[clang] Implement Change scope of lambda trailing-return-type

[cor3ntin]

Summary

Implement P2036R3.

Captured variables by copy (explicitely or not), are deduced
correctly at the point we know whether the lambda is mutable,
and ill-formed before that.

Up until now, the entire lambda declaration up to the start of the body would be parsed in the parent scope, such that capture would not be available to look up.

The scoping is changed to have an outer lambda scope, followed by the lambda prototype and body.

The lambda scope is necessary because there may be a template scope between the start of the lambda (to which we want to attach the captured variable) and the prototype scope.

We also need to introduce a declaration context to attach the captured variable to (and several parts of clang assume captures are handled from the call operator context), before we know the type of the call operator.

The order of operations is as follow:

• Parse the init capture in the lambda's parent scope

• Introduce a lambda scope

• Create the lambda class and call operator

• Add the init captures to the call operator context and the lambda scope. But the variables are not capured yet (because we don't know their type).

Instead, explicit captures are stored in a temporary map that conserves the order of capture (for the purpose of having a stable order in the ast dumps).

• A flag is set on LambdaScopeInfo to indicate that we have not yet injected the captures.

• The parameters are parsed (in the parent context, as lambda mangling recurses in the parent context, we couldn't mangle a lambda that is attached to the context of a lambda whose type is not yet known).

• The lambda qualifiers are parsed, at this point We can switch (for the second time) inside the lambda context, unset the flag indicating that we have not parsed the lambda qualifiers,

record the lambda is mutable and capture the explicit variables.

• We can parse the rest of the lambda type, transform the lambda and call operator's types and also transform the call operator to a template function decl where necessary.

At this point, both captures and parameters can be injected in the body's scope. When trying to capture an implicit variable, if we are before the qualifiers of a lambda, we need to remember that the variables are still in the parent's context (rather than in the call operator's).

[erichkeane]

Attn: @Fznamznon

[ChuanqiXu]

I've just skimmed over the patch and it is a little bit hard to understand for people who don't have a strong background. Is it possible to split this one into several parts?

clang/lib/Sema/Scope.cpp

72

This looks like:

clang/lib/Sema/SemaExpr.cpp

18399

If this is only used in current file, we should mark it as static or put in anonymous namespace. And I think this lacks a comment

[cor3ntin]

• Add missing static

[cor3ntin]

clang/lib/Sema/SemaExpr.cpp

18399

Good catch, thanks

[cor3ntin]

In D119136#3418585, @ChuanqiXu wrote:

I've just skimmed over the patch and it is a little bit hard to understand for people who don't have a strong background. Is it possible to split this one into several parts?

Unfortunately, I don't thing that would be very practical. The patch is not small but what it changes is rather small, and if we tried to split it the intermediate patches would be inconsistent and untested

[ChuanqiXu]

In D119136#3421531, @cor3ntin wrote:

In D119136#3418585, @ChuanqiXu wrote:

I've just skimmed over the patch and it is a little bit hard to understand for people who don't have a strong background. Is it possible to split this one into several parts?

Unfortunately, I don't thing that would be very practical. The patch is not small but what it changes is rather small, and if we tried to split it the intermediate patches would be inconsistent and untested

Yeah, but I indeed found some NFC changes... I think it might be helpful to split it as NFC parts and other parts.

clang/include/clang/AST/DeclCXX.h

1816–1821

It looks like this one is not used, is it?

clang/include/clang/Sema/ScopeInfo.h

840

I suggest a comment to describe this one.

842

I think the name Before* is a little bit confusing. From the context, it would be set to true in ActOnLambdaIntroducerand be set to false in ActOnLambdaClosureQualifiers. So it looks like something IsInRangeOf.... The name before implies that it should be true by default and be false after some point...

I am not sure if my point is clear...

clang/include/clang/Sema/Sema.h

6855

The comment looks like a draft.

clang/lib/Parse/ParseExprCXX.cpp

1388

Unintended change?

1439

It looks a little bit. It looks like if the code is () and HasParameters would still be true.

clang/lib/Sema/Scope.cpp

72

This isn't addressed. I mean the flags would always be true if the last clause get evaluated.

clang/lib/Sema/SemaExpr.cpp

18404

Could we declare it as a boolean.

clang/lib/Sema/SemaLambda.cpp

358–364

The comment says Start while the function name starts with finish. It looks odd...

512–514

How about:

892–980

Add a static qualifier.

[cor3ntin]

Address ChuanqiXu's feedback

[cor3ntin]

Formatting.

clang/include/clang/AST/DeclCXX.h

1816–1821

Thanks., you are right, i wrote that before realizing i did not need it.

clang/include/clang/Sema/ScopeInfo.h

842

I struggle to find a better name, but I added a comment. Does that help?

clang/include/clang/Sema/Sema.h

6855

Thanks for noticing that, I added a longer comment

clang/lib/Parse/ParseExprCXX.cpp

1439

Which is what we want to track. But it was not clear, I renamed the variable to HasParentheses

clang/lib/Sema/Scope.cpp

72

I'm not sure I understand your point. We are checking that it's a function scope that is not a lambda scope.
They are probably other ways to write that but they aren't clearer

clang/lib/Sema/SemaLambda.cpp

358–364

The comment appertain to the startLambdaDefinition below, i fixed it.
I also renamed finishLambdaMethodType to buildTypeForLambdaCallOperator` which is clearer`

[ChuanqiXu]

clang/include/clang/Sema/ScopeInfo.h

842

I'm not good at naming too... I am wondering a name with words like range could be fine.

843

I hope the comment explains the meaning of unsigned here. If it means index, I think SmallVector would make code simpler.

clang/lib/Sema/Scope.cpp

72

I mean the suggested change is equal to the original one and it is shorter. I don't think it would lost any information since they are all in the same line.

clang/lib/Sema/SemaExpr.cpp

18411

clang/lib/Sema/SemaLambda.cpp

892

I guess it is needed to explain the meaning of Unsafe

894–897

1076–1078

I suggest to add a assertion for Var to check it is not null

1127–1133

1182–1184

If DelayedCaptures is SmallVector, we could write:

---

I observed there a lot continue in the loop. So we could hoist the definition of Var to the start of the loop. And we could fill DelayedCaptures by RAII.

1193

Do you think it is better to replace I with std:distance(Intro.Captures.begin(), C)? I prefer it since we could reduce the scope of I in the style.

1194

It looks like we could define PrevCaptureLoc in the loop.

1214

We prefer C++20 than C++2a now.

1311–1312

[cor3ntin]

Address ChuanqiXu's comment

[cor3ntin]

• Address ChuanqiXu's comments
• Add an entry in ReleaseNotes

clang/lib/Sema/Scope.cpp

72

LambdaScope is a constant here
Both FunctionPrototypeScope and FunctionPrototypeScope | LambdaScope are valid values, and we only want the condition to be true when flags & FunctionPrototypeScope is true and flags & LambdaScope is false.
Maybe we misunderstand each other

clang/lib/Sema/SemaLambda.cpp

892

Agreed, I added a comment

1194

I think that would be worse, as this is the location of the previous capture.
it would looks something like.
SourceLocation PrevCaptureLoc = C == Intro.Captures.begin() ? {} : (C-1)->Loc

[cor3ntin]

Add dependent tests

[aaron.ballman]

Some drive by comments, I plan to do a more thorough review when I have the chance.

clang/docs/ReleaseNotes.rst

97 ↗

(On Diff #421461)

Unintended whitespace change?

217 ↗

(On Diff #421461)

clang/include/clang/Basic/DiagnosticSemaKinds.td

7702

clang/include/clang/Sema/Scope.h

47–51

Unintended whitespace changes?

145–147

clang/include/clang/Sema/ScopeInfo.h

840

Can you re-flow the comments to 80 cols?

clang/lib/Parse/ParseExprCXX.cpp

1259

This call surprises me here -- I would expect to see this called from ParseLambdaIntroducer() so that anyone who parses a lambda introducer gets the action.

clang/lib/Sema/Scope.cpp

71–73

Should re-flow comments with the above line.

clang/lib/Sema/SemaDecl.cpp

14352

Unintended whitespace change?

[cor3ntin]

Address Aaron's comments.

[cor3ntin]

Missed a comment in Scope.h

[ChuanqiXu]

I've tried my best to review this one and it looks good to me basically. Since @aaron.ballman plans to review this one. I think it'd better to wait for this respond.

clang/lib/Sema/Scope.cpp

72

Oh, sorry, I misread the code. The code now should be correct.

clang/lib/Sema/SemaLambda.cpp

1127–1135

I feel like my suggested change is simpler.

[aaron.ballman]

clang/include/clang/Basic/DiagnosticSemaKinds.td

7702

It looks like this comment may have been missed.

clang/lib/Parse/ParseExprCXX.cpp

1259

I'd still like to hear why this is being called here instead of when parsing the lambda introducer itself.

[cor3ntin]

clang/include/clang/Basic/DiagnosticSemaKinds.td

7702

That doesn't work

[a]<typename x = decltype(a)>(decltype(a) )
                      ^ nope            ^ double nope

clang/lib/Parse/ParseExprCXX.cpp

1259

This is preexisting that sema for the introducer was done in this function.
The only thing that change is that we are doing it before parsiong the parameter instead of after, in the same action.
And we need to do it here, because of the scope introduction

[aaron.ballman]

clang/include/clang/Basic/DiagnosticSemaKinds.td

7702

Thank you for this! I think we should go with %select to name where it cannot be used. e.g., cannot be used in a lambda %select{parameter list|template parameter list|whatever}1

clang/test/SemaCXX/lambda-capture-type-deduction.cpp

62

Can you also add test coverage for when the requires comes *before* the parens as in: [x] requires(is_same<const int &, decltype((x))>)(){}();?

I think you should also look at the grammar for lambda-expression to make sure we're not missing other coverage (use in attributes also appears to be untested).

[aaron.ballman]

clang/lib/Parse/ParseExprCXX.cpp

1259

This is preexisting that sema for the introducer was done in this function. The only thing that change is that we are doing it before parsiong the parameter instead of after, in the same action. And we need to do it here, because of the scope introduction

Ah, okay, that's a good reason not to muck with it in this patch. I still think that design is a bit suspect, but it's not a new issue (thank you for pointing that out).

[cor3ntin]

After offline discussion with Aaron, we decided that "can not appear here" was a
sufficient diagnostic, so I have implemented that.
A better solution would be to track which captures fail, and collect the diagnostics
to emit them after the parameter list, but this seem to add a lot of complexity
for something few people are likely to encounter.

• Add tests
• A diagnostic for use before capture could be emited twice when parsing

a template parameter list, this was fixed.

[aaron.ballman]

LGTM! When landing, please revert the changes to SemaDecl.cpp (no need to churn that file) and apply the one suggested refactoring (I commented to make it more clear which one I mean).

clang/lib/Sema/SemaLambda.cpp

1127–1135

+1 to this suggestion being a bit easier to read.

[cor3ntin]

clang/lib/Sema/SemaLambda.cpp

1127–1135

With that changes, a capture that is not in the delayed capture list forces us to go through the list of capture anyway, which seems like a waste.
So this is not just about readability. it should still work, and only really matter for error cases, but it's still not identical code.

[cor3ntin]

clang/lib/Sema/SemaLambda.cpp

1127–1135

Alternatively, I guess I can change the order of operation to ensure Var is never null here, and that would probably be cleaner!

[aaron.ballman]

clang/lib/Sema/SemaLambda.cpp

1127–1135

Ah, thank you for pointing out that this was also about performance. If you're able to clean it up by changing the order of operations, that'd be good, but if it turns into a slog, I don't insist.

[cor3ntin]

Cleanup SemaLambda as discussed with Aaron

[cor3ntin]

@aaron.ballman @ChuanqiXu Thanks for the review!

[mehdi_amini]

Seems like this is breaking clang bootstrapping?

llvm/include/llvm/CodeGen/LiveInterval.h:630:53: error: captured variable 'Idx' cannot appear here
              [=](std::remove_reference_t<decltype(*Idx)> V,
                                                    ^

[cor3ntin]

Yes, working on a fix as we speak.
The meaning of that code changed (in all c++ language modes), I'm
currently trying to find if we have any other issue of that sort and will
commit a fix in a few hours

[mehdi_amini]

In D119136#3449325, @cor3ntin wrote:

Yes, working on a fix as we speak.
The meaning of that code changed (in all c++ language modes), I'm
currently trying to find if we have any other issue of that sort and will
commit a fix in a few hours

OK, I'll revert in the meantime to unbreak bots.

[rsmith]

Hey @cor3ntin, sorry, I reverted this again. It's breaking thread-safety annotations on lambdas in a pretty severe way:

Mutex m;
auto lambda = [=]() EXCLUSIVE_LOCKS_REQUIRED(m) {...}

doesn't compile any more. Presumably we should be parsing the attribute in the enclosing context rather than in the half-way-inside-the-lambda context in which we parse parameters?

[cor3ntin]

@rsmith Sorry about that, I will investigate. We found preexisting issues - attributes seem to ODR use when they should maybe not and I could not thing of attributes that would refer to capture, so i didn't spent enough time looking at attributes.

[cor3ntin]

Presumably we should be parsing the attribute in the enclosing context rather than in the half-way-inside-the-lambda context in which we parse parameters?

That's a good question.

void f() {
int y;
(void)[=, x = 1]() 
    __attribute__((diagnose_if(!is_same<decltype((y)), int&>, "wrong type", "warning")))  // do we want this to work?
    __attribute__((diagnose_if(x != 1, "wrong type", "warning"))) // do we want this to work?
    mutable {}();
}

I think we *probably* want both of this to compile and evaluate the conditions consistently with the new rule for trailing return type/requires/noexcept specifier (in that case triggering no warning).
So the consistent approach would be to parse them in the context of the call operator, once the captures and their type is known.

Is there any reason we should not be consistent with the spirit of the c++ spec here?

[cor3ntin]

Properly support GNU Attributes

Perform delayed parsing of GNU attributes (using code that already
existed to delay the parsing of class methods), so that
gnu attributes can refer to captured variables,
with the same semantics as decltype/noexcept/requires.

In order we:
 * Lex the attributes
 * Parse the mutable keyword
 * Inject parameter names in the scope of the call operator
   so they get found during attribute parsing
 * Parse the attributes and attach them to the declarator
   being parsed.

[aaron.ballman]

clang/lib/Parse/ParseExprCXX.cpp

1234

I think this should likely be named ParseLambdaLexedGNUAttributeArgs() instead, because this is pretty specific to GNU-style attribute argument lists and shouldn't be used for other attribute syntaxes, correct?

1253–1256

This comment reads a bit oddly to me. I think it may be better as "After parsing attribute arguments, we've either reached the EOF token (signaling that parsing was successful) or we have tokens we need to consume until we reach the EOF."

1258

Then I think we can assert that the token is EOF here and consume it always.

1341

1363

I think you should add an assert before this call to ensure that all attributes are GNU ones.

1443

Do we have test coverage for this change? (It seems orthogonal to your patch, so this might be worth splitting out.)

[cor3ntin]

clang/lib/Parse/ParseExprCXX.cpp

1253–1256

Agreed

1258

Agreed, that seems better

1341

It's more like there is no __declspec that can refer to expression so this was never implemented

1363

I'm not sure that gains much?
In the future if late parsing is implemented for __declspec this would just work , unless we add an assert. I'd be fine adding it though.

1443

I'm happy doing that, or to add a test somewhere.

[aaron.ballman]

clang/lib/Parse/ParseExprCXX.cpp

1363

Because ParseLambdaLexedAttribute() specifically parses GNU attribute args and not declspec ones, I think that function should be renamed. Once that function is renamed to say GNU explicitly, then there's a question of what happens when we hand it late parsed declspec attribute. Which is why I was suggesting a comment above to mention that declspec attributes don't late parse anyway.

Another way to handle this that's more general would be to change ParseLambdaLexedAttribute() to parse different args depending on the syntax of the attribute (then it can keep its generic name).

