstructor()) &&
            "Copy constructor should not be deleted");
     data().DefaultedCopyConstructorIsDeleted = true;
   }

clang/include/clang/AST/Expr.h

bool EvaluateWithSubstitution(APValue &Value, ASTContext &Ctx,
                                 ArrayRef<const Expr*> Args,
                                 const Expr *This = nullptr) const;
 
   /// Indicates how the constant expression will be used.
   enum ConstExprUsage { EvaluateForCodeGen, EvaluateForMangling };
 
   /// Evaluate an expression that is required to be a constant expression.
   bool EvaluateAsConstantExpr(EvalResult &Result, ConstExprUsage Usage,
-                              const ASTContext &Ctx) const;
+                              const ASTContext &Ctx,
+                              bool InPlace = false) const;
 
   /// If the current Expr is a pointer, this will try to statically
   /// determine the number of bytes available where the pointer is pointing.
   /// Returns true if all of the above holds and we were able to figure out the
   /// size, false otherwise.
   ///
   /// \param Type - How to evaluate the size of the Expr, as defined by the
   /// "type" parameter of __builtin_object_size

 
 public:
   friend TrailingObjects;
   friend class ASTStmtReader;
   friend class ASTStmtWriter;
   static ConstantExpr *Create(const ASTContext &Context, Expr *E,
                               const APValue &Result);
   static ConstantExpr *Create(const ASTContext &Context, Expr *E,
-                              ResultStorageKind Storage = RSK_None);
+                              ResultStorageKind Storage = RSK_None,
+                              bool IsCausedByImmediateInvocation = false);
   static ConstantExpr *CreateEmpty(const ASTContext &Context,
                                    ResultStorageKind StorageKind,
                                    EmptyShell Empty);
 
   static ResultStorageKind getStorageKind(const APValue &Value);
   static ResultStorageKind getStorageKind(const Type *T,
                                           const ASTContext &Context);
 
public:
   void MoveIntoResult(APValue &Value, const ASTContext &Context);
 
   APValue::ValueKind getResultAPValueKind() const {
     return static_cast<APValue::ValueKind>(ConstantExprBits.APValueKind);
   }
   ResultStorageKind getResultStorageKind() const {
     return static_cast<ResultStorageKind>(ConstantExprBits.ResultKind);
   }
+  bool IsCausedByImmediateInvocation() const {
+    return ConstantExprBits.IsCausedByImmediateInvocation;
+  }
   APValue getAPValueResult() const;
   const APValue &getResultAsAPValue() const { return APValueResult(); }
   llvm::APSInt getResultAsAPSInt() const;
   // Iterators
   child_range children() { return child_range(&SubExpr, &SubExpr+1); }
   const_child_range children() const {
     return const_child_range(&SubExpr, &SubExpr + 1);
   }

clang/include/clang/AST/Stmt.h

class ConstantExprBitfields {
     /// When ResultKind == RSK_Int64. the BitWidth of the trail-allocated
     /// integer. 7 bits because it is the minimal number of bit to represent a
     /// value from 0 to 64 (the size of the trail-allocated number).
     unsigned BitWidth : 7;
 
     /// When ResultKind == RSK_APValue. Wether the ASTContext will cleanup the
     /// destructor on the trail-allocated APValue.
     unsigned HasCleanup : 1;
+
+    /// Whether this ConstantExpr was created for immediate invocation.
+    unsigned IsCausedByImmediateInvocation : 1;
   };
 
   class PredefinedExprBitfields {
     friend class ASTStmtReader;
     friend class PredefinedExpr;
 
     unsigned : NumExprBits;
 

clang/include/clang/Basic/DiagnosticASTKinds.td

def note_constexpr_virtual_base : Note<
   "cannot construct object of type %0 with virtual base class "
   "in a constant expression">;
 def note_constexpr_nonliteral : Note<
   "non-literal type %0 cannot be used in a constant expression">;
 def note_constexpr_non_global : Note<
   "%select{pointer|reference}0 to %select{|subobject of }1"
   "%select{temporary|%3}2 is not a constant expression">;
 def note_constexpr_dynamic_alloc : Note<
   "%select{pointer|reference}0 to %select{|subobject of }1"
   "heap-allocated object is not a constant expression">;
+def note_consteval_address_accessible : Note<
+  "%select{pointer|reference}0 to a consteval declaration "
+  "is not a constant expression">;
 def note_constexpr_uninitialized : Note<
   "%select{|sub}0object of type %1 is not initialized">;
 def note_constexpr_subobject_declared_here : Note<
   "subobject declared here">;
 def note_constexpr_array_index : Note<"cannot refer to element %0 of "
   "%select{array of %2 element%plural{1:|:s}2|non-array object}1 "
   "in a constant expression">;
 def note_constexpr_float_arithmetic : Note<

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def warn_cxx98_compat_constexpr : Warning<
   "'constexpr' specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 // FIXME: Maybe this should also go in -Wc++14-compat?
 def warn_cxx14_compat_constexpr_not_const : Warning<
   "'constexpr' non-static member function will not be implicitly 'const' "
   "in C++14; add 'const' to avoid a change in behavior">,
   InGroup<DiagGroup<"constexpr-not-const">>;
+def err_invalid_consteval_take_address : Error<
+  "cannot take address of consteval function %0 outside"
+  " of an immediate invocation">;
+def err_invalid_consteval_call : Error<
+  "call to consteval function '%0' could not be evaluated">;
+def err_invalid_consteval_decl_kind : Error<
+  "%0 cannot be declared consteval">;
 def err_invalid_constexpr : Error<
   "%select{function parameter|typedef}0 "
   "cannot be %sub{select_constexpr_spec_kind}1">;
 def err_invalid_constexpr_member : Error<"non-static data member cannot be "
   "constexpr%select{; did you intend to make it %select{const|static}0?|}1">;
 def err_constexpr_tag : Error<
   "%select{class|struct|interface|union|enum}0 "
   "cannot be marked %sub{select_constexpr_spec_kind}1">;
 def err_constexpr_dtor : Error<
   "destructor cannot be declared %sub{select_constexpr_spec_kind}0">;

 def ext_constexpr_local_var_no_init : ExtWarn<
   "uninitialized variable in a constexpr %select{function|constructor}0 "
   "is a C++20 extension">, InGroup<CXX2a>;
 def warn_cxx17_compat_constexpr_local_var_no_init : Warning<
   "uninitialized variable in a constexpr %select{function|constructor}0 "
   "is incompatible with C++ standards before C++20">,
   InGroup<CXXPre2aCompat>, DefaultIgnore;
 def ext_constexpr_function_never_constant_expr : ExtWarn<
-  "constexpr %select{function|constructor}0 never produces a "
+  "%select{constexpr|consteval}1 %select{function|constructor}0 never produces a "
   "constant expression">, InGroup<DiagGroup<"invalid-constexpr">>, DefaultError;
 def err_attr_cond_never_constant_expr : Error<
   "%0 attribute expression never produces a constant expression">;
 def err_diagnose_if_invalid_diagnostic_type : Error<
   "invalid diagnostic type for 'diagnose_if'; use \"error\" or \"warning\" "
   "instead">;
 def err_constexpr_body_no_return : Error<
   "no return statement in %select{constexpr|consteval}0 function">;
def note_ovl_candidate_disabled_by_function_cond_attr : Note<
     "candidate disabled: %0">;
 def note_ovl_candidate_disabled_by_extension : Note<
     "candidate unavailable as it requires OpenCL extension '%0' to be enabled">;
 def err_addrof_function_disabled_by_enable_if_attr : Error<
     "cannot take address of function %0 because it has one or more "
     "non-tautological enable_if conditions">;
 def note_addrof_ovl_candidate_disabled_by_enable_if_attr : Note<
     "candidate function made ineligible by enable_if">;
 def note_ovl_candidate_deduced_mismatch : Note<
     "candidate template ignored: deduced type "
     "%diff{$ of %select{|element of }4%ordinal0 parameter does not match "
     "adjusted type $ of %select{|element of }4argument"
     "|of %select{|element of }4%ordinal0 parameter does not match "
     "adjusted type of %select{|element of }4argument}1,2%3">;
 def note_ovl_candidate_non_deduced_mismatch : Note<
     "candidate template ignored: could not match %diff{$ against $|types}0,1">;
 // This note is needed because the above note would sometimes print two
 // different types with the same name.  Remove this note when the above note

clang/include/clang/Sema/Sema.h

void pop() {
       SavedContext = nullptr;
     }
 
     ~ContextRAII() {
       pop();
     }
   };
 
+  /// Whether the AST is currently being rebuild to correct immediate
+  /// invocations. Immediate invocation candidates and references to consteval
+  /// functions aren't tracked when this is set.
+  bool RebuildingImmediateInvocation;
+
   /// Used to change context to isConstantEvaluated without pushing a heavy
   /// ExpressionEvaluationContextRecord object.
   bool isConstantEvaluatedOverride;
 
   bool isConstantEvaluated() {
     return ExprEvalContexts.back().isConstantEvaluated() ||
            isConstantEvaluatedOverride;
   }
enum class ExpressionEvaluationContext {
     ///
     /// This value is used when parsing default function arguments, for which
     /// we would like to provide diagnostics (e.g., passing non-POD arguments
     /// through varargs) but do not want to mark declarations as "referenced"
     /// until the default argument is used.
     PotentiallyEvaluatedIfUsed
   };
 
