[C++2a] P0634r3: Down with typename!

[ayzhao]

Summary

This patch implements P0634r3 that removes the need for 'typename' in certain contexts.

For example,

template <typename T>
using foo = T::type; // ok

This is also allowed in previous language versions as an extension, because I think it's pretty useful. :)

[rsmith]

clang/lib/Sema/SemaTemplate.cpp

3995–3997

This FIXME is concerning. Is this a problem with this patch? (Is the FIXME wrong now?)

clang/test/CXX/drs/dr5xx.cpp

488–490

A comment here explaining that A and B stop being aggregates in C++20 would be nice. (Nicer would be changing the testcase so it still tests the relevant rule in C++20 mode, if that's possible...)

clang/test/CXX/temp/temp.res/p5.cpp

1 ↗

(On Diff #200440)

Please add tests for enum-base and conversion-type-id:

template<typename T> struct A {
  enum E : T::type {};
  operator T::type() {}
  void f() { this->operator T::type(); }
};

... which should currently be rejected.

[Rakete1111]

• rebased
• addressed review comments

clang/lib/Parse/ParseDecl.cpp

4754

Do you know why this isn't allowed in operator ids?

5655–5657

Yeah it does the break the second. Would just checking whether a friend is there be good enough? clang doesn't seem to actively propose to add a friend if there's one missing, so if we add a IsFriend parameter and then check if it is set than always return false, it should work. Is that acceptable? It would break the nice error message you get if you write friend C(int);, but if we restrict it when isDeclarationSpecifier return false (with Never) would that be better (that's what I'm doing right now)?

Another solution would be to tentatively parse the whole thing, but that can be pretty expensive I believe.

clang/lib/Sema/SemaTemplate.cpp

3995–3997

Yes you're right, I believe it not longer applies due to the change in ParseDecl.cpp in ParseDeclarationSpecifiers.

[Rakete1111]

clang/lib/Sema/SemaTemplate.cpp

3995–3997

Although I'm not that sure I fully understand what that comments alludes to.

[rsmith]

clang/lib/Parse/ParseDecl.cpp

4754

There's already been discussion of that on the core reflector, and people seem to agree it's an oversight in the wording. If you want to accept that here, I think that's fine, under the assumption that this will be fixed by DR. (If you want to follow the wording-as-moved, that's fine too.)

5655–5657

This seems a little fragile against future grammar changes, but I think the IsFriend check is correct -- I *think* the only way we can see a qualified-id here in a valid non-constructor, non-nested-name-specifier case is in a friend declaration.

clang/lib/Sema/SemaTemplate.cpp

3995–3997

The example would be something like this:

template<typename T> struct X {
  template<typename U> struct Y { Y(); using V = int; };
  template<typename U> typename Y<U>::V f();
};
template<typename T> template<typename U>
X<T>::Y<U>::V X<T>::f() {}

Clang trunk today points out that the typename keyword is missing on the final line, but fails to point out that the template keyword is also missing. The reason is that in the case where that construct is valid:

template<typename T> template<typename U>
X<T>::Y<U>::Y() {}

... we are "entering the context" of the nested-name-specifier, which means we don't need a template keyword.

If the FIXME is fixed, then we should diagnose the missing template in the above program.

Also, because we don't rebuild the nested-name-specifier as a dependent nested name specifier in this case, we fail to match it against the declaration in the class, so in my example above, we also produce an "out-of-line definition does not match" error.

A closely-related issue can be seen in an example such as:

template<typename T> struct X {
  template<typename U> struct Y { Y(); typedef void V; }; 
  template<typename U> typename Y<U>::V::W f();
};
template<typename T> template<typename U>
X<T>::template Y<U>::V::W X<T>::f() { return 0; }

Here, we produce a completely bogus error:

<stdin>:6:13: error: 'X::Y::V' (aka 'void') is not a class, namespace, or enumeration
X<T>::template Y<U>::V::W X<T>::f() { return 0; } 
            ^

... because we parse this in "entering context" mode and so resolve X<T>::Y<U>::V to the type in the primary template (that is, void). That's wrong: we should defer resolving this name to a type until we know what T and U are, or until we know that we're *actually* entering the context. Specifically, the above program can be extended as follows:

template<> template<> struct X<int>::Y<int> {
  struct V { using W = int; };
};
void call(X<int> x) { x.f<int>(); } // OK, f returns int

The above program should be accepted by this patch, if the FIXME is indeed now fixed. Please add it as a testcase :)

[Rakete1111]

clang/lib/Sema/SemaTemplate.cpp

3995–3997

Oh ok, got it thanks. So no, the program is still not accepted in clang, and a pretty dumb side effect of it is that

template<typename T> struct X {
  template<typename U> struct Y { Y(); using V = int; };
  template<typename U> typename Y<U>::V f();
};
template<typename T> template<typename U>
X<T>::Y<U>::V X<T>::f() {}

is accepted without the diagnostic for the missing template. :(

[vivekvpandya]

Minor suggestion, you may want to update http://clang.llvm.org/cxx_status.html page for "typename optional in more contexts" with in this change.