[cor3ntin]

clang/lib/Parse/ParseExprCXX.cpp

1443

This was actually a _very_ stupid change on my part, the previous code was correct. ie, msvc does not support [] __declspec(deprecated) {};
https://godbolt.org/z/cEKP44rY1
and so the omission of __declspec here, which i thought was a bug, is actually very purposeful

[aaron.ballman]

clang/lib/Parse/ParseExprCXX.cpp

1443

Thank you for double-checking that; I thought that might be the case, but I hadn't tested it myself yet.

[cor3ntin]

Address Aaron's comments

[cor3ntin]

clang/lib/Parse/ParseExprCXX.cpp

1363

Okay, after actually reading the code, we can't actually assert on that.

LateParsedAttribute is really LateParsedGNUAttribute. And maybe we want to rename that after this patch. It can never contain anything else, and because __attribute__ was eaten, we don't really have anything to check for.

[aaron.ballman]

clang/lib/Parse/ParseExprCXX.cpp

1363

Oh, well TIL. :-D

Yeah, don't bother with the assert then. Any probably don't bother with the rename I suggested either.

(Someday) we'll either rename LateParsedAttribute to be more correct, or we'll extend it to support more syntaxes and keep the same name. Either way, the interfaces in your patch will be sufficiently clear.

[aaron.ballman]

LGTM, thank you!

[grandinj]

I note that building on Ubuntu, I am now seeing failures like:

env/gbuildtojson/gbuildtojson.cxx:10:
In file included from /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/algorithm:74:
In file included from /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/pstl/glue_algorithm_defs.h:13:
In file included from /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/functional:61:
In file included from /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/unordered_map:52:
In file included from /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/unordered_map:49:
/usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/safe_unordered_container.h:71:28: error: captured variable '__local_end' cannot appear here

[__local_end](__decltype(__local_end) __it)
                         ^

/usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/safe_unordered_container.h:71:4: note: variable '__local_end' is explicitly captured here

[__local_end](__decltype(__local_end) __it)
 ^

/usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/safe_unordered_container.h:69:2: note: '__local_end' declared here

auto __local_end = _M_cont()._M_base().cend(0);
^

/usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/safe_unordered_container.h:79:44: error: captured variable '__end' cannot appear here

this->_M_invalidate_if([__end](__decltype(__end) __it)
                                          ^

/usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/safe_unordered_container.h:79:26: note: variable '__end' is explicitly captured here

this->_M_invalidate_if([__end](__decltype(__end) __it)
                        ^

/usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/debug/safe_unordered_container.h:78:2: note: '__end' declared here

auto __end = _M_cont()._M_base().cend();
^

[cor3ntin]

@grandinj Thanks for notifying me of this issue, and sorry for the inconvenience.

There is a fix here https://reviews.llvm.org/D123909
Should the review of that linger too long, I'll revert this patch.

[cor3ntin]

The remediation has landed, sorry again for the trouble this caused, I'll keep an eye out for further issues

[grandinj]

thanks for the quick response

[rtrieu]

This seems to be acting weird in template instantations. Here's an example where the lambda only errors inside a template.

template <class>
int foo(int x = 0) {
    auto lambda = [x = x+1]() -> decltype(x) {
        return x;
    };
    return -1;
}

// no template
int bar(int x = 0) {
    auto lambda = [x = x+1]() -> decltype(x) {
        return x;
    };
    return -1;
}

int a = foo<int>();  // error
int b = bar();  // no  error

/tmp/lambda.cc:4:16: error: variable 'x' cannot be implicitly captured in a lambda with no capture-default specified
        return x;
               ^
/tmp/lambda.cc:17:9: note: in instantiation of function template specialization 'foo<int>' requested here
int a = foo<int>();  // error
        ^
/tmp/lambda.cc:3:20: note: 'x' declared here
    auto lambda = [x = x+1]() -> decltype(x) {
                   ^
/tmp/lambda.cc:3:19: note: lambda expression begins here
    auto lambda = [x = x+1]() -> decltype(x) {
                  ^
/tmp/lambda.cc:3:27: note: capture 'x' by value
    auto lambda = [x = x+1]() -> decltype(x) {
                          ^
                          , x
/tmp/lambda.cc:3:27: note: capture 'x' by reference
    auto lambda = [x = x+1]() -> decltype(x) {
                          ^
                          , &x
/tmp/lambda.cc:3:20: note: default capture by value
    auto lambda = [x = x+1]() -> decltype(x) {
                   ^
                   =, 
/tmp/lambda.cc:3:20: note: default capture by reference
    auto lambda = [x = x+1]() -> decltype(x) {
                   ^
                   &, 
1 error generated.

[alanphipps]

We've also been seeing failures in our downstream Arm compiler when running the Perennial C++14 compliance tests related to this change. Specifically, the tests expect a diagnostic to be issued when the lambda capture variable is outside of the lambda's {} scope. Another tests uses the lambda capture variable in a decltype-style return type which is also outside of the scope.

Do these failures sound like what others have run into?

[cor3ntin]

In D119136#3457027, @rtrieu wrote:

This seems to be acting weird in template instantations. Here's an example where the lambda only errors inside a template.

Thanks for notifying me of that. I will land a fix shortly https://reviews.llvm.org/D124012

[MaskRay]

Sorry but I've reverted this patch and all its fixups in c79e6007edef4b0044be93c4ffff64dc806ac687 and 0f5dbfd29ae0df215a01aff80d29255bb799fed0 .
See https://reviews.llvm.org/D123909#3461716 for another case not considered.

[aaron.ballman]

In D119136#3462570, @MaskRay wrote:

Sorry but I've reverted this patch and all its fixups in c79e6007edef4b0044be93c4ffff64dc806ac687 and 0f5dbfd29ae0df215a01aff80d29255bb799fed0 .
See https://reviews.llvm.org/D123909#3461716 for another case not considered.

Please double check with the patch author and reviewers before unilaterally reverting multiple commits with no notice and no failing build bots. I do not see a valid justification for these reverts -- the concerns have been true positives so far (or have generated core issues that WG21 is still discussing), and @cor3ntin has been highly responsive with addressing the fallout. This is a significant amount of churn that I don't think should have happened.

[MaskRay]

In D119136#3462579, @aaron.ballman wrote:

In D119136#3462570, @MaskRay wrote:

Sorry but I've reverted this patch and all its fixups in c79e6007edef4b0044be93c4ffff64dc806ac687 and 0f5dbfd29ae0df215a01aff80d29255bb799fed0 .
See https://reviews.llvm.org/D123909#3461716 for another case not considered.

Please double check with the patch author and reviewers before unilaterally reverting multiple commits with no notice and no failing build bots. I do not see a valid justification for these reverts -- the concerns have been true positives so far (or have generated core issues that WG21 is still discussing), and @cor3ntin has been highly responsive with addressing the fallout. This is a significant amount of churn that I don't think should have happened.

Reply to both this message and https://reviews.llvm.org/D123909#3462560:

I am sorry but this thread has gained reports from multiple independent parties about breakage, and I just thought the followups did not fix all issues.
I confess I do not really understand the subtle areas in the language.
My decision to revert was mostly driven by fixups' descriptions like "but did not properly handled dependant context,"

• which made me believe there were indeed bugs in the implementation and new corner cases just emerged.

I'd agree that there is invalid code which is not correctly rejected, and the new code correctly rejects it (if only for such cases there is no justification for reverts), but the fixups mixed with bugfixes made the whole picture unclear.

I apology if I did make wrong reverts.
At this point, the reverts have been made. I hope that starting from a clean state is not too bad. I am grateful to @cor3ntin who is highly responsive with addressing the fallout.

[aaron.ballman]

In D119136#3462609, @MaskRay wrote:

In D119136#3462579, @aaron.ballman wrote:

In D119136#3462570, @MaskRay wrote:

Sorry but I've reverted this patch and all its fixups in c79e6007edef4b0044be93c4ffff64dc806ac687 and 0f5dbfd29ae0df215a01aff80d29255bb799fed0 .
See https://reviews.llvm.org/D123909#3461716 for another case not considered.

Please double check with the patch author and reviewers before unilaterally reverting multiple commits with no notice and no failing build bots. I do not see a valid justification for these reverts -- the concerns have been true positives so far (or have generated core issues that WG21 is still discussing), and @cor3ntin has been highly responsive with addressing the fallout. This is a significant amount of churn that I don't think should have happened.

Reply to both this message and https://reviews.llvm.org/D123909#3462560:

I am sorry but this thread has gained reports from multiple independent parties about breakage, and I just thought the followups did not fix all issues.

AFAIK, all issues have either been addressed, are true positives, or are a matter for WG21 to figure out what to do with. However, I'm not certain it was clear from the reviews that this was the case, so it's understandable there's some amount of "did we get everything?" confusion.

I confess I do not really understand the subtle areas in the language.
My decision to revert was mostly driven by fixups' descriptions like "but did not properly handled dependant context,"

• which made me believe there were indeed bugs in the implementation and new corner cases just emerged.

I'd agree that there is invalid code which is not correctly rejected, but the fixups mixed with bugfixes made the whole picture unclear.

The trouble here was: WG21 adopted a proposal intended as a bugfix, but it was never implemented before (to my knowledge). We went and did the implementation and discovered that the bugfix paper actually introduces different bugs (hence us breaking libstdc++; the diagnostic was valid, but the paper didn't likely intend to make that code invalid). CWG2569 was filed to try to address this, but the issue hasn't been resolved by WG21 yet. So what @cor3ntin has done is to address *only* the libstdc++ failure case (to keep important system headers working) until WG21 figures out whether we need to be relaxed in more cases.

This does mean that there may be some code broken that WG21 will later decide should not be broken, but that's a matter for the Core Issue; these changes are to implement P2036R3 which is a C++23 feature.

I apology if I did make wrong reverts.
At this point, the reverts have been made. I hope that starting from a clean state is not too bad. I am grateful to @cor3ntin who is highly responsive with addressing the fallout.

Thanks for the apology, but I still think the reverts were premature, so hopefully we don't do this again. The issue with reverting all of these is that you put the burden back onto a relatively new contributor to try to figure out how to reapply all these changes when there wasn't actually a need to do so.

In terms of where we go from here, there's a few options:

1. revert the reverts; this is easiest, but causes the most churn
2. merge all three patches into one (squashed) patch and commit that; this requires a little bit more work, but causes less churn
3. assume that P2036R3 is not implementable until WG21 resolves CWG2569 because it breaks too much code

I think I have a preference for #2 as that will hopefully leave us with the most clear git history for when we do a blame later, but I could live with #1. If we find that there is significant broken code still, but it's not in system headers, then I think we should explore #3, but I don't think we have evidence that we're in this situation either.

[amyk]

In D119136#3459738, @alanphipps wrote:

We've also been seeing failures in our downstream Arm compiler when running the Perennial C++14 compliance tests related to this change. Specifically, the tests expect a diagnostic to be issued when the lambda capture variable is outside of the lambda's {} scope. Another tests uses the lambda capture variable in a decltype-style return type which is also outside of the scope.

Do these failures sound like what others have run into?

Hi, I would just like to express that I am also seeing these two issues on Power, that are a result of the original patch (D119136) that @alanphipps has pointed out.

[cor3ntin]

Sorry for the delayed reply.

In D119136#3462660, @aaron.ballman wrote:

In D119136#3462609, @MaskRay wrote:

In D119136#3462579, @aaron.ballman wrote:

In D119136#3462570, @MaskRay wrote:

Sorry but I've reverted this patch and all its fixups in c79e6007edef4b0044be93c4ffff64dc806ac687 and 0f5dbfd29ae0df215a01aff80d29255bb799fed0 .
See https://reviews.llvm.org/D123909#3461716 for another case not considered.

Please double check with the patch author and reviewers before unilaterally reverting multiple commits with no notice and no failing build bots. I do not see a valid justification for these reverts -- the concerns have been true positives so far (or have generated core issues that WG21 is still discussing), and @cor3ntin has been highly responsive with addressing the fallout. This is a significant amount of churn that I don't think should have happened.

Reply to both this message and https://reviews.llvm.org/D123909#3462560:

I am sorry but this thread has gained reports from multiple independent parties about breakage, and I just thought the followups did not fix all issues.

AFAIK, all issues have either been addressed, are true positives, or are a matter for WG21 to figure out what to do with. However, I'm not certain it was clear from the reviews that this was the case, so it's understandable there's some amount of "did we get everything?" confusion.

Yes, as far as I'm aware all the implementation issues with this patch has been addressed (or are undiscovered), so the remaining issues are that a C++ proposal (approved as a Defect Report in C++11 and up) do break existing code.
The issue has been raised in the C++ committee and clang implements https://github.com/cplusplus/CWG/issues/31 to mitigate the issue.
You will note that the resolution addresses decltype(id) but not decltype(*id) - there is no reasonable way to make the later work in a coherent fashion.

I think it would be tremendously helpful if you could share more of the reports you had internally, as it might affect how the C++ committee decides to resolves this.

In the meantime, clang could make that pattern well formed in older language modes, it something we have discussed, however I'm not sure you want to rely on code that will be made ill-formed when switching to C++23.
And the fact that decltype(*id) would have a different meaning in different part of the lambda expression ([id] (decltype(*id)) ->decltype(*id) may be 2 different types) makes me very uncomfortable - it's the reason the C++ committee decided to make that ill-formed to begin with.

It was the intent of WG21 for this change to be breaking. But as Aaron said, Clang is the first implementation to support it, so we are discovering unforeseen issues with it.
I think we should quantify the extent of the problem before taking drastic actions, as that will help both WG21 and Clang decide what are the most effective mitigation strategy, if any.

[lichray]

In D119136#3462660, @aaron.ballman wrote:>

Thanks for the apology, but I still think the reverts were premature, so hopefully we don't do this again. The issue with reverting all of these is that you put the burden back onto a relatively new contributor to try to figure out how to reapply all these changes when there wasn't actually a need to do so.

In terms of where we go from here, there's a few options:

1. revert the reverts; this is easiest, but causes the most churn
2. merge all three patches into one (squashed) patch and commit that; this requires a little bit more work, but causes less churn
3. assume that P2036R3 is not implementable until WG21 resolves CWG2569 because it breaks too much code

I think I have a preference for #2 as that will hopefully leave us with the most clear git history for when we do a blame later, but I could live with #1. If we find that there is significant broken code still, but it's not in system headers, then I think we should explore #3, but I don't think we have evidence that we're in this situation either.

I hope we don't fix bugs in production -- WG21 bug is also a bug.

1. The decision made in the paper p2036r3 section 4.1 brings in a breaking change.
2. The paper is applied as a DR on all previous C++ versions that apply.
3. The paper has no Annex C entry.

These three things do not compose. The paper didn't do enough homework.

I would like to see One patch that allows other parties to try, but not necessarily in the trunk, since doing so couples the trial with other useful work in the trunk.

[cor3ntin]

In D119136#3462707, @amyk wrote:

In D119136#3459738, @alanphipps wrote:

We've also been seeing failures in our downstream Arm compiler when running the Perennial C++14 compliance tests related to this change. Specifically, the tests expect a diagnostic to be issued when the lambda capture variable is outside of the lambda's {} scope. Another tests uses the lambda capture variable in a decltype-style return type which is also outside of the scope.

Do these failures sound like what others have run into?

Hi, I would just like to express that I am also seeing these two issues on Power, that are a result of the original patch (D119136) that @alanphipps has pointed out.

Do you have a code example, just to make sure we are talking about the same thing?

[i = 0]() -> decltype(i) {}

For example now is supposed to be valid with this change. I'm not sure how conformance test suits handle back-ported language changes and how long it takes them to be updated.

[MaskRay]

Thanks for being willing to wait a bit and collect usage. Given current feedback, I am not going to simply undo the reverts.

@cor3ntin To get one patch other folks can try:

git revert -n c79e6007edef4b0044be93c4ffff64dc806ac687
git revert -n 0f5dbfd29ae0df215a01aff80d29255bb799fed0
git commit   # with a description incorporating this revision and all the following-ups. Make sure `Differential Revision: https://reviews.llvm.org/D119136` is included

On this webpage, click "Add Action" -> "Reopen Revision".
Upload a new diff either via web UI or arc diff 'HEAD^'.

Other folks can use curl -L 'https://reviews.llvm.org/D119136?download=1' | patch -p1 to apply the patch locally, or use arc patch D119136.