+  using IICandidate = llvm::PointerIntPair<ConstantExpr *, 1>;
+
   /// Data structure used to record current or nested
   /// expression evaluation contexts.
   struct ExpressionEvaluationContextRecord {
     /// The expression evaluation context.
     ExpressionEvaluationContext Context;
 
     /// Whether the enclosing context needed a cleanup.
     CleanupInfo ParentCleanup;
struct ExpressionEvaluationContextRecord {
     /// If we are processing a decltype type, a set of temporary binding
     /// expressions for which we have deferred checking the destructor.
     SmallVector<CXXBindTemporaryExpr *, 8> DelayedDecltypeBinds;
 
     llvm::SmallPtrSet<const Expr *, 8> PossibleDerefs;
 
     /// Expressions appearing as the LHS of a volatile assignment in this
     /// context. We produce a warning for these when popping the context if
     /// they are not discarded-value expressions nor unevaluated operands.
     SmallVector<Expr*, 2> VolatileAssignmentLHSs;
 
+    /// Set of candidate for starting an immediate invocation.
+    llvm::SmallVector<IICandidate, 4> ImmediateInvocationCandidates;
+
+    /// Set of DeclRefExpr referencing a consteval function when used in a
+    /// context not already know to be immediately invoked.
+    llvm::SmallPtrSet<DeclRefExpr *, 4> ReferenceToConsteval;
+
     /// \brief Describes whether we are in an expression constext which we have
     /// to handle differently.
     enum ExpressionKind {
       EK_Decltype, EK_TemplateArgument, EK_Other
     } ExprContext;
 
     ExpressionEvaluationContextRecord(ExpressionEvaluationContext Context,
                                       unsigned NumCleanupObjects,
public:
   void checkNonTrivialCUnion(QualType QT, SourceLocation Loc,
                              NonTrivialCUnionContext UseContext,
                              unsigned NonTrivialKind);
 
   void AddInitializerToDecl(Decl *dcl, Expr *init, bool DirectInit);
   void ActOnUninitializedDecl(Decl *dcl);
   void ActOnInitializerError(Decl *Dcl);
 
   void ActOnPureSpecifier(Decl *D, SourceLocation PureSpecLoc);
   void ActOnCXXForRangeDecl(Decl *D);
   StmtResult ActOnCXXForRangeIdentifier(Scope *S, SourceLocation IdentLoc,
                                         IdentifierInfo *Ident,
                                         ParsedAttributes &Attrs,
                                         SourceLocation AttrEnd);
   void SetDeclDeleted(Decl *dcl, SourceLocation DelLoc);
   void SetDeclDefaulted(Decl *dcl, SourceLocation DefaultLoc);
   void CheckStaticLocalForDllExport(VarDecl *VD);
public:
   /// \returns true if an error occurred.
   bool checkThisInStaticMemberFunctionAttributes(CXXMethodDecl *Method);
 
   /// MaybeBindToTemporary - If the passed in expression has a record type with
   /// a non-trivial destructor, this will return CXXBindTemporaryExpr. Otherwise
   /// it simply returns the passed in expression.
   ExprResult MaybeBindToTemporary(Expr *E);
 
+  /// Wrap the expression in a ConstantExpr if it is a potention immediate
+  /// invocation
+  ExprResult CheckForImmediateInvocation(ExprResult E, FunctionDecl *Decl);
+
   bool CompleteConstructorCall(CXXConstructorDecl *Constructor,
                                MultiExprArg ArgsPtr,
                                SourceLocation Loc,
                                SmallVectorImpl<Expr*> &ConvertedArgs,
                                bool AllowExplicit = false,
                                bool IsListInitialization = false);
 
   ParsedType getInheritingConstructorName(CXXScopeSpec &SS,

clang/lib/AST/DeclCXX.cpp

: UserDeclaredConstructor(false), UserDeclaredSpecialMembers(0),
       NeedOverloadResolutionForMoveAssignment(false),
       NeedOverloadResolutionForDestructor(false),
       DefaultedCopyConstructorIsDeleted(false),
       DefaultedMoveConstructorIsDeleted(false),
       DefaultedMoveAssignmentIsDeleted(false),
       DefaultedDestructorIsDeleted(false), HasTrivialSpecialMembers(SMF_All),
       HasTrivialSpecialMembersForCall(SMF_All),
       DeclaredNonTrivialSpecialMembers(0),
-      DeclaredNonTrivialSpecialMembersForCall(0), HasIrrelevantDestructor(true),
+      DeclaredNonTrivialSpecialMembersForCall(0),
+      DefaultedSpecialMemberIsConsteval(0), HasIrrelevantDestructor(true),
       HasConstexprNonCopyMoveConstructor(false),
       HasDefaultedDefaultConstructor(false),
       DefaultedDefaultConstructorIsConstexpr(true),
       HasConstexprDefaultConstructor(false),
       DefaultedDestructorIsConstexpr(true),
       HasNonLiteralTypeFieldsOrBases(false),
       UserProvidedDefaultConstructor(false), DeclaredSpecialMembers(0),
       ImplicitCopyConstructorCanHaveConstParamForVBase(true),
if (const auto *Constructor = dyn_cast<CXXConstructorDecl>(D)) {
 
     if (Constructor->isDefaultConstructor()) {
       SMKind |= SMF_DefaultConstructor;
 
       if (Constructor->isUserProvided())
         data().UserProvidedDefaultConstructor = true;
       if (Constructor->isConstexpr())
         data().HasConstexprDefaultConstructor = true;
-      if (Constructor->isDefaulted())
+      if (Constructor->isDefaulted()) {
         data().HasDefaultedDefaultConstructor = true;
+        if (Constructor->isConsteval())
+          data().DefaultedSpecialMemberIsConsteval |= SMF_DefaultConstructor;
+      }
     }
 
     if (!FunTmpl) {
       unsigned Quals;
       if (Constructor->isCopyConstructor(Quals)) {
         SMKind |= SMF_CopyConstructor;
 
         if (Quals & Qualifiers::Const)
           data().HasDeclaredCopyConstructorWithConstParam = true;
-      } else if (Constructor->isMoveConstructor())
+        if (Constructor->isConsteval() && Constructor->isDefaulted())
+          data().DefaultedSpecialMemberIsConsteval |= SMF_CopyConstructor;
+      } else if (Constructor->isMoveConstructor()) {
         SMKind |= SMF_MoveConstructor;
+        if (Constructor->isConsteval() && Constructor->isDefaulted())
+          data().DefaultedSpecialMemberIsConsteval |= SMF_MoveConstructor;
+      }
     }
 
     // C++11 [dcl.init.aggr]p1: DR1518
     //   An aggregate is an array or a class with no user-provided [or]
     //   explicit [...] constructors
     // C++20 [dcl.init.aggr]p1:
     //   An aggregate is an array or a class with no user-declared [...]
     //   constructors
if (const auto *Constructor = dyn_cast<CXXConstructorDecl>(DUnderlying)) {
     if (Constructor->isConstexpr() && !Constructor->isCopyOrMoveConstructor())
       data().HasConstexprNonCopyMoveConstructor = true;
   }
 
   // Handle destructors.
   if (const auto *DD = dyn_cast<CXXDestructorDecl>(D)) {
     SMKind |= SMF_Destructor;
 
+    if (DD->isConsteval() && DD->isDefaulted())
+      data().DefaultedSpecialMemberIsConsteval |= SMF_Destructor;
     if (DD->isUserProvided())
       data().HasIrrelevantDestructor = false;
     // If the destructor is explicitly defaulted and not trivial or not public
     // or if the destructor is deleted, we clear HasIrrelevantDestructor in
     // finishedDefaultedOrDeletedMember.
 