[cjdb]

What's the status of this patch?

[erichkeane]

In D53847#3252460, @cjdb wrote:

What's the status of this patch?

It appears to me that the 'delta' right now is a couple of cases where this patch doesn't properly diagnose/warn (see the convo in SemaTemplate.cpp )

[ilya-biryukov]

Hey! Also wondering what's the status of this.
@Rakete1111 do you plan to finish the patch? Or would it be ok if someone takes it over?

[erichkeane]

In D53847#3706965, @ilya-biryukov wrote:

Hey! Also wondering what's the status of this.
@Rakete1111 do you plan to finish the patch? Or would it be ok if someone takes it over?

Note that this would also let us mark P2324 as complete as well. @ilya-biryukov : Since there is no response, I suspect the answer here is someone should take this over. Were you going to?

[ilya-biryukov]

In D53847#3735704, @erichkeane wrote:

Note that this would also let us mark P2324 as complete as well. @ilya-biryukov : Since there is no response, I suspect the answer here is someone should take this over. Were you going to?

I could, but also busy with other things and happy to give this one away.
Would you be interested to take it over instead?

[erichkeane]

In D53847#3735738, @ilya-biryukov wrote:

In D53847#3735704, @erichkeane wrote:

Note that this would also let us mark P2324 as complete as well. @ilya-biryukov : Since there is no response, I suspect the answer here is someone should take this over. Were you going to?

I could, but also busy with other things and happy to give this one away.
Would you be interested to take it over instead?

Also very busy with other things, but was sorta looking into closing the partial P2324. I haven't dug into this one much in a while, but I was sort of wondering if this is a candidate for being 'split up' into smaller patches/handle 1 case at a time. If i have time next week, I might just start looking into making this work for single-examples, rather than everything.

[erichkeane]

In D53847#3735758, @erichkeane wrote:

In D53847#3735738, @ilya-biryukov wrote:

In D53847#3735704, @erichkeane wrote:

Note that this would also let us mark P2324 as complete as well. @ilya-biryukov : Since there is no response, I suspect the answer here is someone should take this over. Were you going to?

I could, but also busy with other things and happy to give this one away.
Would you be interested to take it over instead?

Also very busy with other things, but was sorta looking into closing the partial P2324. I haven't dug into this one much in a while, but I was sort of wondering if this is a candidate for being 'split up' into smaller patches/handle 1 case at a time. If i have time next week, I might just start looking into making this work for single-examples, rather than everything.

Welp, spoke too soon, my concepts stuff got reverted again, so I'll be workingon that for a while now, so I wont' have time to jump on this today.

[royjacobson]

I looked at this a bit about a week ago and got it down to 3-4 tests failing, but I'm not sure how much time I'll have to continue working on it. If one of you wants to take over I'll be happy to send you what I've currently got. @erichkeane @ilya-biryukov

Also, do you know if having posted this patch is agreement for licensing this code? Or do we need to get explicit agreement from the original author before committing a version of this?

[erichkeane]

In D53847#3745415, @royjacobson wrote:

I looked at this a bit about a week ago and got it down to 3-4 tests failing, but I'm not sure how much time I'll have to continue working on it. If one of you wants to take over I'll be happy to send you what I've currently got. @erichkeane @ilya-biryukov

Also, do you know if having posted this patch is agreement for licensing this code? Or do we need to get explicit agreement from the original author before committing a version of this?

I've never seen that be an issue before, and I don't see enough in https://llvm.org/docs/DeveloperPolicy.html#copyright-license-and-patents to be clear about the answer to that. @tstellar could perhaps answer.

[ayzhao]

In D53847#3745415, @royjacobson wrote:

I looked at this a bit about a week ago and got it down to 3-4 tests failing, but I'm not sure how much time I'll have to continue working on it. If one of you wants to take over I'll be happy to send you what I've currently got. @erichkeane @ilya-biryukov

Also, do you know if having posted this patch is agreement for licensing this code? Or do we need to get explicit agreement from the original author before committing a version of this?

I'm currently looking at this patch; please do send me what you have.

[ayzhao]

Update commit with contributions from @royjacobson

[jyknight]

Also, do you know if having posted this patch is agreement for licensing this code? Or do we need to get explicit agreement from the original author before committing a version of this?

I've never seen that be an issue before, and I don't see enough in https://llvm.org/docs/DeveloperPolicy.html#copyright-license-and-patents to be clear about the answer to that. @tstellar could perhaps answer.

Contributing the patch to phab was agreement to contribute to llvm under the llvm license, you do not need further permission to take over and finish and commit a patch from phab. Per the standard apache2 terms in the llvm license, "Unless You explicitly state otherwise, any Contribution intentionally submitted for inclusion in the Work by You to the Licensor shall be under the terms and conditions of this License, without any additional terms or conditions."