[cor3ntin]

@rsmith @aaron.ballman Do you think it's worth resubmitting this patch with a look ahead of the mutable keyword (this seems to me a better strategy than other flimsy workaround suggested/tried), or would you rather wait for WG21 to come up with something?

[AaronLiu]

When compile the following valid testcase:

void foo()
{
        int x = [x](int y[sizeof x]){return sizeof x;}(0);
}

It will complain:

error: captured variable 'x' cannot appear here
        int x = [x](int y[sizeof x]){return sizeof x;}(0);
                                 ^

The issue is also described in: https://cplusplus.github.io/CWG/issues/2569.html

[rsmith]

In D119136#3463888, @cor3ntin wrote:

@rsmith @aaron.ballman Do you think it's worth resubmitting this patch with a look ahead of the mutable keyword (this seems to me a better strategy than other flimsy workaround suggested/tried), or would you rather wait for WG21 to come up with something?

In principle, I think it would be reasonable to put this change behind a language option with a -f flag, and enable it by default only in C++23 mode, while WG21 sorts out what they want to do. But this patch is making some substantial changes in terms of how much code is moved around -- do you think it's feasible to support both modes without a lot of additional complexity?

[rsmith]

In D119136#3463888, @cor3ntin wrote:

@rsmith @aaron.ballman Do you think it's worth resubmitting this patch with a look ahead of the mutable keyword (this seems to me a better strategy than other flimsy workaround suggested/tried), or would you rather wait for WG21 to come up with something?

To directly answer your question: I'd want a reasonably strong signal from WG21 that that's the direction they want to pursue before we implement something like that, especially because it changes the meaning of some constructs while leaving them valid, which will create some churn for people living at head especially if we then roll that change back again. (I think we did have a strong enough signal to early-adopt the decltype(id) handling.)

[cor3ntin]

In D119136#3465403, @rsmith wrote:

In D119136#3463888, @cor3ntin wrote:

@rsmith @aaron.ballman Do you think it's worth resubmitting this patch with a look ahead of the mutable keyword (this seems to me a better strategy than other flimsy workaround suggested/tried), or would you rather wait for WG21 to come up with something?

To directly answer your question: I'd want a reasonably strong signal from WG21 that that's the direction they want to pursue before we implement something like that, especially because it changes the meaning of some constructs while leaving them valid, which will create some churn for people living at head especially if we then roll that change back again. (I think we did have a strong enough signal to early-adopt the decltype(id) handling.)

I'll wait then, there are enough subtlety as it is, having different behavior in different language modes is not something i can commit time to at the moment (and one of the reason I was hoping we would still get this paper as a DR).
It's certainly doable - in particular getting everything before the end of the parameter declaration to behave as currently is trivial, but getting lookup in trailing return type etc to behave differently in different language mode would require some massaging.
I would also be concerned that putting that behind a flag would not give us meaningful data, so i don't know how useful it would be.

I was hoping to have some kind of implementation experience with look ahead as it's the most palatable fix, in my opinion, but it doesn't seem to be the one likely to be considered.
So I might implement it so that we can show that it's implementable, but no reason to attempt to upstream it - except that we won't get usage experience, nor will we know the extent of the breakage with CWG2569 applied

[cor3ntin]

In D119136#3465361, @AaronLiu wrote:

When compile the following valid testcase:

void foo()
{
        int x = [x](int y[sizeof x]){return sizeof x;}(0);
}

It will complain:

error: captured variable 'x' cannot appear here
        int x = [x](int y[sizeof x]){return sizeof x;}(0);
                                 ^

The issue is also described in: https://cplusplus.github.io/CWG/issues/2569.html

Is that something you encountered in existing code, or where you trying to write test against this change.
I only implemented the decltype part of the proposed resolution, for reason explained in that link (supporting sizeof(unqual-id) and `noexcept(unqual-id) seems extremely arbitrary).
But hopefully WG21 will come up with a more encompassing fix before we attempt to re-land this change

[AaronLiu]

In D119136#3465522, @cor3ntin wrote:

In D119136#3465361, @AaronLiu wrote:

When compile the following valid testcase:

void foo()
{
        int x = [x](int y[sizeof x]){return sizeof x;}(0);
}

It will complain:

error: captured variable 'x' cannot appear here
        int x = [x](int y[sizeof x]){return sizeof x;}(0);
                                 ^

The issue is also described in: https://cplusplus.github.io/CWG/issues/2569.html

Is that something you encountered in existing code, or where you trying to write test against this change.
I only implemented the decltype part of the proposed resolution, for reason explained in that link (supporting sizeof(unqual-id) and `noexcept(unqual-id) seems extremely arbitrary).
But hopefully WG21 will come up with a more encompassing fix before we attempt to re-land this change

It is reduced from the existing code.

clang/include/clang/AST/DeclCXX.h

if (!isLambda())
       return LDK_Unknown;
     return getLambdaData().DependencyKind;
   }
 
   TypeSourceInfo *getLambdaTypeInfo() const {
     return getLambdaData().MethodTyInfo;
   }
 
+  void setLambdaTypeInfo(TypeSourceInfo *TS) {
+    auto *DD = DefinitionData;
+    assert(DD && DD->IsLambda && "setting lambda property of non-lambda class");
+    auto &DL = static_cast<LambdaDefinitionData &>(*DD);
+    DL.MethodTyInfo = TS;
+  }
+
+  void setLambdaIsGeneric(bool IsGeneric) {
+    auto *DD = DefinitionData;
+    assert(DD && DD->IsLambda && "setting lambda property of non-lambda class");
+    auto &DL = static_cast<LambdaDefinitionData &>(*DD);
+    DL.IsGenericLambda = IsGeneric;
+  }
+
   // Determine whether this type is an Interface Like type for
   // __interface inheritance purposes.
   bool isInterfaceLike() const;
 
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) {
     return K >= firstCXXRecord && K <= lastCXXRecord;
   }
   void markAbstract() { data().Abstract = true; }
 };
 
 /// Store information needed for an explicit specifier.
 /// Used by CXXDeductionGuideDecl, CXXConstructorDecl and CXXConversionDecl.
 class ExplicitSpecifier {

clang/include/clang/Basic/DiagnosticSemaKinds.td

let CategoryName = "Lambda Issue" in {
   def err_lambda_capture_anonymous_var : Error<
     "unnamed variable cannot be implicitly captured in a lambda expression">;
   def err_lambda_capture_flexarray_type : Error<
     "variable %0 with flexible array member cannot be captured in "
     "a lambda expression">;
   def err_lambda_impcap : Error<
     "variable %0 cannot be implicitly captured in a lambda with no "
     "capture-default specified">;
+  def err_lambda_used_before_capture: Error<
+    "captured variable %0 cannot appear before the end of the lambda's parameter list">;
   def note_lambda_variable_capture_fixit : Note<
     "capture %0 by %select{value|reference}1">;
   def note_lambda_default_capture_fixit : Note<
     "default capture by %select{value|reference}0">;
   def note_lambda_decl : Note<"lambda expression begins here">;
   def err_lambda_unevaluated_operand : Error<
     "lambda expression in an unevaluated operand">;
   def err_lambda_in_constant_expression : Error<

clang/include/clang/Sema/Scope.h

 /// declaration.
 class Scope {
 public:
   /// ScopeFlags - These are bitfields that are or'd together when creating a
   /// scope, which defines the sorts of things the scope contains.
   enum ScopeFlags {
     /// This indicates that the scope corresponds to a function, which
     /// means that labels are set here.
-    FnScope       = 0x01,
+    FnScope = 0x01,
 
     /// This is a while, do, switch, for, etc that can have break
     /// statements embedded into it.
-    BreakScope    = 0x02,
+    BreakScope = 0x02,
 
     /// This is a while, do, for, which can have continue statements
     /// embedded into it.
     ContinueScope = 0x04,
 
     /// This is a scope that can contain a declaration.  Some scopes
     /// just contain loop constructs but don't contain decls.
     DeclScope = 0x08,
enum ScopeFlags {
 
     /// This is the scope of a C++ catch statement.
     CatchScope = 0x1000000,
 
     /// This is a scope in which a condition variable is currently being
     /// parsed. If such a scope is a ContinueScope, it's invalid to jump to the
     /// continue block from here.
     ConditionVarScope = 0x2000000,
+
+    /// This is the scope for a lambda, after the lambda introducer.
+    /// Lambdas need 2 FunctionPrototypeScope scopes ( because there is a
+    /// template scope in between),
+    /// the outer scope does not increase the depth of recursion.
+    LambdaScope = 0x4000000,
   };
 
 private:
   /// The parent scope for this scope.  This is null for the translation-unit
   /// scope.
   Scope *AnyParent;
 
   /// Flags - This contains a set of ScopeFlags, which indicates how the scope

clang/include/clang/Sema/ScopeInfo.h

class LambdaScopeInfo final :
     public CapturingScopeInfo, public InventedTemplateParameterInfo {
 public:
   /// The class that describes the lambda.
   CXXRecordDecl *Lambda = nullptr;
 
   /// The lambda's compiler-generated \c operator().
   CXXMethodDecl *CallOperator = nullptr;
 
+  struct DelayedCapture {
+    VarDecl *Var;
+    SourceLocation Loc;
+    LambdaCaptureKind Kind;
+  };
+
+  /// Holds the captures until we parsed the qualifiers,
+  /// as the cv qualified type of capture can only be computed at that point,
+  /// and the capture should not be visible before.
+  llvm::DenseMap<unsigned, DelayedCapture> DelayedCaptures;
+
+  /// Whether the current scope when parsing the lambda
+  /// is after the call operator qualifiers,
+  /// which is the point at which the captures are usable
+  /// per [expr.prim.id.unqual]/p3.2 and [expr.prim.lambda.capture]/6.
+  /// This is set to false by default as the lambda can be reconstructed during
+  /// instantiation
+  bool BeforeLambdaQualifiersScope = false;
+
   /// 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

clang/include/clang/Sema/Sema.h

QualType buildLambdaInitCaptureInitialization(
       Optional<unsigned> NumExpansions, IdentifierInfo *Id, bool DirectInit,
       Expr *&Init);
 
   /// Create a dummy variable within the declcontext of the lambda's
   ///  call operator, for name lookup purposes for a lambda init capture.
   ///
   ///  CodeGen handles emission of lambda captures, ignoring these dummy
   ///  variables appropriately.
-  VarDecl *createLambdaInitCaptureVarDecl(SourceLocation Loc,
-                                          QualType InitCaptureType,
-                                          SourceLocation EllipsisLoc,
-                                          IdentifierInfo *Id,
-                                          unsigned InitStyle, Expr *Init);
+  VarDecl *createLambdaInitCaptureVarDecl(
+      SourceLocation Loc, QualType InitCaptureType, SourceLocation EllipsisLoc,
+      IdentifierInfo *Id, unsigned InitStyle, Expr *Init, DeclContext *DeclCtx);
 
   /// Add an init-capture to a lambda scope.
   void addInitCapture(sema::LambdaScopeInfo *LSI, VarDecl *Var);
 
   /// Note that we have finished the explicit captures for the
   /// given lambda.
   void finishLambdaExplicitCaptures(sema::LambdaScopeInfo *LSI);
 
-  /// \brief This is called after parsing the explicit template parameter list
+  /// Deduce a block or lambda's return type based on the return
+  /// statements present in the body.
+  void deduceClosureReturnType(sema::CapturingScopeInfo &CSI);
+
+  /// Once the Lambdas capture are known, we can
+  /// start to create the closure, call operator method,
+  /// and keep track of the captures.
+  /// We do the capture lookup here, but they are not actually captured
+  /// until after we know what the qualifiers of the call operator are.
+  void ActOnLambdaIntroducer(LambdaIntroducer &Intro, Scope *CurContext);
+
+  /// This is called after parsing the explicit template parameter list
   /// on a lambda (if it exists) in C++2a.
-  void ActOnLambdaExplicitTemplateParameterList(SourceLocation LAngleLoc,
+  void ActOnLambdaExplicitTemplateParameterList(LambdaIntroducer &Intro,
+                                                SourceLocation LAngleLoc,
                                                 ArrayRef<NamedDecl *> TParams,
                                                 SourceLocation RAngleLoc,
                                                 ExprResult RequiresClause);
 
-  /// Introduce the lambda parameters into scope.
-  void addLambdaParameters(
-      ArrayRef<LambdaIntroducer::LambdaCapture> Captures,
-      CXXMethodDecl *CallOperator, Scope *CurScope);
-
-  /// Deduce a block or lambda's return type based on the return
-  /// statements present in the body.
-  void deduceClosureReturnType(sema::CapturingScopeInfo &CSI);
+  void ActOnLambdaClosureQualifiers(LambdaIntroducer &Intro,
+                                    SourceLocation MutableLoc,
+                                    SourceLocation EndLoc, const DeclSpec &DS);
 
   /// ActOnStartOfLambdaDefinition - This is called just before we start
   /// parsing the body of a lambda; it analyzes the explicit captures and
   /// arguments, and sets up various data-structures for the body of the
   /// lambda.
   void ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
                                     Declarator &ParamInfo, Scope *CurScope);
 

clang/lib/Parse/ParseExprCXX.cpp

if (ConstevalLoc.isValid()) {
     unsigned DiagID = 0;
     DS.SetConstexprSpec(ConstexprSpecKind::Consteval, ConstevalLoc, PrevSpec,
                         DiagID);
     if (DiagID != 0)
       P.Diag(ConstevalLoc, DiagID) << PrevSpec;
   }
 }
 
 /// ParseLambdaExpressionAfterIntroducer - Parse the rest of a lambda
 /// expression.
 ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
                      LambdaIntroducer &Intro) {
   SourceLocation LambdaBeginLoc = Intro.Range.getBegin();
   Diag(LambdaBeginLoc, diag::warn_cxx98_compat_lambda);
 
   PrettyStackTraceLoc CrashInfo(PP.getSourceManager(), LambdaBeginLoc,
                                 "lambda expression parsing");
 
 
 
   // FIXME: Call into Actions to add any init-capture declarations to the
   // scope while parsing the lambda-declarator and compound-statement.
 