     // C++11 [class.dtor]p5:
     //   A destructor is trivial if [...] the destructor is not virtual.
     if (DD->isVirtual()) {
       data().HasTrivialSpecialMembers &= ~SMF_Destructor;
       data().HasTrivialSpecialMembersForCall &= ~SMF_Destructor;
     }
   }
 
   // Handle member functions.
   if (const auto *Method = dyn_cast<CXXMethodDecl>(D)) {
     if (Method->isCopyAssignmentOperator()) {
       SMKind |= SMF_CopyAssignment;
 
+      if (Method->isConsteval() && Method->isDefaulted())
+        data().DefaultedSpecialMemberIsConsteval |= SMF_CopyAssignment;
+
       const auto *ParamTy =
           Method->getParamDecl(0)->getType()->getAs<ReferenceType>();
       if (!ParamTy || ParamTy->getPointeeType().isConstQualified())
         data().HasDeclaredCopyAssignmentWithConstParam = true;
     }
 
-    if (Method->isMoveAssignmentOperator())
+    if (Method->isMoveAssignmentOperator()) {
       SMKind |= SMF_MoveAssignment;
 
+      if (Method->isConsteval() && Method->isDefaulted())
+        data().DefaultedSpecialMemberIsConsteval |= SMF_CopyAssignment;
+    }
+
     // Keep the list of conversion functions up-to-date.
     if (auto *Conversion = dyn_cast<CXXConversionDecl>(D)) {
       // FIXME: We use the 'unsafe' accessor for the access specifier here,
       // because Sema may not have set it yet. That's really just a misdesign
       // in Sema. However, LLDB *will* have set the access specifier correctly,
       // and adds declarations after the class is technically completed,
       // so completeDefinition()'s overriding of the access specifiers doesn't
       // work.

clang/lib/AST/Expr.cpp

 }
 
 ConstantExpr::ConstantExpr(Expr *subexpr, ResultStorageKind StorageKind)
     : FullExpr(ConstantExprClass, subexpr) {
   DefaultInit(StorageKind);
 }
 
 ConstantExpr *ConstantExpr::Create(const ASTContext &Context, Expr *E,
-                                   ResultStorageKind StorageKind) {
+                                   ResultStorageKind StorageKind,
+                                   bool IsCausedByImmediateInvocation) {
   assert(!isa<ConstantExpr>(E));
   AssertResultStorageKind(StorageKind);
   unsigned Size = totalSizeToAlloc<APValue, uint64_t>(
       StorageKind == ConstantExpr::RSK_APValue,
       StorageKind == ConstantExpr::RSK_Int64);
   void *Mem = Context.Allocate(Size, alignof(ConstantExpr));
   ConstantExpr *Self = new (Mem) ConstantExpr(E, StorageKind);
+  Self->ConstantExprBits.IsCausedByImmediateInvocation =
+      IsCausedByImmediateInvocation;
   return Self;
 }
 
 ConstantExpr *ConstantExpr::Create(const ASTContext &Context, Expr *E,
                                    const APValue &Result) {
   ResultStorageKind StorageKind = getStorageKind(Result);
   ConstantExpr *Self = Create(Context, E, StorageKind);
   Self->SetResult(Result, Context);

clang/lib/AST/ExprConstant.cpp

static bool CheckLValueConstantExpression(EvalInfo &Info, SourceLocation Loc,
                                           QualType Type, const LValue &LVal,
                                           Expr::ConstExprUsage Usage,
                                           CheckedTemporaries &CheckedTemps) {
   bool IsReferenceType = Type->isReferenceType();
 
   APValue::LValueBase Base = LVal.getLValueBase();
   const SubobjectDesignator &Designator = LVal.getLValueDesignator();
 
+  if (auto *VD = LVal.getLValueBase().dyn_cast<const ValueDecl *>())
+    if (auto *FD = dyn_cast<FunctionDecl>(VD))
+      if (FD->isConsteval()) {
+        Info.FFDiag(Loc, diag::note_consteval_address_accessible)
+            << !Type->isAnyPointerType();
+        Info.Note(FD->getLocation(), diag::note_declared_at);
+        return false;
+      }
+
   // Check that the object is a global. Note that the fake 'this' object we
   // manufacture when checking potential constant expressions is conservatively
   // assumed to be global here.
   if (!IsGlobalLValue(Base)) {
     if (Info.getLangOpts().CPlusPlus11) {
       const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>();
       Info.FFDiag(Loc, diag::note_constexpr_non_global, 1)
         << IsReferenceType << !Designator.Entries.empty()
static bool CheckMemberPointerConstantExpression(EvalInfo &Info,
                                                  SourceLocation Loc,
                                                  QualType Type,
                                                  const APValue &Value,
                                                  Expr::ConstExprUsage Usage) {
   const ValueDecl *Member = Value.getMemberPointerDecl();
   const auto *FD = dyn_cast_or_null<CXXMethodDecl>(Member);
   if (!FD)
     return true;
+  if (FD->isConsteval()) {
+    Info.FFDiag(Loc, diag::note_consteval_address_accessible) << /*pointer*/ 0;
+    Info.Note(FD->getLocation(), diag::note_declared_at);
+    return false;
+  }
   return Usage == Expr::EvaluateForMangling || FD->isVirtual() ||
          !FD->hasAttr<DLLImportAttr>();
 }
 
 /// Check that this core constant expression is of literal type, and if not,
 /// produce an appropriate diagnostic.
 static bool CheckLiteralType(EvalInfo &Info, const Expr *E,
                              const LValue *This = nullptr) {
if (!EvaluateLValue(this, LV, Info) || !Info.discardCleanups() ||
                                      Expr::EvaluateForCodeGen, CheckedTemps))
     return false;
 
   LV.moveInto(Result.Val);
   return true;
 }
 
 bool Expr::EvaluateAsConstantExpr(EvalResult &Result, ConstExprUsage Usage,
-                                  const ASTContext &Ctx) const {
+                                  const ASTContext &Ctx, bool InPlace) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
 
   EvalInfo::EvaluationMode EM = EvalInfo::EM_ConstantExpression;
   EvalInfo Info(Ctx, Result, EM);
   Info.InConstantContext = true;
 
-  if (!::Evaluate(Result.Val, Info, this) || Result.HasSideEffects)
+  if (InPlace) {
+    LValue LVal;
+    if (!::EvaluateInPlace(Result.Val, Info, LVal, this) ||
+        Result.HasSideEffects)
+      return false;
+  } else if (!::Evaluate(Result.Val, Info, this) || Result.HasSideEffects)
     return false;
 
   if (!Info.discardCleanups())
     llvm_unreachable("Unhandled cleanup; missing full expression marker?");
 
   return CheckConstantExpression(Info, getExprLoc(), getStorageType(Ctx, this),
                                  Result.Val, Usage) &&
          CheckMemoryLeaks(Info);

clang/lib/Sema/Sema.cpp

: ExternalSource(nullptr), isMultiplexExternalSource(false),
       AccessCheckingSFINAE(false), InNonInstantiationSFINAEContext(false),
       NonInstantiationEntries(0), ArgumentPackSubstitutionIndex(-1),
       CurrentInstantiationScope(nullptr), DisableTypoCorrection(false),
       TyposCorrected(0), AnalysisWarnings(*this),
       ThreadSafetyDeclCache(nullptr), VarDataSharingAttributesStack(nullptr),
       CurScope(nullptr), Ident_super(nullptr), Ident___float128(nullptr) {
   TUScope = nullptr;
   isConstantEvaluatedOverride = false;
+  RebuildingImmediateInvocation = false;
 
   LoadedExternalKnownNamespaces = false;
   for (unsigned I = 0; I != NSAPI::NumNSNumberLiteralMethods; ++I)
     NSNumberLiteralMethods[I] = nullptr;
 
   if (getLangOpts().ObjC)
     NSAPIObj.reset(new NSAPI(Context));
 

clang/lib/Sema/SemaDecl.cpp

if (ConstexprSpecKind ConstexprKind =
             D.getDeclSpec().getConstexprSpecifier()) {
       // C++11 [dcl.constexpr]p2: constexpr functions and constexpr constructors
       // are implicitly inline.
       NewFD->setImplicitlyInline();
 
       // C++11 [dcl.constexpr]p3: functions declared constexpr are required to
       // be either constructors or to return a literal type. Therefore,
       // destructors cannot be declared constexpr.
-      if (isa<CXXDestructorDecl>(NewFD) && !getLangOpts().CPlusPlus2a) {
+      if (isa<CXXDestructorDecl>(NewFD) &&
+          (!getLangOpts().CPlusPlus2a || ConstexprKind == CSK_consteval)) {
         Diag(D.getDeclSpec().getConstexprSpecLoc(), diag::err_constexpr_dtor)
             << ConstexprKind;
+        NewFD->setConstexprKind(CSK_unspecified);
+      }
+      // C++20 [dcl.constexpr]p2: An allocation function, or a
+      // deallocation function shall not be declared with the consteval
+      // specifier.
+      if (ConstexprKind == CSK_consteval)
+        if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewFD))
+          if (MD->getOverloadedOperator() == OO_New ||
+              MD->getOverloadedOperator() == OO_Array_New ||
+              MD->getOverloadedOperator() == OO_Delete ||
+              MD->getOverloadedOperator() == OO_Array_Delete) {
+            Diag(D.getDeclSpec().getConstexprSpecLoc(),
+                 diag::err_invalid_consteval_decl_kind)
+                << MD;
+            NewFD->setConstexprKind(CSK_constexpr);
           }
     }
 
     // If __module_private__ was specified, mark the function accordingly.
     if (D.getDeclSpec().isModulePrivateSpecified()) {
       if (isFunctionTemplateSpecialization) {
         SourceLocation ModulePrivateLoc
           = D.getDeclSpec().getModulePrivateSpecLoc();
         Diag(ModulePrivateLoc, diag::err_module_private_specialization)
Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
   if (FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(D))
     FD = FunTmpl->getTemplatedDecl();
   else
     FD = cast<FunctionDecl>(D);
 
   // Do not push if it is a lambda because one is already pushed when building
   // the lambda in ActOnStartOfLambdaDefinition().
   if (!isLambdaCallOperator(FD))
-    PushExpressionEvaluationContext(ExprEvalContexts.back().Context);
+    PushExpressionEvaluationContext(
+        FD->isConsteval() ? ExpressionEvaluationContext::ConstantEvaluated
+                          : ExprEvalContexts.back().Context);
 