[shafik]

First set of comments.

clang/include/clang/Parse/Parser.h

2418

Why don't we just have isImplicitTypenameContext(...) return an ImplicitTypenameContext?

clang/include/clang/Sema/DeclSpec.h

1809

Since you use Yes for the positive option how about No for the negative option? Yes/Never feel like an odd pair.

clang/lib/Parse/ParseDecl.cpp

3610–3612

3611

clang/lib/Parse/ParseDeclCXX.cpp

1248–1249

1291–1292

1313–1314

clang/lib/Parse/ParseTentative.cpp

446

clang/lib/Parse/Parser.cpp

2035–2036

2036

clang/lib/Sema/DeclSpec.cpp

27

Duplicate include.

1472

Dead code?

[ayzhao]

address code review comments from shafik@

[ayzhao]

clang/lib/Parse/ParseDecl.cpp

5655–5657

I _think_ the first comment in this chain can be marked as done given that the test cases are now in p5.cpp and Clang compiles them without errors with the help of the IsFriend check.

[shafik]

Maybe I missed it but I don't see a test that covers temp.res p5 e.g.:

template <class T> void f(int i) {
  T::x * i;  // This will be assumed to be the expression
             //   T::x multiplied by i
             // Not a declaration of variable i of 
             //   type pointer to T::x?
}

struct Foo {
 typedef int x;
};

struct Bar {
  static int const x = 5;
};

int main() {
  f<Bar>(1);          // OK
  f<Foo>(1);          // error: Foo::x is a type
}

clang/include/clang/Sema/DeclSpec.h

1918

Why do we need this one shot previous lookup result? I am struggling to see the connection between isDeclaratorFunctionLike(...) and where we check hasPrevLookupResult(...) and why it matter.

clang/lib/Parse/ParseDecl.cpp

6484

Looks like there are a few of these left over as debugging lines, the should removed.

clang/lib/Sema/DeclSpec.cpp

1501–1503

clang/test/CXX/temp/temp.res/p5.cpp

1 ↗

(On Diff #458580)

It looks like this is temp.res p4 now

[ayzhao]

Fix failing test in p5.cpp where typename should be required for enum-base

(N.B. due to https://github.com/llvm/llvm-project/commit/c90e198107431f64b73686bdce31c293e3380ac7, the correct error should now be missing 'typename' rather than forbidden enum forward references)

[ayzhao]

remove obsolete FIXME

[ayzhao]

clang/lib/Parse/ParseDecl.cpp

4754

It seems like a wording oversight that we don't assume typename in an enum-base. Probably would be good to raise this on the core reflector.

Marking this as done for now; this patch currently returns an error if typename is assumed in an enum-base.

[ayzhao]

p5.cpp -> p4.cpp, removed debug statements

haven't yet fully grokked this patch. still looking into fixing test failures and PrevLookupResult.

[ayzhao]

Fix typename-specifier-3.cpp test

Things done:

• the warning should also include a check that C++20 is required
• the warning should only show up for precxx17, as %std_cxx17- translates to -std=c++2b

[ayzhao]

clang/lib/Sema/SemaDecl.cpp

362

Status update:

This line is currently causing p2-2.cpp to fail with the following error:

FAIL: Clang :: CXX/module/module.interface/p2-2.cpp (2 of 17555)
******************** TEST 'Clang :: CXX/module/module.interface/p2-2.cpp' FAILED ********************
Script:
--
: 'RUN: at line 3';   /dev/shm/ayzhao_llvm/llvm-project/build/bin/clang -cc1 -internal-isystem /dev/shm/ayzhao_llvm/llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -std=c++20 /dev/shm/ayzhao_llvm/llvm-project/clang/test/CXX/module/module.interface/p2-2.cpp -verify
--
Exit Code: 1
 
Command Output (stderr):
--
error: 'error' diagnostics expected but not seen:
  File /dev/shm/ayzhao_llvm/llvm-project/clang/test/CXX/module/module.interface/p2-2.cpp Line 17: cannot export 'iterator' as it is not at namespace scope
  File /dev/shm/ayzhao_llvm/llvm-project/clang/test/CXX/module/module.interface/p2-2.cpp Line 35: cannot export 'iterator' as it is not at namespace scope
error: 'error' diagnostics seen but not expected:
  File /dev/shm/ayzhao_llvm/llvm-project/clang/test/CXX/module/module.interface/p2-2.cpp Line 17: declaration does not declare anything
  File /dev/shm/ayzhao_llvm/llvm-project/clang/test/CXX/module/module.interface/p2-2.cpp Line 35: declaration does not declare anything
4 errors generated.
 
--
 
********************

A reduced repro is here:

export module X;

export template <typename T>
struct X {
  struct iterator {
    T node;
  };
};

export template <typename T> X<T>::iterator;

Sema::getTypeName(...) is called from these lines, with the result being that TypeRep is null in main but not null in this patch.

I'm still in the process of trying to figure out how to resolve this, but any suggestions/insights would be very welcome.