   // Parse lambda-declarator[opt].
   DeclSpec DS(AttrFactory);
   Declarator D(DS, DeclaratorContext::LambdaExpr);
   TemplateParameterDepthRAII CurTemplateDepthTracker(TemplateParameterDepth);
+
+  ParseScope LambdaScope(this, Scope::LambdaScope | Scope::DeclScope |
+                                   Scope::FunctionDeclarationScope |
+                                   Scope::FunctionPrototypeScope);
+
   Actions.PushLambdaScope();
+  Actions.ActOnLambdaIntroducer(Intro, getCurScope());
 
   ParsedAttributes Attr(AttrFactory);
   if (getLangOpts().CUDA) {
     // In CUDA code, GNU attributes are allowed to appear immediately after the
     // "[...]", even if there is no "(...)" before the lambda body.
     MaybeParseGNUAttributes(D);
   }
 
if (TemplateParams.empty()) {
         RequiresClause =
             Actions.ActOnRequiresClause(ParseConstraintLogicalOrExpression(
                 /*IsTrailingRequiresClause=*/false));
         if (RequiresClause.isInvalid())
           SkipUntil({tok::l_brace, tok::l_paren}, StopAtSemi | StopBeforeMatch);
       }
 
       Actions.ActOnLambdaExplicitTemplateParameterList(
-          LAngleLoc, TemplateParams, RAngleLoc, RequiresClause);
+          Intro, LAngleLoc, TemplateParams, RAngleLoc, RequiresClause);
       ++CurTemplateDepthTracker;
     }
   }
 
   // Implement WG21 P2173, which allows attributes immediately before the
   // lambda declarator and applies them to the corresponding function operator
   // or operator template declaration. We accept this as a conforming extension
   // in all language modes that support lambdas.
   if (isCXX11AttributeSpecifier()) {
     Diag(Tok, getLangOpts().CPlusPlus2b
                   ? diag::warn_cxx20_compat_decl_attrs_on_lambda
                   : diag::ext_decl_attrs_on_lambda);
     MaybeParseCXX11Attributes(D);
   }
 
   TypeResult TrailingReturnType;
   SourceLocation TrailingReturnTypeLoc;
+  SourceLocation LParenLoc, RParenLoc;
+  SourceLocation DeclEndLoc;
+  bool HasParentheses = false;
+  bool HasSpecifiers = false;
+  SourceLocation MutableLoc;
 
-  auto ParseLambdaSpecifiers =
-      [&](SourceLocation LParenLoc, SourceLocation RParenLoc,
-          MutableArrayRef<DeclaratorChunk::ParamInfo> ParamInfo,
-          SourceLocation EllipsisLoc) {
-        SourceLocation DeclEndLoc = RParenLoc;
-
-        // GNU-style attributes must be parsed before the mutable specifier to
-        // be compatible with GCC. MSVC-style attributes must be parsed before
-        // the mutable specifier to be compatible with MSVC.
-        MaybeParseAttributes(PAKM_GNU | PAKM_Declspec, Attr);
-
-        // Parse mutable-opt and/or constexpr-opt or consteval-opt, and update
-        // the DeclEndLoc.
-        SourceLocation MutableLoc;
-        SourceLocation ConstexprLoc;
-        SourceLocation ConstevalLoc;
-        tryConsumeLambdaSpecifierToken(*this, MutableLoc, ConstexprLoc,
-                                       ConstevalLoc, DeclEndLoc);
+  auto ParseConstexprAndMutableSpecifiers = [&] {
 
-        addConstexprToLambdaDeclSpecifier(*this, ConstexprLoc, DS);
-        addConstevalToLambdaDeclSpecifier(*this, ConstevalLoc, DS);
+  };
+
+  auto ParseLambdaSpecifiers =
+      [&](MutableArrayRef<DeclaratorChunk::ParamInfo> ParamInfo,
+          SourceLocation EllipsisLoc) {
         // Parse exception-specification[opt].
         ExceptionSpecificationType ESpecType = EST_None;
         SourceRange ESpecRange;
         SmallVector<ParsedType, 2> DynamicExceptions;
         SmallVector<SourceRange, 2> DynamicExceptionRanges;
         ExprResult NoexceptExpr;
         CachedTokens *ExceptionSpecTokens;
         ESpecType = tryParseExceptionSpecification(
             /*Delayed=*/false, ESpecRange, DynamicExceptions,
             DynamicExceptionRanges, NoexceptExpr, ExceptionSpecTokens);
 
         if (ESpecType != EST_None)
           DeclEndLoc = ESpecRange.getEnd();
 
         // Parse attribute-specifier[opt].
         if (MaybeParseCXX11Attributes(Attr))
           DeclEndLoc = Attr.Range.getEnd();
 
         // Parse OpenCL addr space attribute.
         if (Tok.isOneOf(tok::kw___private, tok::kw___global, tok::kw___local,
                         tok::kw___constant, tok::kw___generic)) {
           ParseOpenCLQualifiers(DS.getAttributes());
           ConsumeToken();
         }
 
         SourceLocation FunLocalRangeEnd = DeclEndLoc;
 
         // Parse trailing-return-type[opt].
         if (Tok.is(tok::arrow)) {
           FunLocalRangeEnd = Tok.getLocation();
           SourceRange Range;
           TrailingReturnType = ParseTrailingReturnType(
               Range, /*MayBeFollowedByDirectInit*/ false);
           TrailingReturnTypeLoc = Range.getBegin();
           if (Range.getEnd().isValid())
             DeclEndLoc = Range.getEnd();
         }
 
         SourceLocation NoLoc;
         D.AddTypeInfo(
             DeclaratorChunk::getFunction(
                 /*HasProto=*/true,
                 /*IsAmbiguous=*/false, LParenLoc, ParamInfo.data(),
                 ParamInfo.size(), EllipsisLoc, RParenLoc,
                 /*RefQualifierIsLvalueRef=*/true,
                 /*RefQualifierLoc=*/NoLoc, MutableLoc, ESpecType, ESpecRange,
                 DynamicExceptions.data(), DynamicExceptionRanges.data(),
                 DynamicExceptions.size(),
                 NoexceptExpr.isUsable() ? NoexceptExpr.get() : nullptr,
                 /*ExceptionSpecTokens*/ nullptr,
                 /*DeclsInPrototype=*/None, LParenLoc, FunLocalRangeEnd, D,
                 TrailingReturnType, TrailingReturnTypeLoc, &DS),
             std::move(Attr), DeclEndLoc);
+
+        if (HasParentheses && Tok.is(tok::kw_requires))
+          ParseTrailingRequiresClause(D);
       };
 
-  if (Tok.is(tok::l_paren)) {
-    ParseScope PrototypeScope(this, Scope::FunctionPrototypeScope |
-                                        Scope::FunctionDeclarationScope |
-                                        Scope::DeclScope);
+  ParseScope Prototype(this, Scope::FunctionPrototypeScope |
 
-    BalancedDelimiterTracker T(*this, tok::l_paren);
-    T.consumeOpen();
-    SourceLocation LParenLoc = T.getOpenLocation();
-
-    // Parse parameter-declaration-clause.
-    SmallVector<DeclaratorChunk::ParamInfo, 16> ParamInfo;
-    SourceLocation EllipsisLoc;
+  if (Tok.is(tok::l_paren)) {
+    BalancedDelimiterTracker T(*this, tok::l_paren);
+    T.consumeOpen();
+    LParenLoc = T.getOpenLocation();
 
     if (Tok.isNot(tok::r_paren)) {
       Actions.RecordParsingTemplateParameterDepth(
           CurTemplateDepthTracker.getOriginalDepth());
 
       ParseParameterDeclarationClause(D.getContext(), Attr, ParamInfo,
                                       EllipsisLoc);
       // For a generic lambda, each 'auto' within the parameter declaration
       // clause creates a template type parameter, so increment the depth.
       // If we've parsed any explicit template parameters, then the depth will
       // have already been incremented. So we make sure that at most a single
       // depth level is added.
       if (Actions.getCurGenericLambda())
         CurTemplateDepthTracker.setAddedDepth(1);
     }
 
     T.consumeClose();
+    DeclEndLoc = RParenLoc = T.getCloseLocation();
+    HasParentheses = true;
+  }
 
-    // Parse lambda-specifiers.
-    ParseLambdaSpecifiers(LParenLoc, /*DeclEndLoc=*/T.getCloseLocation(),
-                          ParamInfo, EllipsisLoc);
-
-    // Parse requires-clause[opt].
-    if (Tok.is(tok::kw_requires))
-      ParseTrailingRequiresClause(D);
-  } else if (Tok.isOneOf(tok::kw_mutable, tok::arrow, tok::kw___attribute,
-                         tok::kw_constexpr, tok::kw_consteval,
-                         tok::kw___private, tok::kw___global, tok::kw___local,
-                         tok::kw___constant, tok::kw___generic,
-                         tok::kw_requires, tok::kw_noexcept) ||
-             (Tok.is(tok::l_square) && NextToken().is(tok::l_square))) {
-    if (!getLangOpts().CPlusPlus2b)
+  if (Tok.isOneOf(tok::kw_mutable, tok::arrow, tok::kw___attribute,
+                  tok::kw_constexpr, tok::kw_consteval, tok::kw___private,
+                  tok::kw___global, tok::kw___local, tok::kw___constant,
+                  tok::kw___generic, tok::kw_requires, tok::kw_noexcept) ||
+    HasSpecifiers = true;
+    if (!HasParentheses && !getLangOpts().CPlusPlus2b) {
       // It's common to forget that one needs '()' before 'mutable', an
       // attribute specifier, the result type, or the requires clause. Deal with
       // this.
       Diag(Tok, diag::ext_lambda_missing_parens)
           << FixItHint::CreateInsertion(Tok.getLocation(), "() ");
+    }
+  }
 
-    SourceLocation NoLoc;
-    // Parse lambda-specifiers.
-    std::vector<DeclaratorChunk::ParamInfo> EmptyParamInfo;
-    ParseLambdaSpecifiers(/*LParenLoc=*/NoLoc, /*RParenLoc=*/NoLoc,
-                          EmptyParamInfo, /*EllipsisLoc=*/NoLoc);
+  if (HasParentheses || HasSpecifiers) {
+    ParseConstexprAndMutableSpecifiers();
   }
 
+  Actions.ActOnLambdaClosureQualifiers(Intro, MutableLoc, DeclEndLoc, DS);
+
+  if (HasSpecifiers || HasParentheses)
+    ParseLambdaSpecifiers(ParamInfo, EllipsisLoc);
+
   WarnIfHasCUDATargetAttr();
 
+  Prototype.Exit();
+
   // FIXME: Rename BlockScope -> ClosureScope if we decide to continue using
   // it.
   unsigned ScopeFlags = Scope::BlockScope | Scope::FnScope | Scope::DeclScope |
                         Scope::CompoundStmtScope;
   ParseScope BodyScope(this, ScopeFlags);
 
   Actions.ActOnStartOfLambdaDefinition(Intro, D, getCurScope());
 
   // Parse compound-statement.
   if (!Tok.is(tok::l_brace)) {
     Diag(Tok, diag::err_expected_lambda_body);
     Actions.ActOnLambdaError(LambdaBeginLoc, getCurScope());
     return ExprError();
   }
 
   StmtResult Stmt(ParseCompoundStatementBody());
   BodyScope.Exit();
   TemplateParamScope.Exit();
+  LambdaScope.Exit();
 
   if (!Stmt.isInvalid() && !TrailingReturnType.isInvalid())
     return Actions.ActOnLambdaExpr(LambdaBeginLoc, Stmt.get(), getCurScope());
 
   Actions.ActOnLambdaError(LambdaBeginLoc, getCurScope());
   return ExprError();
 }
 

clang/lib/Sema/Scope.cpp

if (Flags & (ClassScope | FnScope)) {
     MSCurManglingNumber = 1;
   }
   if (flags & BreakScope)         BreakParent = this;
   if (flags & ContinueScope)      ContinueParent = this;
   if (flags & BlockScope)         BlockParent = this;
   if (flags & TemplateParamScope) TemplateParamParent = this;
 
   // If this is a prototype scope, record that.
-  if (flags & FunctionPrototypeScope) PrototypeDepth++;
+  // Lambdas have an extra prototype scope that doesn't add any depth
+  if (flags & FunctionPrototypeScope && !(flags & LambdaScope))
+    PrototypeDepth++;
 
   if (flags & DeclScope) {
     if (flags & FunctionPrototypeScope)
       ; // Prototype scopes are uninteresting.
     else if ((flags & ClassScope) && getParent()->isClassScope())
       ; // Nested class scopes aren't ambiguous.
     else if ((flags & ClassScope) && getParent()->getFlags() == DeclScope)
       ; // Classes inside of namespaces aren't ambiguous.

clang/lib/Sema/Sema.cpp

for (int e = FunctionScopes.size() - 1; e >= 0; --e) {
     return FunctionScopes[e];
   }
   return nullptr;
 }
 
 LambdaScopeInfo *Sema::getEnclosingLambda() const {
   for (auto *Scope : llvm::reverse(FunctionScopes)) {
     if (auto *LSI = dyn_cast<sema::LambdaScopeInfo>(Scope)) {
-      if (LSI->Lambda && !LSI->Lambda->Encloses(CurContext)) {
+      if (LSI->Lambda && !LSI->Lambda->Encloses(CurContext) &&
+          !LSI->BeforeLambdaQualifiersScope) {
         // We have switched contexts due to template instantiation.
         // FIXME: We should swap out the FunctionScopes during code synthesis
         // so that we don't need to check for this.
         assert(!CodeSynthesisContexts.empty());
         return nullptr;
       }
       return LSI;
     }
LambdaScopeInfo *Sema::getCurLambda(bool IgnoreNonLambdaCapturingScope) {
   if (IgnoreNonLambdaCapturingScope) {
     auto E = FunctionScopes.rend();
     while (I != E && isa<CapturingScopeInfo>(*I) && !isa<LambdaScopeInfo>(*I))
       ++I;
     if (I == E)
       return nullptr;
   }
   auto *CurLSI = dyn_cast<LambdaScopeInfo>(*I);
-  if (CurLSI && CurLSI->Lambda &&
-      !CurLSI->Lambda->Encloses(CurContext)) {
+  if (CurLSI && CurLSI->Lambda && CurLSI->CallOperator &&
+      !CurLSI->Lambda->Encloses(CurContext) &&
+      !CurLSI->BeforeLambdaQualifiersScope) {
     // We have switched contexts due to template instantiation.
     assert(!CodeSynthesisContexts.empty());
     return nullptr;
   }
 
   return CurLSI;
 }
 

clang/lib/Sema/SemaCXXScopeSpec.cpp

bool Sema::ActOnCXXGlobalScopeSpecifier(SourceLocation CCLoc,
                                         CXXScopeSpec &SS) {
   SS.MakeGlobal(Context, CCLoc);
   return false;
 }
 
 bool Sema::ActOnSuperScopeSpecifier(SourceLocation SuperLoc,
                                     SourceLocation ColonColonLoc,
                                     CXXScopeSpec &SS) {
+  if (getCurLambda()) {
+    Diag(SuperLoc, diag::err_super_in_lambda_unsupported);
+    return true;
+  }
+
   CXXRecordDecl *RD = nullptr;
   for (Scope *S = getCurScope(); S; S = S->getParent()) {
     if (S->isFunctionScope()) {
       if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(S->getEntity()))
         RD = MD->getParent();
       break;
     }
     if (S->isClassScope()) {
       RD = cast<CXXRecordDecl>(S->getEntity());
       break;
     }
   }
 
   if (!RD) {
     Diag(SuperLoc, diag::err_invalid_super_scope);
     return true;
-  } else if (RD->isLambda()) {
-    Diag(SuperLoc, diag::err_super_in_lambda_unsupported);
-    return true;
   } else if (RD->getNumBases() == 0) {
     Diag(SuperLoc, diag::err_no_base_classes) << RD->getName();
     return true;
   }
 
   SS.MakeSuper(Context, RD, SuperLoc, ColonColonLoc);
   return false;
 }

clang/lib/Sema/SemaDecl.cpp

static void RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator,
 
   if (LCD == LCD_None)
     LSI->ImpCaptureStyle = CapturingScopeInfo::ImpCap_None;
   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();
 
   // 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()) {

clang/lib/Sema/SemaExpr.cpp

#include "clang/AST/DeclNodes.inc"
 
   case Decl::Binding: {
     // These are always lvalues.
     valueKind = VK_LValue;
     type = type.getNonReferenceType();
     // FIXME: Support lambda-capture of BindingDecls, once CWG actually
     // decides how that's supposed to work.
     auto *BD = cast<BindingDecl>(VD);
-    if (BD->getDeclContext() != CurContext) {
+    if (BD->getDeclContext() != CurContext && !isUnevaluatedContext()) {
       auto *DD = dyn_cast_or_null<VarDecl>(BD->getDecomposedDecl());
       if (DD && DD->hasLocalStorage())
         diagnoseUncapturableValueReference(*this, Loc, BD);
     }
     break;
   }
 
   case Decl::Function: {
if (llvm::none_of(LSI->Captures, [](Capture &C) {
       })) {
     FixBuffer.assign({"&", Separator});
     Sema.Diag(DefaultInsertLoc, diag::note_lambda_default_capture_fixit)
         << /*reference*/ 1
         << FixItHint::CreateInsertion(DefaultInsertLoc, FixBuffer);
   }
 }
 