   // Check for defining attributes before the check for redefinition.
   if (const auto *Attr = FD->getAttr<AliasAttr>()) {
     Diag(Attr->getLocation(), diag::err_alias_is_definition) << FD << 0;
     FD->dropAttr<AliasAttr>();
     FD->setInvalidDecl();
   }
   if (const auto *Attr = FD->getAttr<IFuncAttr>()) {

clang/lib/Sema/SemaDeclCXX.cpp

static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
   //
   // Note that this rule is distinct from the "requirements for a constexpr
   // function", so is not checked in CheckValid mode.
   SmallVector<PartialDiagnosticAt, 8> Diags;
   if (Kind == Sema::CheckConstexprKind::Diagnose &&
       !Expr::isPotentialConstantExpr(Dcl, Diags)) {
     SemaRef.Diag(Dcl->getLocation(),
                  diag::ext_constexpr_function_never_constant_expr)
-        << isa<CXXConstructorDecl>(Dcl);
+        << isa<CXXConstructorDecl>(Dcl) << Dcl->isConsteval();
     for (size_t I = 0, N = Diags.size(); I != N; ++I)
       SemaRef.Diag(Diags[I].first, Diags[I].second);
     // Don't return false here: we allow this for compatibility in
     // system headers.
   }
 
   return true;
 }
if ((getLangOpts().CPlusPlus2a ||
     HadError = true;
   }
 
   if (First) {
     // C++2a [dcl.fct.def.default]p3:
     //   If a function is explicitly defaulted on its first declaration, it is
     //   implicitly considered to be constexpr if the implicit declaration
     //   would be.
-    MD->setConstexprKind(Constexpr ? CSK_constexpr : CSK_unspecified);
+    MD->setConstexprKind(
+        Constexpr ? (MD->isConsteval() ? CSK_consteval : CSK_constexpr)
+                  : CSK_unspecified);
 
     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;
if (getLangOpts().OpenCLCPlusPlus) {
     // space.
     EPI.TypeQuals.addAddressSpace(LangAS::opencl_generic);
   }
 
   auto QT = Context.getFunctionType(ResultTy, Args, EPI);
   SpecialMem->setType(QT);
 }
 
+static ConstexprSpecKind
+DetermineSpecialMemberConstexprKind(bool IsConstexprValid,
+                                    bool HasDefaultedConstevalSM) {
+  return IsConstexprValid
+             ? (HasDefaultedConstevalSM ? CSK_consteval : CSK_constexpr)
+             : CSK_unspecified;
+}
+
 CXXConstructorDecl *Sema::DeclareImplicitDefaultConstructor(
                                                      CXXRecordDecl *ClassDecl) {
   // C++ [class.ctor]p5:
   //   A default constructor for a class X is a constructor of class X
   //   that can be called without an argument. If there is no
   //   user-declared constructor for class X, a default constructor is
   //   implicitly declared. An implicitly-declared default constructor
   //   is an inline public member of its class.
   assert(ClassDecl->needsImplicitDefaultConstructor() &&
          "Should not build implicit default constructor!");
 
   DeclaringSpecialMember DSM(*this, ClassDecl, CXXDefaultConstructor);
   if (DSM.isAlreadyBeingDeclared())
     return nullptr;
 
   bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXDefaultConstructor,
                                                      false);
 
+  ConstexprSpecKind ConstexprKind = DetermineSpecialMemberConstexprKind(
+      Constexpr, ClassDecl->hasDefaultedConstevalDefaultCtor());
+
   // Create the actual constructor declaration.
-  CanQualType ClassType
-    = Context.getCanonicalType(Context.getTypeDeclType(ClassDecl));
+  CanQualType ClassType =
+      Context.getCanonicalType(Context.getTypeDeclType(ClassDecl));
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationName Name
     = Context.DeclarationNames.getCXXConstructorName(ClassType);
   DeclarationNameInfo NameInfo(Name, ClassLoc);
   CXXConstructorDecl *DefaultCon = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, /*Type*/ QualType(),
       /*TInfo=*/nullptr, ExplicitSpecifier(),
-      /*isInline=*/true, /*isImplicitlyDeclared=*/true,
-      Constexpr ? CSK_constexpr : CSK_unspecified);
+      /*isInline=*/true, /*isImplicitlyDeclared=*/true, ConstexprKind);
   DefaultCon->setAccess(AS_public);
   DefaultCon->setDefaulted();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXDefaultConstructor,
                                             DefaultCon,
                                             /* ConstRHS */ false,
                                             /* Diagnose */ false);
if (Const)
     ArgType = ArgType.withConst();
 
   ArgType = Context.getLValueReferenceType(ArgType);
 
   bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXCopyAssignment,
                                                      Const);
 
+  ConstexprSpecKind ConstexprKind = DetermineSpecialMemberConstexprKind(
+      Constexpr, ClassDecl->hasDefaultedConstevalCopyAssignment());
+
   //   An implicitly-declared copy assignment operator is an inline public
   //   member of its class.
   DeclarationName Name = Context.DeclarationNames.getCXXOperatorName(OO_Equal);
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationNameInfo NameInfo(Name, ClassLoc);
-  CXXMethodDecl *CopyAssignment = CXXMethodDecl::Create(
-      Context, ClassDecl, ClassLoc, NameInfo, QualType(),
+  CXXMethodDecl *CopyAssignment =
+      CXXMethodDecl::Create(Context, ClassDecl, ClassLoc, NameInfo, QualType(),
                             /*TInfo=*/nullptr, /*StorageClass=*/SC_None,
-      /*isInline=*/true, Constexpr ? CSK_constexpr : CSK_unspecified,
-      SourceLocation());
+                            /*isInline=*/true, ConstexprKind, SourceLocation());
   CopyAssignment->setAccess(AS_public);
   CopyAssignment->setDefaulted();
   CopyAssignment->setImplicit();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXCopyAssignment,
                                             CopyAssignment,
                                             /* ConstRHS */ Const,
if (Context.getLangOpts().OpenCLCPlusPlus)
     ArgType = Context.getAddrSpaceQualType(ArgType, LangAS::opencl_generic);
   QualType RetType = Context.getLValueReferenceType(ArgType);
   ArgType = Context.getRValueReferenceType(ArgType);
 
   bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXMoveAssignment,
                                                      false);
 
+  ConstexprSpecKind ConstexprKind = DetermineSpecialMemberConstexprKind(
+      Constexpr, ClassDecl->hasDefaultedConstevalMoveAssignment());
+
   //   An implicitly-declared move assignment operator is an inline public
   //   member of its class.
   DeclarationName Name = Context.DeclarationNames.getCXXOperatorName(OO_Equal);
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationNameInfo NameInfo(Name, ClassLoc);
-  CXXMethodDecl *MoveAssignment = CXXMethodDecl::Create(
-      Context, ClassDecl, ClassLoc, NameInfo, QualType(),
+  CXXMethodDecl *MoveAssignment =
+      CXXMethodDecl::Create(Context, ClassDecl, ClassLoc, NameInfo, QualType(),
                             /*TInfo=*/nullptr, /*StorageClass=*/SC_None,
-      /*isInline=*/true, Constexpr ? CSK_constexpr : CSK_unspecified,
-      SourceLocation());
+                            /*isInline=*/true, ConstexprKind, SourceLocation());
   MoveAssignment->setAccess(AS_public);
   MoveAssignment->setDefaulted();
   MoveAssignment->setImplicit();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXMoveAssignment,
                                             MoveAssignment,
                                             /* ConstRHS */ false,
if (Context.getLangOpts().OpenCLCPlusPlus)
     ArgType = Context.getAddrSpaceQualType(ArgType, LangAS::opencl_generic);
 
   ArgType = Context.getLValueReferenceType(ArgType);
 
   bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXCopyConstructor,
                                                      Const);
 
+  ConstexprSpecKind ConstexprKind = DetermineSpecialMemberConstexprKind(
+      Constexpr, ClassDecl->hasDefaultedConstevalCopyCtor());
+
   DeclarationName Name
     = Context.DeclarationNames.getCXXConstructorName(
                                            Context.getCanonicalType(ClassType));
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationNameInfo NameInfo(Name, ClassLoc);
 
   //   An implicitly-declared copy constructor is an inline public
   //   member of its class.
   CXXConstructorDecl *CopyConstructor = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, QualType(), /*TInfo=*/nullptr,
       ExplicitSpecifier(),
       /*isInline=*/true,
-      /*isImplicitlyDeclared=*/true,
-      Constexpr ? CSK_constexpr : CSK_unspecified);
+      /*isImplicitlyDeclared=*/true, ConstexprKind);
   CopyConstructor->setAccess(AS_public);
   CopyConstructor->setDefaulted();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXCopyConstructor,
                                             CopyConstructor,
                                             /* ConstRHS */ Const,
                                             /* Diagnose */ false);
CXXConstructorDecl *Sema::DeclareImplicitMoveConstructor(
   if (Context.getLangOpts().OpenCLCPlusPlus)
     ArgType = Context.getAddrSpaceQualType(ClassType, LangAS::opencl_generic);
   ArgType = Context.getRValueReferenceType(ArgType);
 
   bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXMoveConstructor,
                                                      false);
 