[cor3ntin]

Maybe @ChuanqiXu Can help as it seems module related

[ayzhao]

clang/lib/Sema/SemaDecl.cpp

362

I fixed this with the following diff:

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 1375431d8998..dacba4d18021 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -362,6 +362,11 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
         if (AllowImplicitTypename == ImplicitTypenameContext::No &&
             !isClassName && !IsCtorOrDtorName)
           return nullptr;
+        // FIXME: This hack is required in order to make the test
+        // CXX/module/module.interface/p2-2.cpp pass. Can we get this working
+        // with this method returning a non-null ParsedType?
+        if (isa<ExportDecl>(CurContext))
+          return nullptr;
         bool IsImplicitTypename = !isClassName && !IsCtorOrDtorName;
         if (IsImplicitTypename) {
           SourceLocation QualifiedLoc = SS->getRange().getBegin();

It's very hacky, but it works (for now)

[ayzhao]

hacky fix for the test CXX/module/module.interface/p2-2.cpp

If anyone has a better idea on how to fix this test, feedback is very welcome.

[ayzhao]

clang/lib/Sema/SemaTemplate.cpp

3995–3997

Currently looking into this.

It is interesting to note that for the following example in one of the parent comments:

template<typename T> struct X {
  template<typename U> struct Y { Y(); typedef void V; }; 
  template<typename U> typename Y<U>::V::W f();
};
template<typename T> template<typename U>
X<T>::template Y<U>::V::W X<T>::f() { return 0; }

GCC doesn't like the fact that the template keyword is on the final line if compiled with -pedantic [0]:

<source>:6:16: warning: keyword 'template' not allowed in declarator-id [-Wpedantic]
    6 | X<T>::template Y<U>::V::W X<T>::f() { return 0; }
      |                ^~~~
Compiler returned: 0

[0]: https://godbolt.org/z/dYddW3ErY

[ayzhao]

fix typo

[ayzhao]

remove extra blank line

[ayzhao]

clang/lib/Sema/SemaTemplate.cpp

3995–3997

This is starting to feel like a separate issue. I filed https://github.com/llvm/llvm-project/issues/57853 to track it.

[ayzhao]

rebase + clang-format

[ayzhao]

remove PrevLookupResult + put Declarator(...) and ~Declarator() definitions back in the header files to reduce the diff since we're no longer overloading the constructor.

[ayzhao]

clang/include/clang/Sema/DeclSpec.h

1918

Removed

clang/lib/Sema/DeclSpec.cpp

1501–1503

Ended up removing PrevLookupResult per my other comment.

[ayzhao]

rebase + upload; also friendly ping!

[shafik]

I made mostly small comments but I think @aaron.ballman and/or @erichkeane should take a look as well.

clang/lib/Parse/ParseDecl.cpp

3429–3432

5588

Instead of adding yet another bool flag maybe we can consider using something like enum isFriend : bool {No, Yes}.

I am sure @aaron.ballman will want to chime in here as well.

clang/lib/Parse/ParseTemplate.cpp

20

Do we still need this?

1449

clang/lib/Sema/Sema.cpp

50

Do we need this?

[erichkeane]

I think this is on the right track, but would like to see more work done on that fixme before we commit (or at least better understand what is causing this). I'm afraid of what else can get us into that situation besides the export-decl.

clang/lib/Sema/SemaDecl.cpp

368

Hmm... this scares me quite a bit, I don't really know what other fallout results from this, or other code that would hit here.

We should probably spend more time making sure we understand this better.

[ayzhao]

address some review comments

[aaron.ballman]

clang/include/clang/Parse/Parser.h

2339

Starting with is implies contextual conversion to bool (generally), so I'd suggest renaming.

clang/include/clang/Sema/DeclSpec.h

1808

Not opposed to this construct, but I am worried about how well it will scale. I don't know that we want to add a bunch of named enums that all boil down to a bool. (If someone thinks they have good ideas here, that'd be a good RFC topic for Discourse because we have a ton of interfaces that take a bunch of bools.)

clang/lib/Parse/ParseDecl.cpp

5588

Heh, so this is where I get worried about the scalability of using enums for these. We really want to use three different enums here, but do we really want to *add* three different enums? I'm unconvinced.

However, if we can come up with some template magic to allow for named bool parameters as a generic interface, that would be valuable to use.

[erichkeane]

clang/lib/Parse/ParseDecl.cpp

5588

I prefer enums over bools TBH, even if we end up with a million of then somewhere.

That said, what about:

https://godbolt.org/z/Kz6jdjobj

template<typename SpecificThing>
class Is {
    Is(bool v) : value(v){}
  public:
  bool value;
  static const Is Yes() { return Is{true};}
  static const Is No() { return Is{false};}

  operator bool() { return value; }
};

class Friend{}; // #1
 
void foo(Is<Friend> f) {
    if (f) {
        ///...
    }
}

void baz() {
    foo(Is<Friend>::Yes());
}

Adding a 'new' thing is as simple as just adding #1 for anything we care about. We might want to put them in a namespace of some sort, but perhaps not awful?