+static bool CheckCaptureUseBeforeLambdaQualifiers(Sema &S, VarDecl *Var,
+                                                  SourceLocation ExprLoc,
+                                                  LambdaScopeInfo *LSI) {
+  bool ByCopy = LSI->ImpCaptureStyle == LambdaScopeInfo::ImpCap_LambdaByval;
+  SourceLocation Loc = LSI->IntroducerRange.getBegin();
+  bool Explicitly = false;
+  for (auto &&C : LSI->DelayedCaptures) {
+    VarDecl *CV = C.second.Var;
+    if (Var != CV)
+      continue;
+    ByCopy = C.second.Kind == LambdaCaptureKind::LCK_ByCopy;
+    Loc = C.second.Loc;
+    Explicitly = 1;
+    break;
+  }
+  if (ByCopy && LSI->BeforeLambdaQualifiersScope) {
+    // This can only occur in a non-ODR context, so we need to diagnose eagerly,
+    // even when BuildAndDiagnose is false
+    S.Diag(ExprLoc, diag::err_lambda_used_before_capture) << Var;
+    S.Diag(Loc, diag::note_var_explicitly_captured_here) << Var << Explicitly;
+    if (!Var->isInitCapture())
+      S.Diag(Var->getBeginLoc(), diag::note_entity_declared_at) << Var;
+    Var->setInvalidDecl();
+    return false;
+  }
+  return true;
+}
+
 bool Sema::tryCaptureVariable(
     VarDecl *Var, SourceLocation ExprLoc, TryCaptureKind Kind,
     SourceLocation EllipsisLoc, bool BuildAndDiagnose, QualType &CaptureType,
     QualType &DeclRefType, const unsigned *const FunctionScopeIndexToStopAt) {
   // An init-capture is notionally from the context surrounding its
   // declaration, but its parent DC is the lambda class.
   DeclContext *VarDC = Var->getDeclContext();
   if (Var->isInitCapture())
     VarDC = VarDC->getParent();
 
   DeclContext *DC = CurContext;
   const unsigned MaxFunctionScopesIndex = FunctionScopeIndexToStopAt
       ? *FunctionScopeIndexToStopAt : FunctionScopes.size() - 1;
   // We need to sync up the Declaration Context with the
   // FunctionScopeIndexToStopAt
   if (FunctionScopeIndexToStopAt) {
     unsigned FSIndex = FunctionScopes.size() - 1;
     while (FSIndex != MaxFunctionScopesIndex) {
       DC = getLambdaAwareParentOfDeclContext(DC);
       --FSIndex;
     }
   }
 
-
-  // If the variable is declared in the current context, there is no need to
-  // capture it.
-  if (VarDC == DC) return true;
-
   // Capture global variables if it is required to use private copy of this
   // variable.
   bool IsGlobal = !Var->hasLocalStorage();
   if (IsGlobal &&
       !(LangOpts.OpenMP && isOpenMPCapturedDecl(Var, /*CheckScopeInfo=*/true,
                                                 MaxFunctionScopesIndex)))
     return true;
   Var = Var->getCanonicalDecl();
 
   // Walk up the stack to determine whether we can capture the variable,
   // performing the "simple" checks that don't depend on type. We stop when
   // we've either hit the declared scope of the variable or find an existing
   // capture of that variable.  We start from the innermost capturing-entity
   // (the DC) and ensure that all intervening capturing-entities
   // (blocks/lambdas etc.) between the innermost capturer and the variable`s
   // declcontext can either capture the variable or have already captured
   // the variable.
   CaptureType = Var->getType();
   DeclRefType = CaptureType.getNonReferenceType();
   bool Nested = false;
   bool Explicit = (Kind != TryCapture_Implicit);
   unsigned FunctionScopesIndex = MaxFunctionScopesIndex;
+  bool IsInLambdaBeforeQualifiers;
   do {
+    IsInLambdaBeforeQualifiers = false;
+
+    LambdaScopeInfo *LSI = nullptr;
+    if (!FunctionScopes.empty())
+      LSI = dyn_cast_or_null<LambdaScopeInfo>(
+          FunctionScopes[FunctionScopesIndex]);
+    if (LSI && LSI->BeforeLambdaQualifiersScope) {
+      if (isa<ParmVarDecl>(Var))
+        return true;
+      IsInLambdaBeforeQualifiers = true;
+      if (!CheckCaptureUseBeforeLambdaQualifiers(*this, Var, ExprLoc, LSI)) {
+        break;
+      }
+    }
+
+    // If the variable is declared in the current context, there is no need to
+    // capture it.
+    if (!IsInLambdaBeforeQualifiers &&
+        FunctionScopesIndex == MaxFunctionScopesIndex && VarDC == DC)
+      return true;
+
     // Only block literals, captured statements, and lambda expressions can
     // capture; other scopes don't work.
-    DeclContext *ParentDC = getParentOfCapturingContextOrNull(DC, Var,
-                                                              ExprLoc,
-                                                              BuildAndDiagnose,
-                                                              *this);
+    DeclContext *ParentDC =
+        IsInLambdaBeforeQualifiers
+            ? DC->getParent()
+            : getParentOfCapturingContextOrNull(DC, Var, ExprLoc,
+                                                BuildAndDiagnose, *this);
     // We need to check for the parent *first* because, if we *have*
     // private-captured a global variable, we need to recursively capture it in
     // intermediate blocks, lambdas, etc.
     if (!ParentDC) {
       if (IsGlobal) {
         FunctionScopesIndex = MaxFunctionScopesIndex - 1;
         break;
       }
       return true;
     }
 
     FunctionScopeInfo  *FSI = FunctionScopes[FunctionScopesIndex];
     CapturingScopeInfo *CSI = cast<CapturingScopeInfo>(FSI);
 
-
     // Check whether we've already captured it.
-    if (isVariableAlreadyCapturedInScopeInfo(CSI, Var, Nested, CaptureType,
+    if (!IsInLambdaBeforeQualifiers &&
+        isVariableAlreadyCapturedInScopeInfo(CSI, Var, Nested, CaptureType,
                                              DeclRefType)) {
       CSI->getCapture(Var).markUsed(BuildAndDiagnose);
       break;
     }
     // If we are instantiating a generic lambda call operator body,
     // we do not want to capture new variables.  What was captured
     // during either a lambdas transformation or initial parsing
     // should be used.
-    if (isGenericLambdaCallOperatorSpecialization(DC)) {
+    if (!IsInLambdaBeforeQualifiers &&
+        isGenericLambdaCallOperatorSpecialization(DC)) {
       if (BuildAndDiagnose) {
         LambdaScopeInfo *LSI = cast<LambdaScopeInfo>(CSI);
         if (LSI->ImpCaptureStyle == CapturingScopeInfo::ImpCap_None) {
           Diag(ExprLoc, diag::err_lambda_impcap) << Var;
           Diag(Var->getLocation(), diag::note_previous_decl) << Var;
           Diag(LSI->Lambda->getBeginLoc(), diag::note_lambda_decl);
           buildLambdaCaptureFixit(*this, LSI, Var);
         } else
           diagnoseUncapturableValueReference(*this, ExprLoc, Var);
       }
       return true;
     }
 
     // Try to capture variable-length arrays types.
-    if (Var->getType()->isVariablyModifiedType()) {
+    if (!IsInLambdaBeforeQualifiers &&
+        Var->getType()->isVariablyModifiedType()) {
       // We're going to walk down into the type and look for VLA
       // expressions.
       QualType QTy = Var->getType();
       if (ParmVarDecl *PVD = dyn_cast_or_null<ParmVarDecl>(Var))
         QTy = PVD->getOriginalType();
       captureVariablyModifiedType(Context, QTy, CSI);
     }
 
-    if (getLangOpts().OpenMP) {
+    if (!IsInLambdaBeforeQualifiers && getLangOpts().OpenMP) {
       if (auto *RSI = dyn_cast<CapturedRegionScopeInfo>(CSI)) {
         // OpenMP private variables should not be captured in outer scope, so
         // just break here. Similarly, global variables that are captured in a
         // target region should not be captured outside the scope of the region.
         if (RSI->CapRegionKind == CR_OpenMP) {
           OpenMPClauseKind IsOpenMPPrivateDecl = isOpenMPPrivateDecl(
               Var, RSI->OpenMPLevel, RSI->OpenMPCaptureLevel);
           // If the variable is private (i.e. not captured) and has variably
if (CSI->ImpCaptureStyle == CapturingScopeInfo::ImpCap_None && !Explicit) {
         // explicitly.  Suggestion:
         //  - create const bool VariableCaptureWasInitiallyExplicit = Explicit
         //    at the function head
         //  - cache the StartingDeclContext - this must be a lambda
         //  - captureInLambda in the innermost lambda the variable.
       }
       return true;
     }
-
+    Explicit = false;
     FunctionScopesIndex--;
+    if (!IsInLambdaBeforeQualifiers)
       DC = ParentDC;
-    Explicit = false;
-  } while (!VarDC->Equals(DC));
+  } while (IsInLambdaBeforeQualifiers || !VarDC->Equals(DC));
 
   // Walk back down the scope stack, (e.g. from outer lambda to inner lambda)
   // computing the type of the capture at each step, checking type-specific
   // requirements, and adding captures if requested.
   // If the variable had already been captured previously, we start capturing
   // at the lambda nested within that one.
   bool Invalid = false;
   for (unsigned I = ++FunctionScopesIndex, N = MaxFunctionScopesIndex + 1; I != N;
if (BlockScopeInfo *BSI = dyn_cast<BlockScopeInfo>(CSI)) {
       Nested = true;
     } else if (CapturedRegionScopeInfo *RSI = dyn_cast<CapturedRegionScopeInfo>(CSI)) {
       Invalid = !captureInCapturedRegion(
           RSI, Var, ExprLoc, BuildAndDiagnose, CaptureType, DeclRefType, Nested,
           Kind, /*IsTopScope*/ I == N - 1, *this, Invalid);
       Nested = true;
     } else {
       LambdaScopeInfo *LSI = cast<LambdaScopeInfo>(CSI);
+      if (!CheckCaptureUseBeforeLambdaQualifiers(*this, Var, ExprLoc, LSI)) {
+        return true;
+      }
       Invalid =
           !captureInLambda(LSI, Var, ExprLoc, BuildAndDiagnose, CaptureType,
                            DeclRefType, Nested, Kind, EllipsisLoc,
                            /*IsTopScope*/ I == N - 1, *this, Invalid);
       Nested = true;
     }
 
     if (Invalid && !BuildAndDiagnose)

clang/lib/Sema/SemaExprCXX.cpp

for (int I = FunctionScopes.size();
 
     if (!CurLSI->isCXXThisCaptured())
         continue;
 
     auto C = CurLSI->getCXXThisCapture();
 
     if (C.isCopyCapture()) {
       ClassType.removeLocalCVRQualifiers(Qualifiers::CVRMask);
-      if (CurLSI->CallOperator->isConst())
+      if (!CurLSI->Mutable)
         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).

clang/lib/Sema/SemaLambda.cpp

 
 CXXRecordDecl *
 Sema::createLambdaClosureType(SourceRange IntroducerRange, TypeSourceInfo *Info,
                               unsigned LambdaDependencyKind,
                               LambdaCaptureDefault CaptureDefault) {
   DeclContext *DC = CurContext;
   while (!(DC->isFunctionOrMethod() || DC->isRecord() || DC->isFileContext()))
     DC = DC->getParent();
-  bool IsGenericLambda = getGenericLambdaTemplateParameterList(getCurLambda(),
-                                                               *this);
+
+  bool IsGenericLambda =
+      Info && getGenericLambdaTemplateParameterList(getCurLambda(), *this);
   // Start constructing the lambda class.
   CXXRecordDecl *Class = CXXRecordDecl::CreateLambda(
       Context, DC, Info, IntroducerRange.getBegin(), LambdaDependencyKind,
       IsGenericLambda, CaptureDefault);
   DC->addDecl(Class);
 
   return Class;
 }
return std::make_tuple(
         &Context.getManglingNumberContext(ASTContext::NeedExtraManglingDecl,
                                           ManglingContextDecl),
         ManglingContextDecl);
   }
 
   llvm_unreachable("unexpected context");
 }
 
-CXXMethodDecl *Sema::startLambdaDefinition(CXXRecordDecl *Class,
-                                           SourceRange IntroducerRange,
-                                           TypeSourceInfo *MethodTypeInfo,
-                                           SourceLocation EndLoc,
-                                           ArrayRef<ParmVarDecl *> Params,
-                                           ConstexprSpecKind ConstexprKind,
-                                           Expr *TrailingRequiresClause) {
+static QualType
+buildTypeForLambdaCallOperator(Sema &S, clang::CXXRecordDecl *Class,
+                               TemplateParameterList *TemplateParams,
+                               TypeSourceInfo *MethodTypeInfo) {
+  assert(MethodTypeInfo && "expected a non null type");
+
   QualType MethodType = MethodTypeInfo->getType();
-  TemplateParameterList *TemplateParams =
-      getGenericLambdaTemplateParameterList(getCurLambda(), *this);
   // If a lambda appears in a dependent context or is a generic lambda (has
   // template parameters) and has an 'auto' return type, deduce it to a
   // dependent type.
   if (Class->isDependentContext() || TemplateParams) {
     const FunctionProtoType *FPT = MethodType->castAs<FunctionProtoType>();
     QualType Result = FPT->getReturnType();
     if (Result->isUndeducedType()) {
-      Result = SubstAutoTypeDependent(Result);
-      MethodType = Context.getFunctionType(Result, FPT->getParamTypes(),
+      Result = S.SubstAutoTypeDependent(Result);
+      MethodType = S.Context.getFunctionType(Result, FPT->getParamTypes(),
                                              FPT->getExtProtoInfo());
     }
   }
+  return MethodType;
+}
+
+/// Start the definition of a lambda expression.
+/// In this overload, we do not know the type yet
+CXXMethodDecl *Sema::startLambdaDefinition(CXXRecordDecl *Class,
+                                           SourceRange IntroducerRange,
+                                           TypeSourceInfo *MethodTypeInfo,
+                                           SourceLocation EndLoc,
+                                           ArrayRef<ParmVarDecl *> Params,
+                                           ConstexprSpecKind ConstexprKind,
+                                           Expr *TrailingRequiresClause) {
+
+  LambdaScopeInfo *LSI = getCurLambda();
+
+  TemplateParameterList *TemplateParams =
+      getGenericLambdaTemplateParameterList(LSI, *this);
+
+  // At this point, we may not know the type of the lambda, if we have not
+  // parsed a trailing return type yet
+  QualType MethodType = MethodTypeInfo
+                            ? buildTypeForLambdaCallOperator(
+                                  *this, Class, TemplateParams, MethodTypeInfo)
+                            : QualType();
 
   // C++11 [expr.prim.lambda]p5:
   //   The closure type for a lambda-expression has a public inline function
-  //   call operator (13.5.4) whose parameters and return type are described by
-  //   the lambda-expression's parameter-declaration-clause and
+  //   call operator (13.5.4) whose parameters and return type are described
+  //   by the lambda-expression's parameter-declaration-clause and
   //   trailing-return-type respectively.
-  DeclarationName MethodName
-    = Context.DeclarationNames.getCXXOperatorName(OO_Call);
+  DeclarationName MethodName =
+      Context.DeclarationNames.getCXXOperatorName(OO_Call);
   DeclarationNameLoc MethodNameLoc =
       DeclarationNameLoc::makeCXXOperatorNameLoc(IntroducerRange);
   CXXMethodDecl *Method = CXXMethodDecl::Create(
       Context, Class, EndLoc,
       DeclarationNameInfo(MethodName, IntroducerRange.getBegin(),
                           MethodNameLoc),
       MethodType, MethodTypeInfo, SC_None, getCurFPFeatures().isFPConstrained(),
       /*isInline=*/true, ConstexprKind, EndLoc, TrailingRequiresClause);
   Method->setAccess(AS_public);
   if (!TemplateParams)
     Class->addDecl(Method);
 
   // Temporarily set the lexical declaration context to the current
   // context, so that the Scope stack matches the lexical nesting.
   Method->setLexicalDeclContext(CurContext);
   // Create a function template if we have a template parameter list
-  FunctionTemplateDecl *const TemplateMethod = TemplateParams ?
-            FunctionTemplateDecl::Create(Context, Class,
-                                         Method->getLocation(), MethodName,
-                                         TemplateParams,
-                                         Method) : nullptr;
+  FunctionTemplateDecl *const TemplateMethod =
+      TemplateParams
+          ? FunctionTemplateDecl::Create(Context, Class, Method->getLocation(),
+                                         MethodName, TemplateParams, Method)
+          : nullptr;
   if (TemplateMethod) {
     TemplateMethod->setAccess(AS_public);
     Method->setDescribedFunctionTemplate(TemplateMethod);
     Class->addDecl(TemplateMethod);
     TemplateMethod->setLexicalDeclContext(CurContext);
   }
 