+  ConstexprSpecKind ConstexprKind = DetermineSpecialMemberConstexprKind(
+      Constexpr, ClassDecl->hasDefaultedConstevalMoveCtor());
+
   DeclarationName Name
     = Context.DeclarationNames.getCXXConstructorName(
                                            Context.getCanonicalType(ClassType));
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationNameInfo NameInfo(Name, ClassLoc);
 
   // C++11 [class.copy]p11:
   //   An implicitly-declared copy/move constructor is an inline public
   //   member of its class.
   CXXConstructorDecl *MoveConstructor = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, QualType(), /*TInfo=*/nullptr,
       ExplicitSpecifier(),
       /*isInline=*/true,
-      /*isImplicitlyDeclared=*/true,
-      Constexpr ? CSK_constexpr : CSK_unspecified);
+      /*isImplicitlyDeclared=*/true, ConstexprKind);
   MoveConstructor->setAccess(AS_public);
   MoveConstructor->setDefaulted();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXMoveConstructor,
                                             MoveConstructor,
                                             /* ConstRHS */ false,
                                             /* Diagnose */ false);

clang/lib/Sema/SemaExpr.cpp

 #include "clang/Sema/Overload.h"
 #include "clang/Sema/ParsedTemplate.h"
 #include "clang/Sema/Scope.h"
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/SemaFixItUtils.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/Template.h"
 #include "llvm/Support/ConvertUTF.h"
+#include "llvm/Support/SaveAndRestore.h"
 using namespace clang;
 using namespace sema;
 
 /// Determine whether the use of this declaration is valid, without
 /// emitting diagnostics.
 bool Sema::CanUseDecl(NamedDecl *D, bool TreatUnavailableAsInvalid) {
   // See if this is an auto-typed variable whose initializer we are parsing.
   if (ParsingInitForAutoVars.count(D))
if (!EnclosingFunctionCtx->isDependentContext()) {
     // ... since this is an implicit member reference, that might potentially
     // involve a 'this' capture, mark 'this' for potential capture in
     // enclosing lambdas.
     if (CurLSI->ImpCaptureStyle != CurLSI->ImpCap_None)
       CurLSI->addPotentialThisCapture(CallLoc);
   }
 }
 
 ExprResult Sema::ActOnCallExpr(Scope *Scope, Expr *Fn, SourceLocation LParenLoc,
                                MultiExprArg ArgExprs, SourceLocation RParenLoc,
                                Expr *ExecConfig) {
   ExprResult Call =
       BuildCallExpr(Scope, Fn, LParenLoc, ArgExprs, RParenLoc, ExecConfig);
   if (Call.isInvalid())
     return Call;
 
   // Diagnose uses of the C++20 "ADL-only template-id call" feature in earlier
   // language modes.
   if (auto *ULE = dyn_cast<UnresolvedLookupExpr>(Fn)) {
     if (ULE->hasExplicitTemplateArgs() &&
         ULE->decls_begin() == ULE->decls_end()) {
       Diag(Fn->getExprLoc(), getLangOpts().CPlusPlus2a
                                  ? diag::warn_cxx17_compat_adl_only_template_id
                                  : diag::ext_adl_only_template_id)
           << ULE->getName();
     }
   }
 
   return Call;
 }
 
 /// BuildCallExpr - Handle a call to Fn with the specified array of arguments.
 /// This provides the location of the left/right parens and a list of comma
 /// locations.
 ExprResult Sema::BuildCallExpr(Scope *Scope, Expr *Fn, SourceLocation LParenLoc,
                                MultiExprArg ArgExprs, SourceLocation RParenLoc,
                                Expr *ExecConfig, bool IsExecConfig) {
   // Since this might be a postfix expression, get rid of ParenListExprs.
   ExprResult Result = MaybeConvertParenListExprToParenExpr(Scope, Fn);
   if (Result.isInvalid()) return ExprError();
   Fn = Result.get();
 
   if (checkArgsForPlaceholders(*this, ArgExprs))
     return ExprError();
 
   if (getLangOpts().CPlusPlus) {
     // If this is a pseudo-destructor expression, build the call immediately.
     if (isa<CXXPseudoDestructorExpr>(Fn)) {
       if (!ArgExprs.empty()) {
         // Pseudo-destructor calls should not have any arguments.
         Diag(Fn->getBeginLoc(), diag::err_pseudo_dtor_call_with_args)
             << FixItHint::CreateRemoval(
                    SourceRange(ArgExprs.front()->getBeginLoc(),
                                ArgExprs.back()->getEndLoc()));
       }
 
       return CallExpr::Create(Context, Fn, /*Args=*/{}, Context.VoidTy,
                               VK_RValue, RParenLoc);
     }
     if (Fn->getType() == Context.PseudoObjectTy) {
       ExprResult result = CheckPlaceholderExpr(Fn);
       if (result.isInvalid()) return ExprError();
       Fn = result.get();
     }
 
     // Determine whether this is a dependent call inside a C++ template,
     // in which case we won't do any semantic analysis now.
     if (Fn->isTypeDependent() || Expr::hasAnyTypeDependentArguments(ArgExprs)) {
       if (ExecConfig) {
         return CUDAKernelCallExpr::Create(
             Context, Fn, cast<CallExpr>(ExecConfig), ArgExprs,
             Context.DependentTy, VK_RValue, RParenLoc);
       } else {
if (auto *DRE = dyn_cast<DeclRefExpr>(NakedFn)) {
     NDecl = DRE->getDecl();
 
     FunctionDecl *FDecl = dyn_cast<FunctionDecl>(NDecl);
     if (FDecl && FDecl->getBuiltinID()) {
       // Rewrite the function decl for this builtin by replacing parameters
       // with no explicit address space with the address space of the arguments
       // in ArgExprs.
       if ((FDecl =
                rewriteBuiltinFunctionDecl(this, Context, FDecl, ArgExprs))) {
         NDecl = FDecl;
         Fn = DeclRefExpr::Create(
             Context, FDecl->getQualifierLoc(), SourceLocation(), FDecl, false,
             SourceLocation(), FDecl->getType(), Fn->getValueKind(), FDecl,
             nullptr, DRE->isNonOdrUse());
       }
     }
   } else if (isa<MemberExpr>(NakedFn))
     NDecl = cast<MemberExpr>(NakedFn)->getMemberDecl();
ExprResult Sema::BuildResolvedCallExpr(Expr *Fn, NamedDecl *NDecl,
   } else if (NDecl) {
     if (CheckPointerCall(NDecl, TheCall, Proto))
       return ExprError();
   } else {
     if (CheckOtherCall(TheCall, Proto))
       return ExprError();
   }
 
-  return MaybeBindToTemporary(TheCall);
+  return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall), FDecl);
 }
 
 ExprResult
 Sema::ActOnCompoundLiteral(SourceLocation LParenLoc, ParsedType Ty,
                            SourceLocation RParenLoc, Expr *InitExpr) {
   assert(Ty && "ActOnCompoundLiteral(): missing type");
   assert(InitExpr && "ActOnCompoundLiteral(): missing expression");
 
void Sema::WarnOnPendingNoDerefs(ExpressionEvaluationContextRecord &Rec) {
   }
   Rec.PossibleDerefs.clear();
 }
 
 /// Check whether E, which is either a discarded-value expression or an
 /// unevaluated operand, is a simple-assignment to a volatlie-qualified lvalue,
 /// and if so, remove it from the list of volatile-qualified assignments that
 /// we are going to warn are deprecated.
 void Sema::CheckUnusedVolatileAssignment(Expr *E) {
   if (!E->getType().isVolatileQualified() || !getLangOpts().CPlusPlus2a)
     return;
 
   // Note: ignoring parens here is not justified by the standard rules, but
   // ignoring parentheses seems like a more reasonable approach, and this only
   // drives a deprecation warning so doesn't affect conformance.
   if (auto *BO = dyn_cast<BinaryOperator>(E->IgnoreParenImpCasts())) {
     if (BO->getOpcode() == BO_Assign) {
       auto &LHSs = ExprEvalContexts.back().VolatileAssignmentLHSs;
       LHSs.erase(std::remove(LHSs.begin(), LHSs.end(), BO->getLHS()),
                  LHSs.end());
     }
   }
 }
 