[aaron.ballman]

clang/lib/Parse/ParseDecl.cpp

5588

Yeah, this is along the lines of what I was thinking of! However, I'm still concerned about that approach because it involves adding a new type for every situation we have a bool. Empty classes to use as a tag definitely works, but I was hoping we could use a string literal rather than a tag type so that we don't have the extra compile time overhead of adding hundreds of new empty classes. e.g.,

void foo(Is<"Friend"> f) {
  if (f) {
     // ...
  }
}

void baz() {
  foo(Is<"Friend">::Yes); // Yay
  foo(Is<"Enemy">::Yes); // Error for type mismatch with Is<"Friend">
}

However, that might require compiling with C++20 (I don't recall), so it may not be a viable idea.

[ayzhao]

address some comments

[ayzhao]

clang/include/clang/Sema/DeclSpec.h

1808

Ack. IIRC this enum was created as a result of this comment by @rsmith expressing concern over adding an additional bool parameter to a function with a lot of preexisting bool args.

[erichkeane]

clang/lib/Parse/ParseDecl.cpp

5588

Yeah, referring to stringliterals is troublesome in C++17. However, we COULD do that like:

void foo(Is<"Friend"_Is> f) {
  if (f) {
     // ...
  }
}

void baz() {
  foo(Is<"Friend"_Is>::Yes); // Yay
  foo(Is<"Enemy"_Is>::Yes); // Error for type mismatch with Is<"Friend">
}

by making operator _Is return an integer_sequence.

[aaron.ballman]

clang/include/clang/Sema/DeclSpec.h

1808

Ack. IIRC this enum was created as a result of this comment by @rsmith expressing concern over adding an additional bool parameter to a function with a lot of preexisting bool args.

Yeah, I spotted his suggestion (and am totally fine with implementing it here as you've done). I was mostly thinking about the future when someone tries to do this for every set of 2 or more bool parameters. :-D

clang/lib/Parse/ParseDecl.cpp

5588

That's a really neat idea! If you want to work it up into something that could be plausible to add to ADT, I think it's worth an RFC to add the interface. I'm guessing the diagnostic behavior of that would be kind of gross, but once we move to C++20 we'd be able to use string literal template arguments directly and get better diagnostic behavior. The critical part is that the code is readable and we get diagnostics when passing an argument to the wrong parameter.

[ayzhao]

Add FriendSpecified enum

[ayzhao]

fix spacing

[ayzhao]

clang/lib/Parse/ParseDecl.cpp

5588

I added a FriendSpecified enum for now.

I'm going to mark the comment chain as done so that they don't unnecessarily block the review.

I agree with the idea of a generic type to represent boolean arguments, but I think it would be out of scope of this patch and that Discourse would be a more appropriate place to discuss this.

[ayzhao]

address some more review comments + rebase

[ayzhao]

clang/lib/Sema/SemaDecl.cpp

368

Following up with my previous comment about the failing test that this hack was supposed to fix:

I added the typename keyword to the export template statement and lo and behold, Clang emits the same "declaration does not declare anything" diagnostic instead of the namespace scope diagnostic [0].

To me, this feels like that the original test case was wrong:

1. There should've been a typename keyword in the export statement. Perhaps the author of the test wasn't aware that the typename keyword was required, or perhaps the author already assumed that P0634r3 was already implemented in clang.
2. The test currently passing (with Clang emitting the namespace diagnostic instead of the declaration diagnostic) seems to be a freak coincidence.

I'm not familiar at all with C++ modules - is this a bug with exporting types?

[0]: https://godbolt.org/z/PYvh68Tq7

[ayzhao]

clang/lib/Sema/SemaDecl.cpp

368

small fix: I linked to the wrong line when referencing the original test case. The link should be https://github.com/llvm/llvm-project/blob/a707675dbba9ca3ec6e668f86fea2240a85ca171/clang/test/CXX/module/module.interface/p2-2.cpp#L17

[ayzhao]

remove the hack for the test p2-2.cpp and patch in patch in D134578 which fixes the test

[ayzhao]

clang/lib/Sema/SemaDecl.cpp

368

So my belief now is that the test p2-2.cpp is broken. I've removed the FIXME hack, and I created D134578 to fix the test. To keep the bots happy, I also cherry-picked D134578 into this patch.

[ayzhao]

In D53847#3809072, @erichkeane wrote:

I think this is on the right track, but would like to see more work done on that fixme before we commit (or at least better understand what is causing this). I'm afraid of what else can get us into that situation besides the export-decl.

The FIXME has been removed; the problem seems to be that the test is broken. See https://reviews.llvm.org/D53847?id=462032#inline-1297092

[erichkeane]

Thanks for looking into it. I think you're right that it is a mistake that we accepted that without the 'struct' keyword after a quick look, so I think I am OK with this. Please give @aaron.ballman a day or so to take a look if he wishes.

[ayzhao]

clang-format + rebase

[cor3ntin]

It's great to see this make progress.
Can you update cxx_status.html and ReleaseNotes.txt?