   // Add parameters.
void Sema::handleLambdaNumbering(
   if (MCtx) {
     unsigned ManglingNumber = MCtx->getManglingNumber(Method);
     Class->setLambdaMangling(ManglingNumber, ManglingContextDecl,
                              HasKnownInternalLinkage);
     Class->setDeviceLambdaManglingNumber(MCtx->getDeviceManglingNumber(Method));
   }
 }
 
-void Sema::buildLambdaScope(LambdaScopeInfo *LSI,
-                                        CXXMethodDecl *CallOperator,
-                                        SourceRange IntroducerRange,
-                                        LambdaCaptureDefault CaptureDefault,
-                                        SourceLocation CaptureDefaultLoc,
-                                        bool ExplicitParams,
-                                        bool ExplicitResultType,
-                                        bool Mutable) {
+static void buildLambdaScopeReturnType(Sema &S, LambdaScopeInfo *LSI,
+                                       CXXMethodDecl *CallOperator,
+                                       bool ExplicitResultType) {
+  if (ExplicitResultType) {
+    LSI->HasImplicitReturnType = false;
+    LSI->ReturnType = CallOperator->getReturnType();
+    if (!LSI->ReturnType->isDependentType() && !LSI->ReturnType->isVoidType()) {
+      S.RequireCompleteType(CallOperator->getBeginLoc(), LSI->ReturnType,
+                            diag::err_lambda_incomplete_result);
+    }
+  } else {
+    LSI->HasImplicitReturnType = true;
+  }
+}
+
+static void buildLambdaScopeCaptures(LambdaScopeInfo *LSI,
+                                     bool ExplicitParams, bool Mutable) {
   LSI->CallOperator = CallOperator;
   CXXRecordDecl *LambdaClass = CallOperator->getParent();
   LSI->Lambda = LambdaClass;
   if (CaptureDefault == LCD_ByCopy)
     LSI->ImpCaptureStyle = LambdaScopeInfo::ImpCap_LambdaByval;
   else if (CaptureDefault == LCD_ByRef)
     LSI->ImpCaptureStyle = LambdaScopeInfo::ImpCap_LambdaByref;
   LSI->CaptureDefaultLoc = CaptureDefaultLoc;
   LSI->IntroducerRange = IntroducerRange;
   LSI->ExplicitParams = ExplicitParams;
   LSI->Mutable = Mutable;
-
-  if (ExplicitResultType) {
-    LSI->ReturnType = CallOperator->getReturnType();
-
-    if (!LSI->ReturnType->isDependentType() &&
-        !LSI->ReturnType->isVoidType()) {
-      if (RequireCompleteType(CallOperator->getBeginLoc(), LSI->ReturnType,
-                              diag::err_lambda_incomplete_result)) {
-        // Do nothing.
-      }
-    }
-  } else {
-    LSI->HasImplicitReturnType = true;
-  }
+
+void Sema::buildLambdaScope(LambdaScopeInfo *LSI, CXXMethodDecl *CallOperator,
+                            SourceRange IntroducerRange,
+                            LambdaCaptureDefault CaptureDefault,
+                            SourceLocation CaptureDefaultLoc,
+                            bool ExplicitParams, bool ExplicitResultType,
+                            bool Mutable) {
+  buildLambdaScopeCaptures(LSI, CallOperator, IntroducerRange, CaptureDefault,
+                           CaptureDefaultLoc, ExplicitParams, Mutable);
+  buildLambdaScopeReturnType(*this, LSI, CallOperator, ExplicitResultType);
 }
 
 void Sema::finishLambdaExplicitCaptures(LambdaScopeInfo *LSI) {
   LSI->finishedExplicitCaptures();
 }
 
-void Sema::ActOnLambdaExplicitTemplateParameterList(SourceLocation LAngleLoc,
-                                                    ArrayRef<NamedDecl *> TParams,
-                                                    SourceLocation RAngleLoc,
-                                                    ExprResult RequiresClause) {
+void Sema::ActOnLambdaExplicitTemplateParameterList(
+    LambdaIntroducer &Intro, SourceLocation LAngleLoc,
+    ArrayRef<NamedDecl *> TParams, SourceLocation RAngleLoc,
   LambdaScopeInfo *LSI = getCurLambda();
   assert(LSI && "Expected a lambda scope");
   assert(LSI->NumExplicitTemplateParams == 0 &&
          "Already acted on explicit template parameters");
   assert(LSI->TemplateParams.empty() &&
          "Explicit template parameters should come "
          "before invented (auto) ones");
   assert(!TParams.empty() &&
          "No template parameters to act on");
   LSI->TemplateParams.append(TParams.begin(), TParams.end());
   LSI->NumExplicitTemplateParams = TParams.size();
   LSI->ExplicitTemplateParamsRange = {LAngleLoc, RAngleLoc};
   LSI->RequiresClause = RequiresClause;
 }
 
-void Sema::addLambdaParameters(
-    ArrayRef<LambdaIntroducer::LambdaCapture> Captures,
-    CXXMethodDecl *CallOperator, Scope *CurScope) {
-  // Introduce our parameters into the function scope
-  for (unsigned p = 0, NumParams = CallOperator->getNumParams();
-       p < NumParams; ++p) {
-    ParmVarDecl *Param = CallOperator->getParamDecl(p);
-
-    // If this has an identifier, add it to the scope stack.
-    if (CurScope && Param->getIdentifier()) {
-      bool Error = false;
-      // Resolution of CWG 2211 in C++17 renders shadowing ill-formed, but we
-      // retroactively apply it.
-      for (const auto &Capture : Captures) {
-        if (Capture.Id == Param->getIdentifier()) {
-          Error = true;
-          Diag(Param->getLocation(), diag::err_parameter_shadow_capture);
-          Diag(Capture.Loc, diag::note_var_explicitly_captured_here)
-              << Capture.Id << true;
-        }
-      }
-      if (!Error)
-        CheckShadow(CurScope, Param);
-
-      PushOnScopeChains(Param, CurScope);
-    }
-  }
-}
-
 /// If this expression is an enumerator-like expression of some type
 /// T, return the type T; otherwise, return null.
 ///
 /// Pointer comparisons on the result here should always work because
 /// it's derived from either the parent of an EnumConstantDecl
 /// (i.e. the definition) or the declaration returned by
 /// EnumType::getDecl() (i.e. the definition).
 static EnumDecl *findEnumForBlockReturn(Expr *E) {
QualType Sema::buildLambdaInitCaptureInitialization(
 
   if (Result.isInvalid())
     return QualType();
 
   Init = Result.getAs<Expr>();
   return DeducedType;
 }
 
-VarDecl *Sema::createLambdaInitCaptureVarDecl(SourceLocation Loc,
-                                              QualType InitCaptureType,
-                                              SourceLocation EllipsisLoc,
-                                              IdentifierInfo *Id,
-                                              unsigned InitStyle, Expr *Init) {
+VarDecl *Sema::createLambdaInitCaptureVarDecl(
+    SourceLocation Loc, QualType InitCaptureType, SourceLocation EllipsisLoc,
+    IdentifierInfo *Id, unsigned InitStyle, Expr *Init, DeclContext *DeclCtx) {
   // FIXME: Retain the TypeSourceInfo from buildLambdaInitCaptureInitialization
   // rather than reconstructing it here.
   TypeSourceInfo *TSI = Context.getTrivialTypeSourceInfo(InitCaptureType, Loc);
   if (auto PETL = TSI->getTypeLoc().getAs<PackExpansionTypeLoc>())
     PETL.setEllipsisLoc(EllipsisLoc);
 
   // Create a dummy variable representing the init-capture. This is not actually
   // used as a variable, and only exists as a way to name and refer to the
   // init-capture.
   // FIXME: Pass in separate source locations for '&' and identifier.
-  VarDecl *NewVD = VarDecl::Create(Context, CurContext, Loc,
-                                   Loc, Id, InitCaptureType, TSI, SC_Auto);
+  VarDecl *NewVD = VarDecl::Create(Context, DeclCtx, Loc, Loc, Id,
+                                   InitCaptureType, TSI, SC_Auto);
   NewVD->setInitCapture(true);
   NewVD->setReferenced(true);
   // FIXME: Pass in a VarDecl::InitializationStyle.
   NewVD->setInitStyle(static_cast<VarDecl::InitializationStyle>(InitStyle));
   NewVD->markUsed(Context);
   NewVD->setInit(Init);
   if (NewVD->isParameterPack())
     getCurLambda()->LocalPacks.push_back(NewVD);
   return NewVD;
 }
 
 void Sema::addInitCapture(LambdaScopeInfo *LSI, VarDecl *Var) {
   assert(Var->isInitCapture() && "init capture flag should be set");
   LSI->addCapture(Var, /*isBlock*/false, Var->getType()->isReferenceType(),
                   /*isNested*/false, Var->getLocation(), SourceLocation(),
                   Var->getType(), /*Invalid*/false);
 }
 
-void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
-                                        Declarator &ParamInfo,
-                                        Scope *CurScope) {
-  LambdaScopeInfo *const LSI = getCurLambda();
-  assert(LSI && "LambdaScopeInfo should be on stack!");
-
-  // Determine if we're within a context where we know that the lambda will
-  // be dependent, because there are template parameters in scope.
-  CXXRecordDecl::LambdaDependencyKind LambdaDependencyKind =
-      CXXRecordDecl::LDK_Unknown;
-  if (LSI->NumExplicitTemplateParams > 0) {
-    auto *TemplateParamScope = CurScope->getTemplateParamParent();
-    assert(TemplateParamScope &&
-           "Lambda with explicit template param list should establish a "
-           "template param scope");
-    assert(TemplateParamScope->getParent());
-    if (TemplateParamScope->getParent()->getTemplateParamParent() != nullptr)
-      LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
-  } else if (CurScope->getTemplateParamParent() != nullptr) {
-    LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
-  }
+static LambdaScopeInfo *getCurrentLambdaScopeUnsafe(Sema &S) {
+  assert(!S.FunctionScopes.empty());
+  auto *CurLSI =
+      dyn_cast<LambdaScopeInfo>(S.FunctionScopes[S.FunctionScopes.size() - 1]);
+  assert(CurLSI);
+  return CurLSI;
 
-  // Determine the signature of the call operator.
-  TypeSourceInfo *MethodTyInfo;
-  bool ExplicitParams = true;
-  bool ExplicitResultType = true;
-  bool ContainsUnexpandedParameterPack = false;
-  SourceLocation EndLoc;
-  SmallVector<ParmVarDecl *, 8> Params;
-  if (ParamInfo.getNumTypeObjects() == 0) {
-    // C++11 [expr.prim.lambda]p4:
-    //   If a lambda-expression does not include a lambda-declarator, it is as
-    //   if the lambda-declarator were ().
-    FunctionProtoType::ExtProtoInfo EPI(Context.getDefaultCallingConvention(
-        /*IsVariadic=*/false, /*IsCXXMethod=*/true));
-    EPI.HasTrailingReturn = true;
-    EPI.TypeQuals.addConst();
-    LangAS AS = getDefaultCXXMethodAddrSpace();
-    if (AS != LangAS::Default)
-      EPI.TypeQuals.addAddressSpace(AS);
+static TypeSourceInfo *
+getDummyLambdaType(Sema &S, SourceLocation Loc = SourceLocation()) {
+  FunctionProtoType::ExtProtoInfo EPI(S.Context.getDefaultCallingConvention(
+  LangAS AS = S.getDefaultCXXMethodAddrSpace();
 
-    // C++1y [expr.prim.lambda]:
-    //   The lambda return type is 'auto', which is replaced by the
-    //   trailing-return type if provided and/or deduced from 'return'
-    //   statements
-    // We don't do this before C++1y, because we don't support deduced return
-    // types there.
-    QualType DefaultTypeForNoTrailingReturn =
-        getLangOpts().CPlusPlus14 ? Context.getAutoDeductType()
-                                  : Context.DependentTy;
-    QualType MethodTy =
-        Context.getFunctionType(DefaultTypeForNoTrailingReturn, None, EPI);
-    MethodTyInfo = Context.getTrivialTypeSourceInfo(MethodTy);
-    ExplicitParams = false;
-    ExplicitResultType = false;
-    EndLoc = Intro.Range.getEnd();
+  QualType DefaultTypeForNoTrailingReturn = S.getLangOpts().CPlusPlus14
+                                                ? S.Context.getAutoDeductType()
+                                                : S.Context.DependentTy;
+      S.Context.getFunctionType(DefaultTypeForNoTrailingReturn, None, EPI);
+  return S.Context.getTrivialTypeSourceInfo(MethodTy, Loc);
+}
+
+static TypeSourceInfo *getLambdaType(Sema &S, LambdaIntroducer &Intro,
+                                     Declarator &ParamInfo, Scope *CurScope,
+                                     SourceLocation Loc,
+                                     bool &ExplicitResultType) {
+
+
+  TypeSourceInfo *MethodTyInfo;
+
+  if (ParamInfo.getNumTypeObjects() == 0) {
+    MethodTyInfo = getDummyLambdaType(S, Loc);
   } else {
-    assert(ParamInfo.isFunctionDeclarator() &&
-           "lambda-declarator is a function");
     DeclaratorChunk::FunctionTypeInfo &FTI = ParamInfo.getFunctionTypeInfo();
-
-    // C++11 [expr.prim.lambda]p5:
-    //   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. It is neither virtual nor declared volatile. [...]
+    ExplicitResultType = FTI.hasTrailingReturnType();
     if (!FTI.hasMutableQualifier()) {
-      FTI.getOrCreateMethodQualifiers().SetTypeQual(DeclSpec::TQ_const,
-                                                    SourceLocation());
+      FTI.getOrCreateMethodQualifiers().SetTypeQual(DeclSpec::TQ_const, Loc);
     }
 
-    MethodTyInfo = GetTypeForDeclarator(ParamInfo, CurScope);
-    assert(MethodTyInfo && "no type from lambda-declarator");
-    EndLoc = ParamInfo.getSourceRange().getEnd();
+    MethodTyInfo = S.GetTypeForDeclarator(ParamInfo, CurScope);
 
-    ExplicitResultType = FTI.hasTrailingReturnType();
-
-    if (FTIHasNonVoidParameters(FTI)) {
-      Params.reserve(FTI.NumParams);
-      for (unsigned i = 0, e = FTI.NumParams; i != e; ++i)
-        Params.push_back(cast<ParmVarDecl>(FTI.Params[i].Param));
-    }
+    assert(MethodTyInfo && "no type from lambda-declarator");
 
     // Check for unexpanded parameter packs in the method type.
     if (MethodTyInfo->getType()->containsUnexpandedParameterPack())
-      DiagnoseUnexpandedParameterPack(Intro.Range.getBegin(), MethodTyInfo,
-                                      UPPC_DeclarationType);
-  }
+      S.DiagnoseUnexpandedParameterPack(Intro.Range.getBegin(), MethodTyInfo,
+                                        S.UPPC_DeclarationType);
+  }
+  return MethodTyInfo;
 
-  CXXRecordDecl *Class = createLambdaClosureType(
-      Intro.Range, MethodTyInfo, LambdaDependencyKind, Intro.Default);
-  CXXMethodDecl *Method =
-      startLambdaDefinition(Class, Intro.Range, MethodTyInfo, EndLoc, Params,
-                            ParamInfo.getDeclSpec().getConstexprSpecifier(),
-                            ParamInfo.getTrailingRequiresClause());
-  if (ExplicitParams)
-    CheckCXXDefaultArguments(Method);
+static CXXMethodDecl *CreateMethod(Sema &S, SourceRange IntroducerRange,
+                                   CXXRecordDecl *Class) {
+  // C++11 [expr.prim.lambda]p5:
+  //   The closure type for a lambda-expression has a public inline function
+  //   call operator (13.5.4) whose parameters and return type are described
+  //   by the lambda-expression's parameter-declaration-clause and
+  //   trailing-return-type respectively.
+  DeclarationName MethodName =
+      S.Context.DeclarationNames.getCXXOperatorName(OO_Call);
+  DeclarationNameLoc MethodNameLoc =
+      DeclarationNameLoc::makeCXXOperatorNameLoc(IntroducerRange.getBegin());
+  CXXMethodDecl *Method = CXXMethodDecl::Create(
+      S.Context, Class, SourceLocation(),
+      DeclarationNameInfo(MethodName, IntroducerRange.getBegin(),
+                          MethodNameLoc),
+      QualType(), nullptr, SC_None, S.getCurFPFeatures().isFPConstrained(),
+      /*isInline=*/true, ConstexprSpecKind::Unspecified, SourceLocation(),
+      nullptr);
+  Method->setAccess(AS_public);
+  return Method;
+}
 