+ExprResult Sema::CheckForImmediateInvocation(ExprResult E, FunctionDecl *Decl) {
+  if (!E.isUsable() || !Decl || !Decl->isConsteval() || isConstantEvaluated() ||
+      RebuildingImmediateInvocation)
+    return E;
+
+  /// Opportunistically remove the callee from ReferencesToConsteval if we can.
+  /// It's OK if this fails; we'll also remove this in
+  /// HandleImmediateInvocations, but catching it here allows us to avoid
+  /// walking the AST looking for it in simple cases.
+  if (auto *Call = dyn_cast<CallExpr>(E.get()->IgnoreImplicit()))
+    if (auto *DeclRef =
+            dyn_cast<DeclRefExpr>(Call->getCallee()->IgnoreImplicit()))
+      ExprEvalContexts.back().ReferenceToConsteval.erase(DeclRef);
+
+  E = MaybeCreateExprWithCleanups(E);
+
+  ConstantExpr *Res = ConstantExpr::Create(
+      getASTContext(), E.get(),
+      ConstantExpr::getStorageKind(E.get()->getType().getTypePtr(),
+                                   getASTContext()),
+      /*IsCausedByImmediateInvocation*/ true);
+  ExprEvalContexts.back().ImmediateInvocationCandidates.emplace_back(Res, 0);
+  return Res;
+}
+
+static void
+EvaluateAndDiagnoseImmediateInvocation(Sema &SemaRef,
+                                       Sema::IICandidate Candidate) {
+  llvm::SmallVector<PartialDiagnosticAt, 8> Notes;
+  Expr::EvalResult Eval;
+  Eval.Diag = &Notes;
+  ConstantExpr *CE = Candidate.getPointer();
+  if (!CE->EvaluateAsConstantExpr(Eval, Expr::EvaluateForCodeGen,
+                                  SemaRef.getASTContext(), true)) {
+    Expr *InnerExpr = CE->getSubExpr()->IgnoreImplicit();
+    FunctionDecl *FD = nullptr;
+    if (auto *Call = dyn_cast<CallExpr>(InnerExpr))
+      FD = cast<FunctionDecl>(Call->getCalleeDecl());
+    else if (auto *Call = dyn_cast<CXXMemberCallExpr>(InnerExpr))
+      FD = Call->getMethodDecl();
+    else if (auto *Call = dyn_cast<CXXConstructExpr>(InnerExpr))
+      FD = Call->getConstructor();
+    else
+      llvm_unreachable("unhandeld decl kind");
+    assert(FD->isConsteval());
+    SmallString<256> Buffer;
+    llvm::raw_svector_ostream OS(Buffer);
+    FD->printQualifiedName(OS, SemaRef.getASTContext().getPrintingPolicy());
+    SemaRef.Diag(CE->getBeginLoc(), diag::err_invalid_consteval_call) << Buffer;
+    for (auto &Note : Notes)
+      SemaRef.Diag(Note.first, Note.second);
+    return;
+  }
+  CE->MoveIntoResult(Eval.Val, SemaRef.getASTContext());
+}
+
+static void RemoveNestedImmediateInvocation(
+    Sema &SemaRef, Sema::ExpressionEvaluationContextRecord &Rec,
+    llvm::SmallVector<Sema::IICandidate, 4>::reverse_iterator It) {
+  struct ComplexRemove : TreeTransform<ComplexRemove> {
+    using Base = TreeTransform<ComplexRemove>;
+    llvm::SmallPtrSetImpl<DeclRefExpr *> &DRSet;
+    llvm::SmallVector<Sema::IICandidate, 4> &IISet;
+    llvm::SmallVector<Sema::IICandidate, 4>::reverse_iterator CurrentII;
+    ComplexRemove(
+        Sema &SemaRef, llvm::SmallPtrSetImpl<DeclRefExpr *> &DR,
+        llvm::SmallVector<Sema::IICandidate, 4> &II,
+        llvm::SmallVector<Sema::IICandidate, 4>::reverse_iterator Current)
+        : Base(SemaRef), DRSet(DR), IISet(II), CurrentII(Current) {}
+    ExprResult TransformConstantExpr(ConstantExpr *E) {
+      if (!E->IsCausedByImmediateInvocation())
+        return Base::TransformConstantExpr(E);
+      auto It =
+          std::find_if(CurrentII, IISet.rend(), [E](Sema::IICandidate Elem) {
+            return Elem.getPointer() == E;
+          });
+      assert(It != IISet.rend() &&
+             "ConstantExpr marked IsCausedByImmediateInvocation should "
+             "be present");
+      It->setInt(1); // Mark as deleted
+      return Base::TransformExpr(E->getSubExpr());
+    }
+    /// Base::TransfromCXXOperatorCallExpr doesn't traverse the callee so
+    /// we need to remove its DeclRefExpr from the DRSet.
+    ExprResult TransformCXXOperatorCallExpr(CXXOperatorCallExpr *E) {
+      DRSet.erase(cast<DeclRefExpr>(E->getCallee()->IgnoreImplicit()));
+      return Base::TransformCXXOperatorCallExpr(E);
+    }
+    ExprResult TransformDeclRefExpr(DeclRefExpr *E) {
+      DRSet.erase(E);
+      return E;
+    }
+    bool AlwaysRebuild() { return false; }
+    bool ReplacingOriginal() { return true; }
+  } Transfromer(SemaRef, Rec.ReferenceToConsteval,
+                Rec.ImmediateInvocationCandidates, It);
+  ExprResult Res = Transfromer.TransformExpr(It->getPointer()->getSubExpr());
+  assert(Res.isUsable());
+  Res = SemaRef.MaybeCreateExprWithCleanups(Res);
+  It->getPointer()->setSubExpr(Res.get());
+}
+
+static void
+HandleImmediateInvocations(Sema &SemaRef,
+                           Sema::ExpressionEvaluationContextRecord &Rec) {
+  if ((Rec.ImmediateInvocationCandidates.size() == 0 &&
+       Rec.ReferenceToConsteval.size() == 0) ||
+      SemaRef.RebuildingImmediateInvocation)
+    return;
+
+  /// When we have more then 1 ImmediateInvocationCandidates we need to check
+  /// for nested ImmediateInvocationCandidates. when we have only 1 we only
+  /// need to remove ReferenceToConsteval in the immediate invocation.
+  if (Rec.ImmediateInvocationCandidates.size() > 1) {
+
+    /// Prevent sema calls during the tree transform from adding pointers that
+    /// are already in the sets.
+    llvm::SaveAndRestore<bool> DisableIITracking(
+        SemaRef.RebuildingImmediateInvocation, true);
+
+    /// Prevent diagnostic during tree transfrom as they are duplicates
+    Sema::TentativeAnalysisScope DisableDiag(SemaRef);
+
+    for (auto It = Rec.ImmediateInvocationCandidates.rbegin();
+         It != Rec.ImmediateInvocationCandidates.rend(); It++)
+      if (!It->getInt())
+        RemoveNestedImmediateInvocation(SemaRef, Rec, It);
+  } else if (Rec.ImmediateInvocationCandidates.size() == 1 &&
+             Rec.ReferenceToConsteval.size()) {
+    struct SimpleRemove : RecursiveASTVisitor<SimpleRemove> {
+      llvm::SmallPtrSetImpl<DeclRefExpr *> &DRSet;
+      SimpleRemove(llvm::SmallPtrSetImpl<DeclRefExpr *> &S) : DRSet(S) {}
+      bool VisitDeclRefExpr(DeclRefExpr *E) {
+        DRSet.erase(E);
+        return DRSet.size();
+      }
+    } Visitor(Rec.ReferenceToConsteval);
+    Visitor.TraverseStmt(
+        Rec.ImmediateInvocationCandidates.front().getPointer()->getSubExpr());
+  }
+  for (auto CE : Rec.ImmediateInvocationCandidates)
+    if (!CE.getInt())
+      EvaluateAndDiagnoseImmediateInvocation(SemaRef, CE);
+  for (auto DR : Rec.ReferenceToConsteval) {
+    auto *FD = cast<FunctionDecl>(DR->getDecl());
+    SemaRef.Diag(DR->getBeginLoc(), diag::err_invalid_consteval_take_address)
+        << FD;
+    SemaRef.Diag(FD->getLocation(), diag::note_declared_at);
+  }
+}
+
 void Sema::PopExpressionEvaluationContext() {
   ExpressionEvaluationContextRecord& Rec = ExprEvalContexts.back();
   unsigned NumTypos = Rec.NumTypos;
 
   if (!Rec.Lambdas.empty()) {
     using ExpressionKind = ExpressionEvaluationContextRecord::ExpressionKind;
     if (Rec.ExprContext == ExpressionKind::EK_TemplateArgument || Rec.isUnevaluated() ||
         (Rec.isConstantEvaluated() && !getLangOpts().CPlusPlus17)) {
if (Rec.ExprContext == ExpressionKind::EK_TemplateArgument || Rec.isUnevaluated() ||
         llvm_unreachable("Couldn't infer lambda error message.");
 
       for (const auto *L : Rec.Lambdas)
         Diag(L->getBeginLoc(), D);
     }
   }
 
   WarnOnPendingNoDerefs(Rec);
+  HandleImmediateInvocations(*this, Rec);
 
   // Warn on any volatile-qualified simple-assignments that are not discarded-
   // value expressions nor unevaluated operands (those cases get removed from
   // this list by CheckUnusedVolatileAssignment).
   for (auto *BO : Rec.VolatileAssignmentLHSs)
     Diag(BO->getBeginLoc(), diag::warn_deprecated_simple_assign_volatile)
         << BO->getType();
 
void Sema::MarkDeclRefReferenced(DeclRefExpr *E, const Expr *Base) {
   // TODO: update this with DR# once a defect report is filed.
   // C++11 defect. The address of a pure member should not be an ODR use, even
   // if it's a qualified reference.
   bool OdrUse = true;
   if (const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(E->getDecl()))
     if (Method->isVirtual() &&
         !Method->getDevirtualizedMethod(Base, getLangOpts().AppleKext))
       OdrUse = false;
+
+  if (auto *FD = dyn_cast<FunctionDecl>(E->getDecl()))
+    if (!isConstantEvaluated() && FD->isConsteval() &&
+        !RebuildingImmediateInvocation)
+      ExprEvalContexts.back().ReferenceToConsteval.insert(E);
   MarkExprReferenced(*this, E->getLocation(), E->getDecl(), E, OdrUse);
 }
 