[ayzhao]

update cxx_status.html and ReleaseNotes.rst + rebase

[ayzhao]

In D53847#3817376, @cor3ntin wrote:

It's great to see this make progress.
Can you update cxx_status.html and ReleaseNotes.txt?

Done.

I also saw that P2092R0 was marked as partially supported in cxx_status.html due to missing support for P0634R3, but I'm not familiar enough with P2092R0 to update that entry.

[cor3ntin]

Good catch

template<typename T>
concept K = requires (typename T::Type X) { // (3)
X.next();
}

That typename should be optional. Can you add a test for that? If it works you can mark P2092 as fully supported.

[ayzhao]

In D53847#3819200, @cor3ntin wrote:

Good catch

template<typename T>
concept K = requires (typename T::Type X) { // (3)
X.next();
}

That typename should be optional. Can you add a test for that? If it works you can mark P2092 as fully supported.

Done.

The test originally failed, but it was a simple fix to get it to work.

[ayzhao]

add test and fix for P2092

[cor3ntin]

In D53847#3819344, @ayzhao wrote:

add test and fix for P2092

These changes look good to me, thanks

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def warn_cxx98_compat_typename_outside_of_template : Warning<
   "use of 'typename' outside of a template is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 def err_typename_refers_to_using_value_decl : Error<
   "typename specifier refers to a dependent using declaration for a value "
   "%0 in %1">;
 def note_using_value_decl_missing_typename : Note<
   "add 'typename' to treat this using declaration as a type">;
+def warn_cxx17_compat_implicit_typename : Warning<"use of implicit 'typename' is "
+  "incompatible with C++ standards before C++20">, InGroup<CXX20Compat>,
+  DefaultIgnore;
+def ext_implicit_typename : ExtWarn<"missing 'typename' prior to dependent "
+  "type name %0%1; implicit 'typename' is a C++20 extension">,
+  InGroup<CXX20>;
 
 def err_template_kw_refers_to_non_template : Error<
   "%0%select{| following the 'template' keyword}1 "
   "does not refer to a template">;
 def note_template_kw_refers_to_non_template : Note<
   "declared as a non-template here">;
 def err_template_kw_refers_to_dependent_non_template : Error<
   "%0%select{| following the 'template' keyword}1 "

clang/include/clang/Parse/Parser.h

private:
   /// token.
   static void setExprAnnotation(Token &Tok, ExprResult ER) {
     Tok.setAnnotationValue(ER.getAsOpaquePointer());
   }
 
 public:
   // If NeedType is true, then TryAnnotateTypeOrScopeToken will try harder to
   // find a type name by attempting typo correction.
-  bool TryAnnotateTypeOrScopeToken();
-  bool TryAnnotateTypeOrScopeTokenAfterScopeSpec(CXXScopeSpec &SS,
-                                                 bool IsNewScope);
+  bool
+  TryAnnotateTypeOrScopeToken(ImplicitTypenameContext AllowImplicitTypename =
+                                  ImplicitTypenameContext::No);
+  bool TryAnnotateTypeOrScopeTokenAfterScopeSpec(
+      CXXScopeSpec &SS, bool IsNewScope,
+      ImplicitTypenameContext AllowImplicitTypename);
   bool TryAnnotateCXXScopeToken(bool EnteringContext = false);
 
   bool MightBeCXXScopeToken() {
     return Tok.is(tok::identifier) || Tok.is(tok::coloncolon) ||
            (Tok.is(tok::annot_template_id) &&
             NextToken().is(tok::coloncolon)) ||
            Tok.is(tok::kw_decltype) || Tok.is(tok::kw___super);
   }
enum AnnotatedNameKind {
     ANK_TentativeDecl,
     /// The identifier is a template name. FIXME: Add an annotation for that.
     ANK_TemplateName,
     /// The identifier can't be resolved.
     ANK_Unresolved,
     /// Annotation was successful.
     ANK_Success
   };
-  AnnotatedNameKind TryAnnotateName(CorrectionCandidateCallback *CCC = nullptr);
+
+  AnnotatedNameKind
+  TryAnnotateName(CorrectionCandidateCallback *CCC = nullptr,
+                  ImplicitTypenameContext AllowImplicitTypename =
+                      ImplicitTypenameContext::No);
 
   /// Push a tok::annot_cxxscope token onto the token stream.
   void AnnotateScopeToken(CXXScopeSpec &SS, bool IsNewAnnotation);
 
   /// TryAltiVecToken - Check for context-sensitive AltiVec identifier tokens,
   /// replacing them with the non-context-sensitive keywords.  This returns
   /// true if the token was replaced.
   bool TryAltiVecToken(DeclSpec &DS, SourceLocation Loc,
private:
   // C++ 5.2.3: Explicit type conversion (functional notation)
   ExprResult ParseCXXTypeConstructExpression(const DeclSpec &DS);
 
   /// ParseCXXSimpleTypeSpecifier - [C++ 7.1.5.2] Simple type specifiers.
   /// This should only be called when the current token is known to be part of
   /// simple-type-specifier.
   void ParseCXXSimpleTypeSpecifier(DeclSpec &DS);
 