-  // This represents the function body for the lambda function, check if we
-  // have to apply optnone due to a pragma.
-  AddRangeBasedOptnone(Method);
+void Sema::ActOnLambdaIntroducer(LambdaIntroducer &Intro, Scope *CurrentScope) {
 
-  // code_seg attribute on lambda apply to the method.
-  if (Attr *A = getImplicitCodeSegOrSectionAttrForFunction(Method, /*IsDefinition=*/true))
-    Method->addAttr(A);
+  LambdaScopeInfo *const LSI = getCurLambda();
+  assert(LSI && "LambdaScopeInfo should be on stack!");
 
-  // Attributes on the lambda apply to the method.
-  ProcessDeclAttributes(CurScope, Method, ParamInfo);
+  if (Intro.Default == LCD_ByCopy)
+    LSI->ImpCaptureStyle = LambdaScopeInfo::ImpCap_LambdaByval;
+  else if (Intro.Default == LCD_ByRef)
+    LSI->ImpCaptureStyle = LambdaScopeInfo::ImpCap_LambdaByref;
+  LSI->CaptureDefaultLoc = Intro.DefaultLoc;
+  LSI->IntroducerRange = Intro.Range;
+  LSI->BeforeLambdaQualifiersScope = true;
 
-  // CUDA lambdas get implicit host and device attributes.
-  if (getLangOpts().CUDA)
-    CUDASetLambdaAttrs(Method);
+  assert(LSI->NumExplicitTemplateParams == 0);
 
-  // OpenMP lambdas might get assumumption attributes.
-  if (LangOpts.OpenMP)
-    ActOnFinishedFunctionDefinitionInOpenMPAssumeScope(Method);
+  // Determine if we're within a context where we know that the lambda will
+  // be dependent, because there are template parameters in scope.
+  CXXRecordDecl::LambdaDependencyKind LambdaDependencyKind =
+      CXXRecordDecl::LDK_Unknown;
+  if (LSI->NumExplicitTemplateParams > 0) {
+    auto *TemplateParamScope = CurScope->getTemplateParamParent();
+    assert(TemplateParamScope &&
+           "Lambda with explicit template param list should establish a "
+           "template param scope");
+    assert(TemplateParamScope->getParent());
+    if (TemplateParamScope->getParent()->getTemplateParamParent() != nullptr)
+      LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
+  } else if (CurScope->getTemplateParamParent() != nullptr) {
+    LambdaDependencyKind = CXXRecordDecl::LDK_AlwaysDependent;
+  }
 
-  // Number the lambda for linkage purposes if necessary.
-  handleLambdaNumbering(Class, Method);
+  CXXRecordDecl *Class = createLambdaClosureType(
+      Intro.Range, nullptr, LambdaDependencyKind, Intro.Default);
+  LSI->Lambda = Class;
 
-  // Introduce the function call operator as the current declaration context.
-  PushDeclContext(CurScope, Method);
+  // C++11 [expr.prim.lambda]p5:
+  //   The closure type for a lambda-expression has a public inline function
+  //   call operator (13.5.4) whose parameters and return type are described
+  //   by the lambda-expression's parameter-declaration-clause and
+  //   trailing-return-type respectively.
 
-  // Build the lambda scope.
-  buildLambdaScope(LSI, Method, Intro.Range, Intro.Default, Intro.DefaultLoc,
-                   ExplicitParams, ExplicitResultType, !Method->isConst());
+  CXXMethodDecl *Method = CreateMethod(*this, Intro.Range, Class);
+  LSI->CallOperator = Method;
+  Method->setLexicalDeclContext(CurContext);
 
-  // C++11 [expr.prim.lambda]p9:
-  //   A lambda-expression whose smallest enclosing scope is a block scope is a
-  //   local lambda expression; any other lambda expression shall not have a
-  //   capture-default or simple-capture in its lambda-introducer.
-  //
-  // For simple-captures, this is covered by the check below that any named
-  // entity is a variable that can be captured.
-  //
-  // For DR1632, we also allow a capture-default in any context where we can
-  // odr-use 'this' (in particular, in a default initializer for a non-static
-  // data member).
-  if (Intro.Default != LCD_None && !Class->getParent()->isFunctionOrMethod() &&
-      (getCurrentThisType().isNull() ||
-       CheckCXXThisCapture(SourceLocation(), /*Explicit*/true,
-                           /*BuildAndDiagnose*/false)))
-    Diag(Intro.DefaultLoc, diag::err_capture_default_non_local);
+  PushDeclContext(CurScope, Method);
+
+  bool ContainsUnexpandedParameterPack = false;
 
   // Distinct capture names, for diagnostics.
-  llvm::SmallSet<IdentifierInfo*, 8> CaptureNames;
+  llvm::SmallSet<IdentifierInfo *, 8> CaptureNames;
 
   // Handle explicit captures.
-  SourceLocation PrevCaptureLoc
-    = Intro.Default == LCD_None? Intro.Range.getBegin() : Intro.DefaultLoc;
+  SourceLocation PrevCaptureLoc =
+      Intro.Default == LCD_None ? Intro.Range.getBegin() : Intro.DefaultLoc;
+  unsigned I = 0;
   for (auto C = Intro.Captures.begin(), E = Intro.Captures.end(); C != E;
-       PrevCaptureLoc = C->Loc, ++C) {
+       PrevCaptureLoc = C->Loc, ++C, ++I) {
     if (C->Kind == LCK_This || C->Kind == LCK_StarThis) {
-      if (C->Kind == LCK_StarThis)
-        Diag(C->Loc, !getLangOpts().CPlusPlus17
-                             ? diag::ext_star_this_lambda_capture_cxx17
-                             : diag::warn_cxx14_compat_star_this_lambda_capture);
-
-      // C++11 [expr.prim.lambda]p8:
-      //   An identifier or this shall not appear more than once in a
-      //   lambda-capture.
-      if (LSI->isCXXThisCaptured()) {
-        Diag(C->Loc, diag::err_capture_more_than_once)
-            << "'this'" << SourceRange(LSI->getCXXThisCapture().getLocation())
-            << FixItHint::CreateRemoval(
-                   SourceRange(getLocForEndOfToken(PrevCaptureLoc), C->Loc));
-        continue;
-      }
-
-      // C++2a [expr.prim.lambda]p8:
-      //  If a lambda-capture includes a capture-default that is =,
-      //  each simple-capture of that lambda-capture shall be of the form
-      //  "&identifier", "this", or "* this". [ Note: The form [&,this] is
-      //  redundant but accepted for compatibility with ISO C++14. --end note ]
-      if (Intro.Default == LCD_ByCopy && C->Kind != LCK_StarThis)
-        Diag(C->Loc, !getLangOpts().CPlusPlus20
-                         ? diag::ext_equals_this_lambda_capture_cxx20
-                         : diag::warn_cxx17_compat_equals_this_lambda_capture);
-
-      // C++11 [expr.prim.lambda]p12:
-      //   If this is captured by a local lambda expression, its nearest
-      //   enclosing function shall be a non-static member function.
-      QualType ThisCaptureType = getCurrentThisType();
-      if (ThisCaptureType.isNull()) {
-        Diag(C->Loc, diag::err_this_capture) << true;
         continue;
-      }
-
-      CheckCXXThisCapture(C->Loc, /*Explicit=*/true, /*BuildAndDiagnose*/ true,
-                          /*FunctionScopeIndexToStopAtPtr*/ nullptr,
-                          C->Kind == LCK_StarThis);
-      if (!LSI->Captures.empty())
-        LSI->ExplicitCaptureRanges[LSI->Captures.size() - 1] = C->ExplicitRange;
-      continue;
-    }
-
     assert(C->Id && "missing identifier for capture");
-
     if (C->Init.isInvalid())
       continue;
 
     VarDecl *Var = nullptr;
     if (C->Init.isUsable()) {
       Diag(C->Loc, getLangOpts().CPlusPlus14
                        ? diag::warn_cxx11_compat_init_capture
                        : diag::ext_init_capture);
if (C->Init.isUsable()) {
         break;
       case LambdaCaptureInitKind::DirectInit:
         InitStyle = VarDecl::CallInit;
         break;
       case LambdaCaptureInitKind::ListInit:
         InitStyle = VarDecl::ListInit;
         break;
       }
       Var = createLambdaInitCaptureVarDecl(C->Loc, C->InitCaptureType.get(),
                                            C->EllipsisLoc, C->Id, InitStyle,
-                                           C->Init.get());
-      // C++1y [expr.prim.lambda]p11:
-      //   An init-capture behaves as if it declares and explicitly
-      //   captures a variable [...] whose declarative region is the
-      //   lambda-expression's compound-statement
-      if (Var)
-        PushOnScopeChains(Var, CurScope, false);
+                                           C->Init.get(), Method);
+      CheckShadow(CurrentScope, Var);
+      PushOnScopeChains(Var, CurrentScope, false);
     } else {
       assert(C->InitKind == LambdaCaptureInitKind::NoInit &&
              "init capture has valid but null init?");
 
       // C++11 [expr.prim.lambda]p8:
       //   If a lambda-capture includes a capture-default that is &, the
       //   identifiers in the lambda-capture shall not be preceded by &.
       //   If a lambda-capture includes a capture-default that is =, [...]
if (C->Init.isUsable()) {
       if (Var && DiagnoseUseOfDecl(Var, C->Loc))
         continue;
     }
 
     // C++11 [expr.prim.lambda]p8:
     //   An identifier or this shall not appear more than once in a
     //   lambda-capture.
     if (!CaptureNames.insert(C->Id).second) {
-      if (Var && LSI->isCaptured(Var)) {
+      auto FindVar = [LSI](VarDecl *V) {
+        auto it = std::find_if(
+            LSI->DelayedCaptures.begin(), LSI->DelayedCaptures.end(),
+            [&](auto &&Pair) { return Pair.second.Var == V; });
+        return it;
+      };
+      auto it = FindVar(Var);
+      if (Var && it != LSI->DelayedCaptures.end()) {
         Diag(C->Loc, diag::err_capture_more_than_once)
-            << C->Id << SourceRange(LSI->getCapture(Var).getLocation())
+            << C->Id << SourceRange(it->second.Loc)
             << FixItHint::CreateRemoval(
                    SourceRange(getLocForEndOfToken(PrevCaptureLoc), C->Loc));
       } else
         // Previous capture captured something different (one or both was
         // an init-cpature): no fixit.
         Diag(C->Loc, diag::err_capture_more_than_once) << C->Id;
       continue;
     }
if (C->EllipsisLoc.isValid()) {
                                    : SourceRange(C->Loc));
 
         // Just ignore the ellipsis.
       }
     } else if (Var->isParameterPack()) {
       ContainsUnexpandedParameterPack = true;
     }
 
-    if (C->Init.isUsable()) {
-      addInitCapture(LSI, Var);
+    if (Var)
+      LSI->DelayedCaptures[I] = LambdaScopeInfo::DelayedCapture{
+          Var, C->ExplicitRange.getBegin(), C->Kind};
+  }
+
+  LSI->ContainsUnexpandedParameterPack |= ContainsUnexpandedParameterPack;
+  PopDeclContext();
+}
+
+static void AddExplicitCapturesToContext(Sema &S, LambdaScopeInfo *LSI,
+                                         LambdaIntroducer &Intro) {
+  unsigned I = 0;
+  SourceLocation PrevCaptureLoc;
+  for (auto C = Intro.Captures.begin(), E = Intro.Captures.end(); C != E;
+       PrevCaptureLoc = C->Loc, ++C, ++I) {
+    if (C->Kind == LCK_This || C->Kind == LCK_StarThis) {
+      if (C->Kind == LCK_StarThis)
+        S.Diag(C->Loc, !S.getLangOpts().CPlusPlus17
+                           ? diag::ext_star_this_lambda_capture_cxx17
+                           : diag::warn_cxx14_compat_star_this_lambda_capture);
+
+      // C++11 [expr.prim.lambda]p8:
+      //   An identifier or this shall not appear more than once in a
+      //   lambda-capture.
+      if (LSI->isCXXThisCaptured()) {
+        S.Diag(C->Loc, diag::err_capture_more_than_once)
+            << "'this'" << SourceRange(LSI->getCXXThisCapture().getLocation())
+            << FixItHint::CreateRemoval(
+                   SourceRange(S.getLocForEndOfToken(PrevCaptureLoc), C->Loc));
+        continue;
+      }
+
+      // C++2a [expr.prim.lambda]p8:
+      //  If a lambda-capture includes a capture-default that is =,
+      //  each simple-capture of that lambda-capture shall be of the form
+      //  "&identifier", "this", or "* this". [ Note: The form [&,this] is
+      //  redundant but accepted for compatibility with ISO C++14. --end note ]
+      if (Intro.Default == LCD_ByCopy && C->Kind != LCK_StarThis)
+        S.Diag(C->Loc,
+               !S.getLangOpts().CPlusPlus20
+                   ? diag::ext_equals_this_lambda_capture_cxx20
+                   : diag::warn_cxx17_compat_equals_this_lambda_capture);
+
+      // C++11 [expr.prim.lambda]p12:
+      //   If this is captured by a local lambda expression, its nearest
+      //   enclosing function shall be a non-static member function.
+      QualType ThisCaptureType = S.getCurrentThisType();
+      if (ThisCaptureType.isNull()) {
+        S.Diag(C->Loc, diag::err_this_capture) << true;
+        continue;
+      }
+      S.CheckCXXThisCapture(C->Loc, true, true, nullptr,
+                            C->Kind == LCK_StarThis);
+    } else {
+      VarDecl *Var = LSI->DelayedCaptures[I].Var;
+      if (!Var)
+        continue;
+      if (Var->isInitCapture() && C->Init.isUsable()) {
+        S.addInitCapture(LSI, Var);
+        S.PushOnScopeChains(Var, S.getCurScope(), false);
       } else {
-      TryCaptureKind Kind = C->Kind == LCK_ByRef ? TryCapture_ExplicitByRef :
-                                                   TryCapture_ExplicitByVal;
-      tryCaptureVariable(Var, C->Loc, Kind, EllipsisLoc);
+        Sema::TryCaptureKind Kind = C->Kind == LCK_ByRef
+                                        ? Sema::TryCapture_ExplicitByRef
+                                        : Sema::TryCapture_ExplicitByVal;
+        S.tryCaptureVariable(Var, C->Loc, Kind, C->EllipsisLoc);
+      }
     }
     if (!LSI->Captures.empty())
       LSI->ExplicitCaptureRanges[LSI->Captures.size() - 1] = C->ExplicitRange;
   }
-  finishLambdaExplicitCaptures(LSI);
+  S.finishLambdaExplicitCaptures(LSI);
+}
 