 /// Perform reference-marking and odr-use handling for a MemberExpr.
 void Sema::MarkMemberReferenced(MemberExpr *E) {
   // C++11 [basic.def.odr]p2:
   //   A non-overloaded function whose name appears as a potentially-evaluated
   //   expression or a member of a set of candidate functions, if selected by

clang/lib/Sema/SemaLambda.cpp

void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
   LSI->ContainsUnexpandedParameterPack |= ContainsUnexpandedParameterPack;
 
   // Add lambda parameters into scope.
   addLambdaParameters(Intro.Captures, Method, CurScope);
 
   // Enter a new evaluation context to insulate the lambda from any
   // cleanups from the enclosing full-expression.
   PushExpressionEvaluationContext(
-      ExpressionEvaluationContext::PotentiallyEvaluated);
+      LSI->CallOperator->isConsteval()
+          ? ExpressionEvaluationContext::ConstantEvaluated
+          : ExpressionEvaluationContext::PotentiallyEvaluated);
 }
 
 void Sema::ActOnLambdaError(SourceLocation StartLoc, Scope *CurScope,
                             bool IsInstantiation) {
   LambdaScopeInfo *LSI = cast<LambdaScopeInfo>(FunctionScopes.back());
 
   // Leave the expression-evaluation context.
   DiscardCleanupsInEvaluationContext();

clang/lib/Sema/SemaOverload.cpp

 /// \param InOverloadResolution - For the purposes of emitting a diagnostic, are
 ///   we in overload resolution?
 /// \param Loc - The location of the statement we're complaining about. Ignored
 ///   if we're not complaining, or if we're in overload resolution.
 static bool checkAddressOfFunctionIsAvailable(Sema &S, const FunctionDecl *FD,
                                               bool Complain,
                                               bool InOverloadResolution,
                                               SourceLocation Loc) {
   if (!isFunctionAlwaysEnabled(S.Context, FD)) {
     if (Complain) {
       if (InOverloadResolution)
         S.Diag(FD->getBeginLoc(),
                diag::note_addrof_ovl_candidate_disabled_by_enable_if_attr);
       else
         S.Diag(Loc, diag::err_addrof_function_disabled_by_enable_if_attr) << FD;
     }
     return false;
   }
 
   auto I = llvm::find_if(FD->parameters(), [](const ParmVarDecl *P) {
     return P->hasAttr<PassObjectSizeAttr>();
   });
   if (I == FD->param_end())
     return true;
 
   if (Complain) {
     // Add one to ParamNo because it's user-facing
     unsigned ParamNo = std::distance(FD->param_begin(), I) + 1;
if (FnDecl) {
           FPOptions(), Best->IsADLCandidate);
 
       if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall, FnDecl))
         return ExprError();
 
       if (CheckFunctionCall(FnDecl, TheCall,
                             FnDecl->getType()->castAs<FunctionProtoType>()))
         return ExprError();
-
-      return MaybeBindToTemporary(TheCall);
+      return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall), FnDecl);
     } else {
       // We matched a built-in operator. Convert the arguments, then
       // break out so that we will build the appropriate built-in
       // operator node.
       ExprResult InputRes = PerformImplicitConversion(
           Input, Best->BuiltinParamTypes[0], Best->Conversions[0], AA_Passing,
           CCK_ForBuiltinOverloadedOp);
       if (InputRes.isInvalid())
case OR_Success: {
         } else {
           assert(ChosenOp == Op && "unexpected operator name");
         }
 
         // Make a note in the AST if we did any rewriting.
         if (Best->RewriteKind != CRK_None)
           R = new (Context) CXXRewrittenBinaryOperator(R.get(), IsReversed);
 
-        return R;
+        return CheckForImmediateInvocation(R, FnDecl);
       } else {
         // We matched a built-in operator. Convert the arguments, then
         // break out so that we will build the appropriate built-in
         // operator node.
         ExprResult ArgsRes0 = PerformImplicitConversion(
             Args[0], Best->BuiltinParamTypes[0], Best->Conversions[0],
             AA_Passing, CCK_ForBuiltinOverloadedOp);
         if (ArgsRes0.isInvalid())
if (CXXDestructorDecl *DD =
           dyn_cast<CXXDestructorDecl>(TheCall->getMethodDecl())) {
     // 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 MaybeBindToTemporary(TheCall);
+  return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall),
+                                     TheCall->getMethodDecl());
 }
 
 /// 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,
CXXOperatorCallExpr *TheCall =
                                   ResultTy, VK, RParenLoc, FPOptions());
 
   if (CheckCallReturnType(Method->getReturnType(), LParenLoc, TheCall, Method))
     return true;
 
   if (CheckFunctionCall(Method, TheCall, Proto))
     return true;
 
-  return MaybeBindToTemporary(TheCall);
+  return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall), Method);
 }
 
 /// BuildOverloadedArrowExpr - Build a call to an overloaded @c operator->
 ///  (if one exists), where @c Base is an expression of class type and
 /// @c Member is the name of the member we're trying to find.
 ExprResult
 Sema::BuildOverloadedArrowExpr(Scope *S, Expr *Base, SourceLocation OpLoc,
                                bool *NoArrowOperatorFound) {
UserDefinedLiteral *UDL = UserDefinedLiteral::Create(
       VK, LitEndLoc, UDSuffixLoc);
 
   if (CheckCallReturnType(FD->getReturnType(), UDSuffixLoc, UDL, FD))
     return ExprError();
 
   if (CheckFunctionCall(FD, UDL, nullptr))
     return ExprError();
 
-  return MaybeBindToTemporary(UDL);
+  return CheckForImmediateInvocation(MaybeBindToTemporary(UDL), FD);
 }
 
 /// Build a call to 'begin' or 'end' for a C++11 for-range statement. If the
 /// given LookupResult is non-empty, it is assumed to describe a member which
 /// will be invoked. Otherwise, the function will be found via argument
 /// dependent lookup.
 /// CallExpr is set to a valid expression and FRS_Success returned on success,
 /// otherwise CallExpr is set to ExprError() and some non-success value

clang/lib/Sema/TreeTransform.h

 template<typename Derived>
 ExprResult TreeTransform<Derived>::TransformInitializer(Expr *Init,
                                                         bool NotCopyInit) {
   // Initializers are instantiated like expressions, except that various outer
   // layers are stripped.
   if (!Init)
     return Init;
 
-  if (auto *FE = dyn_cast<FullExpr>(Init))
-    Init = FE->getSubExpr();
-
   if (auto *AIL = dyn_cast<ArrayInitLoopExpr>(Init))
     Init = AIL->getCommonExpr();
 
   if (MaterializeTemporaryExpr *MTE = dyn_cast<MaterializeTemporaryExpr>(Init))
     Init = MTE->GetTemporaryExpr();
 
   while (CXXBindTemporaryExpr *Binder = dyn_cast<CXXBindTemporaryExpr>(Init))
     Init = Binder->getSubExpr();

clang/test/SemaCXX/cxx2a-consteval.cpp

-// RUN: %clang_cc1 -std=c++2a -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++2a -fsyntax-only -Wno-unused-value %s -verify
 
 namespace basic_sema {
 
 consteval int f1(int i) {
   return i;
 }
 
 consteval constexpr int f2(int i) { 
   //expected-error@-1 {{cannot combine}}
   return i;
 }
 
 constexpr auto l_eval = [](int i) consteval {
+// expected-note@-1+ {{declared here}}
 
   return i;
 };
 
 constexpr consteval int f3(int i) {
   //expected-error@-1 {{cannot combine}}
   return i;
 }
 
 struct A {
   consteval int f1(int i) const {
+// expected-note@-1 {{declared here}}
     return i;
   }
   consteval A(int i);
   consteval A() = default;
-  consteval ~A() = default;
+  consteval ~A() = default; // expected-error {{destructor cannot be declared consteval}}
 };
 
 consteval struct B {}; // expected-error {{struct cannot be marked consteval}}
 
 consteval typedef B b; // expected-error {{typedef cannot be consteval}}
 
 consteval int redecl() {return 0;} // expected-note {{previous declaration is here}}
 constexpr int redecl() {return 0;} // expected-error {{constexpr declaration of 'redecl' follows consteval declaration}}
 
 consteval int i = 0; // expected-error {{consteval can only be used in function declarations}}
 
 consteval int; // expected-error {{consteval can only be used in function declarations}}
 
 consteval int f1() {} // expected-error {{no return statement in consteval function}}
 
 struct C {
   C() {}
   ~C() {}
 };
 
 struct D {
   C c;
   consteval D() = default; // expected-error {{cannot be consteval}}
-  consteval ~D() = default; // expected-error {{cannot be consteval}}
+  consteval ~D() = default; // expected-error {{destructor cannot be declared consteval}}
 };
 