-  bool ParseCXXTypeSpecifierSeq(DeclSpec &DS);
+  bool ParseCXXTypeSpecifierSeq(
+      DeclSpec &DS,
+      DeclaratorContext Context = DeclaratorContext::TypeName);
 
   //===--------------------------------------------------------------------===//
   // C++ 5.3.4 and 5.3.5: C++ new and delete
   bool ParseExpressionListOrTypeId(SmallVectorImpl<Expr*> &Exprs,
                                    Declarator &D);
   void ParseDirectNewDeclarator(Declarator &D);
   ExprResult ParseCXXNewExpression(bool UseGlobal, SourceLocation Start);
   ExprResult ParseCXXDeleteExpression(bool UseGlobal,
private:
   /// out, there are other significant restrictions on specifiers than
   /// would be best implemented in the parser.
   enum class DeclSpecContext {
     DSC_normal, // normal context
     DSC_class,  // class context, enables 'friend'
     DSC_type_specifier, // C++ type-specifier-seq or C specifier-qualifier-list
     DSC_trailing, // C++11 trailing-type-specifier in a trailing return type
     DSC_alias_declaration, // C++11 type-specifier-seq in an alias-declaration
+    DSC_conv_operator, // C++ type-specifier-seq in an conversion operator
     DSC_top_level, // top-level/namespace declaration context
     DSC_template_param, // template parameter context
+    DSC_template_arg, // template argument context
     DSC_template_type_arg, // template type argument context
     DSC_objc_method_result, // ObjC method result context, enables 'instancetype'
     DSC_condition, // condition declaration context
     DSC_association // A _Generic selection expression's type association
   };
 
   /// Is this a context in which we are parsing just a type-specifier (or
   /// trailing-type-specifier)?
   static bool isTypeSpecifier(DeclSpecContext DSC) {
     switch (DSC) {
     case DeclSpecContext::DSC_normal:
     case DeclSpecContext::DSC_template_param:
+    case DeclSpecContext::DSC_template_arg:
     case DeclSpecContext::DSC_class:
     case DeclSpecContext::DSC_top_level:
     case DeclSpecContext::DSC_objc_method_result:
     case DeclSpecContext::DSC_condition:
       return false;
 
     case DeclSpecContext::DSC_template_type_arg:
     case DeclSpecContext::DSC_type_specifier:
+    case DeclSpecContext::DSC_conv_operator:
     case DeclSpecContext::DSC_trailing:
     case DeclSpecContext::DSC_alias_declaration:
     case DeclSpecContext::DSC_association:
       return true;
     }
     llvm_unreachable("Missing DeclSpecContext case");
   }
 
isDefiningTypeSpecifierContext(DeclSpecContext DSC, bool IsCPlusPlus) {
     case DeclSpecContext::DSC_type_specifier:
       return AllowDefiningTypeSpec::NoButErrorRecovery;
 
     case DeclSpecContext::DSC_association:
       return IsCPlusPlus ? AllowDefiningTypeSpec::NoButErrorRecovery
                          : AllowDefiningTypeSpec::Yes;
 
     case DeclSpecContext::DSC_trailing:
+    case DeclSpecContext::DSC_conv_operator:
+    case DeclSpecContext::DSC_template_arg:
       return AllowDefiningTypeSpec::No;
     }
     llvm_unreachable("Missing DeclSpecContext case");
   }
 
   /// Is this a context in which an opaque-enum-declaration can appear?
   static bool isOpaqueEnumDeclarationContext(DeclSpecContext DSC) {
     switch (DSC) {
     case DeclSpecContext::DSC_normal:
     case DeclSpecContext::DSC_class:
     case DeclSpecContext::DSC_top_level:
       return true;
 
     case DeclSpecContext::DSC_alias_declaration:
     case DeclSpecContext::DSC_objc_method_result:
     case DeclSpecContext::DSC_condition:
     case DeclSpecContext::DSC_template_param:
     case DeclSpecContext::DSC_template_type_arg:
     case DeclSpecContext::DSC_type_specifier:
     case DeclSpecContext::DSC_trailing:
     case DeclSpecContext::DSC_association:
+    case DeclSpecContext::DSC_conv_operator:
+    case DeclSpecContext::DSC_template_arg:
+
       return false;
     }
     llvm_unreachable("Missing DeclSpecContext case");
   }
 
   /// Is this a context in which we can perform class template argument
   /// deduction?
   static bool isClassTemplateDeductionContext(DeclSpecContext DSC) {
     switch (DSC) {
     case DeclSpecContext::DSC_normal:
     case DeclSpecContext::DSC_template_param:
+    case DeclSpecContext::DSC_template_arg:
     case DeclSpecContext::DSC_class:
     case DeclSpecContext::DSC_top_level:
     case DeclSpecContext::DSC_condition:
     case DeclSpecContext::DSC_type_specifier:
     case DeclSpecContext::DSC_association:
+    case DeclSpecContext::DSC_conv_operator:
       return true;
 
     case DeclSpecContext::DSC_objc_method_result:
     case DeclSpecContext::DSC_template_type_arg:
     case DeclSpecContext::DSC_trailing:
     case DeclSpecContext::DSC_alias_declaration:
       return false;
     }
     llvm_unreachable("Missing DeclSpecContext case");
   }
 