-  LSI->ContainsUnexpandedParameterPack |= ContainsUnexpandedParameterPack;
+void Sema::ActOnLambdaClosureQualifiers(LambdaIntroducer &Intro,
+                                        SourceLocation MutableLoc,
+                                        SourceLocation EndLoc,
+                                        const DeclSpec &DS) {
+
+  LambdaScopeInfo *const LSI = getCurrentLambdaScopeUnsafe(*this);
+  LSI->Mutable = MutableLoc.isValid();
+  LSI->BeforeLambdaQualifiersScope = false;
+  LSI->CallOperator->setConstexprKind(DS.getConstexprSpecifier());
+
+  // C++11 [expr.prim.lambda]p9:
+  //   A lambda-expression whose smallest enclosing scope is a block scope is a
+  //   local lambda expression; any other lambda expression shall not have a
+  //   capture-default or simple-capture in its lambda-introducer.
+  //
+  // For simple-captures, this is covered by the check below that any named
+  // entity is a variable that can be captured.
+  //
+  // For DR1632, we also allow a capture-default in any context where we can
+  // odr-use 'this' (in particular, in a default initializer for a non-static
+  // data member).
+  if (Intro.Default != LCD_None &&
+      !LSI->Lambda->getParent()->isFunctionOrMethod() &&
+      (getCurrentThisType().isNull() ||
+       CheckCXXThisCapture(SourceLocation(), /*Explicit*/ true,
+                           /*BuildAndDiagnose*/ false)))
+    Diag(Intro.DefaultLoc, diag::err_capture_default_non_local);
+
+  PushDeclContext(CurScope, LSI->CallOperator);
+  AddExplicitCapturesToContext(*this, LSI, Intro);
+}
+
+void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
+                                        Declarator &ParamInfo,
+                                        Scope *CurScope) {
+
+  LambdaScopeInfo *const LSI = getCurrentLambdaScopeUnsafe(*this);
+
+  SmallVector<ParmVarDecl *, 8> Params;
+  bool ExplicitResultType;
+
+  SourceLocation TypeLoc, LambdaLoc;
+  if (ParamInfo.getNumTypeObjects() == 0) {
+    LambdaLoc = TypeLoc = Intro.Range.getEnd();
+  } else {
+    unsigned index;
+    ParamInfo.isFunctionDeclarator(index);
+    const auto &Object = ParamInfo.getTypeObject(index);
+    TypeLoc =
+        Object.Loc.isValid() ? Object.Loc : ParamInfo.getSourceRange().getEnd();
+    LambdaLoc = ParamInfo.getSourceRange().getEnd();
+  }
+
+  CXXRecordDecl *Class = LSI->Lambda;
+  CXXMethodDecl *Method = LSI->CallOperator;
+
+  if (ParamInfo.getTrailingRequiresClause())
+    Method->setTrailingRequiresClause(ParamInfo.getTrailingRequiresClause());
+
+  TemplateParameterList *TemplateParams =
+      getGenericLambdaTemplateParameterList(LSI, *this);
+
+  auto DC = Method->getLexicalDeclContext();
+  Method->setLexicalDeclContext(Class);
+  if (TemplateParams) {
+    FunctionTemplateDecl *const TemplateMethod = FunctionTemplateDecl::Create(
+        Context, Class, Method->getLocation(), Method->getDeclName(),
+        TemplateParams, Method);
+    TemplateMethod->setAccess(AS_public);
+    Method->setDescribedFunctionTemplate(TemplateMethod);
+    Class->addDecl(TemplateMethod);
+    TemplateMethod->setLexicalDeclContext(DC);
+  } else {
+    Class->addDecl(Method);
+  }
+  Method->setLexicalDeclContext(DC);
+  Class->setLambdaIsGeneric(TemplateParams);
+
+  TypeSourceInfo *MethodTyInfo = getLambdaType(
+      *this, Intro, ParamInfo, getCurScope(), TypeLoc, ExplicitResultType);
+
+  Class->setLambdaTypeInfo(MethodTyInfo);
+  Method->setInnerLocStart(LambdaLoc);
+  Method->setLocation(Intro.Range.getBegin());
+  Method->setTypeSourceInfo(MethodTyInfo);
+  Method->setType(buildTypeForLambdaCallOperator(*this, Class, TemplateParams,
+                                                 MethodTyInfo));
+  Method->setConstexprKind(ParamInfo.getDeclSpec().getConstexprSpecifier());
+  buildLambdaScopeReturnType(*this, LSI, Method, ExplicitResultType);
+
+  LSI->ExplicitParams = ParamInfo.getNumTypeObjects() != 0;
+
+  if (ParamInfo.isFunctionDeclarator() != 0 &&
+      !FTIHasSingleVoidParameter(ParamInfo.getFunctionTypeInfo())) {
+    const auto &FTI = ParamInfo.getFunctionTypeInfo();
+    Params.reserve(Params.size());
+    for (unsigned I = 0; I < FTI.NumParams; ++I) {
+      auto *Param = cast<ParmVarDecl>(FTI.Params[I].Param);
+      Param->setScopeInfo(0, Params.size());
+      Param->setOwningFunction(Method);
+      Params.push_back(Param);
+    }
+  }
+
+  ContextRAII ManglingContext(*this, Class->getDeclContext());
+
+  CheckParmsForFunctionDef(Params, /*CheckParameterNames=*/false);
 
-  // Add lambda parameters into scope.
-  addLambdaParameters(Intro.Captures, Method, CurScope);
+  if (LSI->ExplicitParams) {
+    Method->setParams(Params);
+    CheckCXXDefaultArguments(Method);
+  }
+
+  // This represents the function body for the lambda function, check if we
+  // have to apply optnone due to a pragma.
+  AddRangeBasedOptnone(Method);
+
+  // code_seg attribute on lambda apply to the method.
+  if (Attr *A = getImplicitCodeSegOrSectionAttrForFunction(
+          Method, /*IsDefinition=*/true))
+    Method->addAttr(A);
+
+  // Attributes on the lambda apply to the method.
+  ProcessDeclAttributes(CurScope, Method, ParamInfo);
+
+  // CUDA lambdas get implicit host and device attributes.
+  if (getLangOpts().CUDA)
+    CUDASetLambdaAttrs(Method);
+
+  // OpenMP lambdas might get assumumption attributes.
+  if (LangOpts.OpenMP)
+    ActOnFinishedFunctionDefinitionInOpenMPAssumeScope(Method);
+
+  handleLambdaNumbering(Class, Method);
+
+  ManglingContext.pop();
+
+  for (auto &&C : LSI->DelayedCaptures) {
+    VarDecl *Var = C.second.Var;
+    if (Var && Var->isInitCapture()) {
+      PushOnScopeChains(Var, CurScope, false);
+    }
+  }
+
+  LSI->DelayedCaptures.clear();
+
+  auto CheckRedefinition = [&](ParmVarDecl *Param) {
+    if (!Param->getIdentifier())
+      return true;
+    for (const auto &Capture : Intro.Captures) {
+      if (Capture.Id == Param->getIdentifier()) {
+        Diag(Param->getLocation(), diag::err_parameter_shadow_capture);
+        Diag(Capture.Loc, diag::note_var_explicitly_captured_here)
+            << Capture.Id << true;
+        return false;
+      }
+    }
+    return true;
+  };
+
+  for (auto &&P : Params) {
+    if (!P->getIdentifier())
+      continue;
+    if (CheckRedefinition(P))
+      CheckShadow(CurScope, P);
+    PushOnScopeChains(P, CurScope);
+  }
 
   // Enter a new evaluation context to insulate the lambda from any
   // cleanups from the enclosing full-expression.
   PushExpressionEvaluationContext(
       LSI->CallOperator->isConsteval()
           ? ExpressionEvaluationContext::ImmediateFunctionContext
           : ExpressionEvaluationContext::PotentiallyEvaluated);
 }

clang/lib/Sema/TreeTransform.h

if (E->isInitCapture(C)) {
         ExprResult Init = Info.first;
         QualType InitQualType = Info.second;
         if (Init.isInvalid() || InitQualType.isNull()) {
           Invalid = true;
           break;
         }
         VarDecl *NewVD = getSema().createLambdaInitCaptureVarDecl(
             OldVD->getLocation(), InitQualType, NewC.EllipsisLoc,
-            OldVD->getIdentifier(), OldVD->getInitStyle(), Init.get());
+            OldVD->getIdentifier(), OldVD->getInitStyle(), Init.get(),
+            getSema().CurContext);
         if (!NewVD) {
           Invalid = true;
           break;
         }
         NewVDs.push_back(NewVD);
         getSema().addInitCapture(LSI, NewVD);
       }
 

clang/test/CXX/expr/expr.prim/expr.prim.lambda/p11-1y.cpp

 // RUN: %clang_cc1 -std=c++1y %s -verify
 
 const char *has_no_member = [x("hello")] {}.x; // expected-error {{no member named 'x'}}
 
 double f;
 auto with_float = [f(1.0f)] {
   using T = decltype(f);
   using T = float;
 };
 auto with_float_2 = [&f(f)] { // ok, refers to outer f
   using T = decltype(f);
   using T = double&;
 };
 
-// Within the lambda-expression's compound-statement,
-// the identifier in the init-capture hides any declaration
-// of the same name in scopes enclosing the lambda-expression.
+// Within the lambda-expression the identifier in the init-capture
+// hides any declaration of the same name in scopes enclosing
+// the lambda-expression.
 void hiding() {
   char c;
   (void) [c("foo")] {
     static_assert(sizeof(c) == sizeof(const char*), "");
   };
-  (void) [c("bar")] () -> decltype(c) { // outer c, not init-capture
-    return "baz"; // expected-error {{cannot initialize}}
+  (void)[c("bar")]()->decltype(c) { // inner c
+    return "baz";
   };
 }
 
 struct ExplicitCopy {
   ExplicitCopy(); // expected-note 2{{not viable}}
   explicit ExplicitCopy(const ExplicitCopy&); // expected-note 2{{not a candidate}}
 };
 auto init_kind_1 = [ec(ExplicitCopy())] {};

clang/test/SemaCXX/lambda-capture-type-deduction.cpp

+// RUN: %clang_cc1 -std=c++2b -verify -fsyntax-only %s
+
+template <typename T, typename U>
+constexpr bool is_same = false;
+
+template <typename T>
+constexpr bool is_same<T, T> = true;
+
+void f() {
+
+  int y;
+
+  static_assert(is_same<const int &,
+                        decltype([x = 1] -> decltype((x)) { return x; }())>);
+
+  static_assert(is_same<int &,
+                        decltype([x = 1] mutable -> decltype((x)) { return x; }())>);
+
+  static_assert(is_same<const int &,
+                        decltype([=] -> decltype((y)) { return y; }())>);
+
+  static_assert(is_same<int &,
+                        decltype([=] mutable -> decltype((y)) { return y; }())>);
+
+  static_assert(is_same<const int &,
+                        decltype([=] -> decltype((y)) { return y; }())>);
+
+  static_assert(is_same<int &,
+                        decltype([=] mutable -> decltype((y)) { return y; }())>);
+
+  auto ref = [&x = y](
+                 decltype([&](decltype(x)) { return 0; }) y) {
+    return x;
+  };
+}
+
+void test_noexcept() {
+
+  int y;
+
+  static_assert(noexcept([x = 1] noexcept(is_same<const int &, decltype((x))>) {}()));
+  static_assert(noexcept([x = 1] mutable noexcept(is_same<int &, decltype((x))>) {}()));
+  static_assert(noexcept([y] noexcept(is_same<const int &, decltype((y))>) {}()));
+  static_assert(noexcept([y] mutable noexcept(is_same<int &, decltype((y))>) {}()));
+  static_assert(noexcept([=] noexcept(is_same<const int &, decltype((y))>) {}()));
+  static_assert(noexcept([=] mutable noexcept(is_same<int &, decltype((y))>) {}()));
+  static_assert(noexcept([&] noexcept(is_same<int &, decltype((y))>) {}()));
+  static_assert(noexcept([&] mutable noexcept(is_same<int &, decltype((y))>) {}()));
+
+  static_assert(noexcept([&] mutable noexcept(!is_same<int &, decltype((y))>) {}())); //expected-error {{static_assert failed due}}
+}
+
+void test_requires() {
+
+  int x;
+
+  [x = 1]() requires is_same<const int &, decltype((x))> {}
+  ();
+  [x = 1]() mutable requires is_same<int &, decltype((x))> {}
+  ();
+  [x]() requires is_same<const int &, decltype((x))> {}
+  ();
+  [x]() mutable requires is_same<int &, decltype((x))> {}
+  ();
+  [=]() requires is_same<const int &, decltype((x))> {}
+  ();
+  [=]() mutable requires is_same<int &, decltype((x))> {}
+  ();
+  [&]() requires is_same<int &, decltype((x))> {}
+  ();
+  [&]() mutable requires is_same<int &, decltype((x))> {}
+  ();
+  [&x]() requires is_same<int &, decltype((x))> {}
+  ();
+  [&x]() mutable requires is_same<int &, decltype((x))> {}
+  ();
+
+  [x = 1]() requires is_same<int &, decltype((x))> {} (); //expected-error {{no matching function for call to object of type}} \
+                                                           // expected-note {{candidate function not viable}} \
+                                                           // expected-note {{'is_same<int &, decltype((x))>' evaluated to false}}
+  [x = 1]() mutable requires is_same<const int &, decltype((x))> {} (); // expected-error {{no matching function for call to object of type}} \
+                                                                     // expected-note {{candidate function not viable}} \
+                                                                     // expected-note {{'is_same<const int &, decltype((x))>' evaluated to false}}
+}
+
+void err() {
+  int y, z;                // expected-note 2{{declared here}}
+  auto implicit_tpl = [=]( // expected-note {{variable 'y' is captured here}}
+                          decltype(
+                              [&]<decltype(y)> { return 0; }) y) { //expected-error{{captured variable 'y' cannot appear}}
+    return y;
+  };
+
+  auto init_tpl = [x = 1](                                          // expected-note{{explicitly captured here}}
+                      decltype([&]<decltype(x)> { return 0; }) y) { // expected-error {{captured variable 'x' cannot appear}}
+    return x;
+  };
+
+  auto implicit = [=]( // expected-note {{variable 'z' is captured here}}
+                      decltype(
+                          [&](decltype(z)) { return 0; }) z) { //expected-error{{captured variable 'z' cannot appear}}
+    return z;
+  };
+
+  auto init = [x = 1](                                          // expected-note{{explicitly captured here}}
+                  decltype([&](decltype(x)) { return 0; }) y) { // expected-error {{captured variable 'x' cannot appear}}
+    return x;
+  };
+}
+
+void nested() {
+  int x, y, z; //expected-note {{'x' declared here}} expected-note {{'z' declared here}}
+  (void)[&](
+      decltype([&](
+                   decltype([=]( // expected-note {{variable 'x' is captured here}}
+                                decltype([&](
+                                             decltype([&](decltype(x)) {}) // expected-error{{captured variable 'x' cannot appear}}
+                                         ) {})) {})) {})){};
+
+  (void)[&](
+      decltype([&](
+                   decltype([&](
+                                decltype([&](
+                                             decltype([&](decltype(y)) {})) {})) {})) {})){};
+
+  (void)[=](
+      decltype([=](
+                   decltype([=](
+                                decltype([=](                              // expected-note {{variable 'z' is captured here}}
+                                             decltype([&]<decltype(z)> {}) // expected-error{{captured variable 'z' cannot appear}}
+                                         ) {})) {})) {})){};
+}

clang/test/SemaCXX/warn-shadow-in-lambdas.cpp

 #else
     int var = 1; // expected-warning {{declaration shadows a local variable}}
 #endif
     { int z = 1; } // expected-warning {{declaration shadows a local variable}}
   };
 #ifdef AVOID
   auto l4 = [var = param] (int param) { ; }; // no warning
 #else
-  auto l4 = [var = param] (int param) { ; }; // expected-warning {{declaration shadows a local variable}}
+  auto l4 = [var = param](int param) { ; }; // expected-warning 2{{declaration shadows a local variable}}
 #endif
 
   // Make sure that inner lambdas work as well.
   auto l5 = [var, l1] { // expected-note {{variable 'l1' is explicitly captured here}}
     auto l1 = [] { // expected-warning {{declaration shadows a local variable}}
 #ifdef AVOID
       int var = 1; // no warning
 #else

clang/www/cxx_status.html

</tr>
     <tr>
       <td>Deducing this</td>
       <td><a href="https://wg21.link/P0847R7">P0847R7</a></td>
       <td class="none" align="center">No</td>
     </tr>
     <tr>
       <td>Change scope of lambda trailing-return-type</td>
       <td><a href="https://wg21.link/P2036R3">P2036R3</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 15</td>
     </tr>
     <tr>
       <td>Multidimensional subscript operator</td>
       <td><a href="https://wg21.link/P2128R6">P2128R6</a></td>
       <td class="unreleased" align="center">Clang 15</td>
     </tr>
     <tr>
       <td>Non-literal variables (and labels and gotos) in constexpr functions</td>