-struct E : C { // expected-note {{here}}
-  consteval ~E() {} // expected-error {{cannot be declared consteval because base class 'basic_sema::C' does not have a constexpr destructor}}
+struct E : C {
+  consteval ~E() {} // expected-error {{cannot be declared consteval}}
 };
 }
 
 consteval int main() { // expected-error {{'main' is not allowed to be declared consteval}}
   return 0;
 }
+
+consteval int f_eval(int i) {
+// expected-note@-1+ {{declared here}}
+  return i;
+}
+
+namespace taking_address {
+
+using func_type = int(int);
+
+func_type* p1 = (&f_eval);
+// expected-error@-1 {{take address}}
+func_type* p7 = __builtin_addressof(f_eval);
+// expected-error@-1 {{take address}}
+
+auto p = f_eval;
+// expected-error@-1 {{take address}}
+
+auto m1 = &basic_sema::A::f1;
+// expected-error@-1 {{take address}}
+auto l1 = &decltype(basic_sema::l_eval)::operator();
+// expected-error@-1 {{take address}}
+
+consteval int f(int i) {
+// expected-note@-1+ {{declared here}}
+  return i;
+}
+
+auto ptr = &f;
+// expected-error@-1 {{take address}}
+
+auto f1() {
+  return &f;
+// expected-error@-1 {{take address}}
+}
+
+}
+
+namespace invalid_function {
+using size_t = unsigned long;
+struct A {
+  consteval void *operator new(size_t count);
+  // expected-error@-1 {{'operator new' cannot be declared consteval}}
+  consteval void *operator new[](size_t count);
+  // expected-error@-1 {{'operator new[]' cannot be declared consteval}}
+  consteval void operator delete(void* ptr);
+  // expected-error@-1 {{'operator delete' cannot be declared consteval}}
+  consteval void operator delete[](void* ptr);
+  // expected-error@-1 {{'operator delete[]' cannot be declared consteval}}
+  consteval ~A() {}
+  // expected-error@-1 {{destructor cannot be declared consteval}}
+};
+
+}
+
+namespace nested {
+consteval int f() {
+  return 0;
+}
+
+consteval int f1(...) {
+  return 1;
+}
+
+enum E {};
+
+using T = int(&)();
+
+consteval auto operator+ (E, int(*a)()) {
+  return 0;
+}
+
+void d() {
+  auto i = f1(E() + &f);
+}
+
+auto l0 = [](auto) consteval {
+  return 0;
+};
+
+int i0 = l0(&f1);
+
+int i1 = f1(l0(4));
+
+int i2 = f1(&f1, &f1, &f1, &f1, &f1, &f1, &f1);
+
+int i3 = f1(f1(f1(&f1, &f1), f1(&f1, &f1), f1(f1(&f1, &f1), &f1)));
+
+}
+
+namespace user_defined_literal {
+
+consteval int operator"" _test(unsigned long long i) {
+// expected-note@-1+ {{declared here}}
+  return 0;
+}
+
+int i = 0_test;
+
+auto ptr = &operator"" _test;
+// expected-error@-1 {{take address}}
+
+consteval auto operator"" _test1(unsigned long long i) {
+  return &f_eval;
+}
+
+auto i1 = 0_test1; // expected-error {{could not be evaluated}}
+// expected-note@-1 {{is not a constant expression}}
+
+}
+
+namespace return_address {
+
+consteval int f() {
+// expected-note@-1 {{declared here}}
+  return 0;
+}
+
+consteval int(*ret1(int i))() {
+  return &f;
+}
+
+auto ptr = ret1(0);
+// expected-error@-1 {{could not be evaluated}}
+// expected-note@-2 {{pointer to a consteval}}
+
+struct A {
+  consteval int f(int) {
+    // expected-note@-1+ {{declared here}}
+    return 0;    
+  }
+};
+
+using mem_ptr_type = int (A::*)(int);
+
+template<mem_ptr_type ptr>
+struct C {};
+
+C<&A::f> c;
+// expected-error@-1 {{is not a constant expression}}
+// expected-note@-2 {{pointer to a consteval}}
+
+consteval mem_ptr_type ret2() {
+  return &A::f;
+}
+
+C<ret2()> c1;
+// expected-error@-1 {{is not a constant expression}}
+// expected-note@-2 {{pointer to a consteval}}
+
+}
+
+namespace context {
+
+int g_i;
+// expected-note@-1 {{declared here}}
+
+consteval int f(int) {
+  return 0;
+}
+
+constexpr int c_i = 0;
+
+int t1 = f(g_i);
+// expected-error@-1 {{could not be evaluated}}
+// expected-note@-2 {{read of non-const variable}}
+int t3 = f(c_i);
+
+constexpr int f_c(int i) {
+// expected-note@-1 {{declared here}}
+  int t = f(i);
+// expected-error@-1 {{could not be evaluated}}
+// expected-note@-2 {{read of non-const variable}}
+  return f(0);  
+}
+
+consteval int f_eval(int i) {
+  return f(i);
+}
+
+auto l0 = [](int i) consteval {
+  return f(i);
+};
+
+auto l1 = [](int i) constexpr {
+// expected-note@-1 {{declared here}}
+  int t = f(i);
+// expected-error@-1 {{could not be evaluated}}
+// expected-note@-2 {{read of non-const variable}}
+  return f(0);  
+};
+
+}
+
+namespace temporaries {
+
+struct A {
+  consteval int ret_i() const { return 0; }
+  consteval A ret_a() const { return A{}; }
+  constexpr ~A() { }
+};
+
+consteval int by_value_a(A a) { return a.ret_i(); }
+
+consteval int const_a_ref(const A &a) {
+  return a.ret_i();
+}
+
+consteval int rvalue_ref(const A &&a) {
+  return a.ret_i();
+}
+
+consteval const A &to_lvalue_ref(const A &&a) {
+  return a;
+}
+
+void test() {
+  constexpr A a {};
+  { int k = A().ret_i(); }
+  { A k = A().ret_a(); }
+  { A k = to_lvalue_ref(A()); }// expected-error {{could not be evaluated}}
+  // expected-note@-1 {{is not a constant expression}} expected-note@-1 {{temporary created here}}
+  { A k = to_lvalue_ref(A().ret_a()); } // expected-error {{could not be evaluated}}
+  // expected-note@-1 {{is not a constant expression}} expected-note@-1 {{temporary created here}}
+  { int k = A().ret_a().ret_i(); }
+  { int k = by_value_a(A()); }
+  { int k = const_a_ref(A()); }
+  { int k = const_a_ref(a); }
+  { int k = rvalue_ref(A()); }
+  { int k = rvalue_ref(static_cast<const A&&>(a)); }
+  { int k = const_a_ref(A().ret_a()); }
+  { int k = const_a_ref(to_lvalue_ref(A().ret_a())); }
+  { int k = const_a_ref(to_lvalue_ref(static_cast<const A&&>(a))); }
+  { int k = by_value_a(A().ret_a()); }
+  { int k = by_value_a(to_lvalue_ref(static_cast<const A&&>(a))); }
+  { int k = (A().ret_a(), A().ret_i()); }
+  { int k = (const_a_ref(A().ret_a()), A().ret_i()); }//
+}
+
+}
+
+namespace alloc {
+
+struct A {
+  int* p = new int(42);
+  // expected-note@-1+ {{heap allocation performed here}}
+  consteval int ret_i() const { return p ? *p : 0; }
+  consteval A ret_a() const { return A{}; }
+  constexpr ~A() { delete p; }
+};
+
+consteval int by_value_a(A a) { return a.ret_i(); }
+
+consteval int const_a_ref(const A &a) {
+  return a.ret_i();
+}
+
+consteval int rvalue_ref(const A &&a) {
+  return a.ret_i();
+}
+
+consteval const A &to_lvalue_ref(const A &&a) {
+  return a;
+}
+
+void test() {
+  constexpr A a{ nullptr };
+  { int k = A().ret_i(); }
+  { A k = A().ret_a(); } // expected-error {{could not be evaluated}}
+  // expected-note@-1 {{is not a constant expression}}
+  { A k = to_lvalue_ref(A()); } // expected-error {{could not be evaluated}}
+  // expected-note@-1 {{is not a constant expression}} expected-note@-1 {{temporary created here}}
+  { A k = to_lvalue_ref(A().ret_a()); } // expected-error {{could not be evaluated}}
+  // expected-note@-1 {{is not a constant expression}} expected-note@-1 {{temporary created here}}
+  { int k = A().ret_a().ret_i(); }
+  { int k = by_value_a(A()); }
+  { int k = const_a_ref(A()); }
+  { int k = const_a_ref(a); }
+  { int k = rvalue_ref(A()); }
+  { int k = rvalue_ref(static_cast<const A&&>(a)); }
+  { int k = const_a_ref(A().ret_a()); }
+  { int k = const_a_ref(to_lvalue_ref(A().ret_a())); }
+  { int k = const_a_ref(to_lvalue_ref(static_cast<const A&&>(a))); }
+  { int k = by_value_a(A().ret_a()); }
+  { int k = by_value_a(to_lvalue_ref(static_cast<const A&&>(a))); }
+  { int k = (A().ret_a(), A().ret_i()); }// expected-error {{could not be evaluated}}
+  // expected-note@-1 {{is not a constant expression}}
+  { int k = (const_a_ref(A().ret_a()), A().ret_i()); }
+}
+
+}
 No newline at end of file