+  // Is this a context in which an implicit 'typename' is allowed?
+  static ImplicitTypenameContext isImplicitTypenameContext(DeclSpecContext DSC) {
+    switch (DSC) {
+    case DeclSpecContext::DSC_class:
+    case DeclSpecContext::DSC_top_level:
+    case DeclSpecContext::DSC_type_specifier:
+    case DeclSpecContext::DSC_template_type_arg:
+    case DeclSpecContext::DSC_trailing:
+    case DeclSpecContext::DSC_alias_declaration:
+    case DeclSpecContext::DSC_template_param:
+      return ImplicitTypenameContext::Yes;
+
+    case DeclSpecContext::DSC_normal:
+    case DeclSpecContext::DSC_objc_method_result:
+    case DeclSpecContext::DSC_condition:
+    case DeclSpecContext::DSC_template_arg:
+    case DeclSpecContext::DSC_conv_operator:
+    case DeclSpecContext::DSC_association:
+      return ImplicitTypenameContext::No;
+    }
+    llvm_unreachable("Missing DeclSpecContext case");
+  }
+
   /// Information on a C++0x for-range-initializer found while parsing a
   /// declaration which turns out to be a for-range-declaration.
   struct ForRangeInit {
     SourceLocation ColonLoc;
     ExprResult RangeExpr;
 
     bool ParsedForRangeDecl() { return !ColonLoc.isInvalid(); }
   };
bool ParseImplicitInt(DeclSpec &DS, CXXScopeSpec *SS,
                         ParsedAttributes &Attrs);
   DeclSpecContext
   getDeclSpecContextFromDeclaratorContext(DeclaratorContext Context);
   void ParseDeclarationSpecifiers(
       DeclSpec &DS,
       const ParsedTemplateInfo &TemplateInfo = ParsedTemplateInfo(),
       AccessSpecifier AS = AS_none,
       DeclSpecContext DSC = DeclSpecContext::DSC_normal,
-      LateParsedAttrList *LateAttrs = nullptr);
+      LateParsedAttrList *LateAttrs = nullptr) {
+    return ParseDeclarationSpecifiers(
+        DS, TemplateInfo, AS, DSC, LateAttrs, isImplicitTypenameContext(DSC));
+  }
+  void ParseDeclarationSpecifiers(
+      DeclSpec &DS,
+      const ParsedTemplateInfo &TemplateInfo,
+      AccessSpecifier AS,
+      DeclSpecContext DSC,
+      LateParsedAttrList *LateAttrs,
+      ImplicitTypenameContext AllowImplicitTypename);
+
   bool DiagnoseMissingSemiAfterTagDefinition(
       DeclSpec &DS, AccessSpecifier AS, DeclSpecContext DSContext,
       LateParsedAttrList *LateAttrs = nullptr);
 
   void ParseSpecifierQualifierList(
       DeclSpec &DS, AccessSpecifier AS = AS_none,
+      DeclSpecContext DSC = DeclSpecContext::DSC_normal) {
+    ParseSpecifierQualifierList(DS, isImplicitTypenameContext(DSC), AS, DSC);
+  }
+
+  void ParseSpecifierQualifierList(
+      DeclSpec &DS, ImplicitTypenameContext AllowImplicitTypename,
+      AccessSpecifier AS = AS_none,
       DeclSpecContext DSC = DeclSpecContext::DSC_normal);
 
   void ParseObjCTypeQualifierList(ObjCDeclSpec &DS,
                                   DeclaratorContext Context);
 
   void ParseEnumSpecifier(SourceLocation TagLoc, DeclSpec &DS,
                           const ParsedTemplateInfo &TemplateInfo,
                           AccessSpecifier AS, DeclSpecContext DSC);
   void ParseEnumBody(SourceLocation StartLoc, Decl *TagDecl);
   void ParseStructUnionBody(SourceLocation StartLoc, DeclSpec::TST TagType,
                             RecordDecl *TagDecl);
 
   void ParseStructDeclaration(
       ParsingDeclSpec &DS,
       llvm::function_ref<void(ParsingFieldDeclarator &)> FieldsCallback);
 
-  bool isDeclarationSpecifier(bool DisambiguatingWithExpression = false);
+  bool isDeclarationSpecifier(ImplicitTypenameContext AllowImplicitTypename,
+                              bool DisambiguatingWithExpression = false);
   bool isTypeSpecifierQualifier();
 
   /// isKnownToBeTypeSpecifier - Return true if we know that the specified token
   /// is definitely a type-specifier.  Return false if it isn't part of a type
   /// specifier or if we're not sure.
   bool isKnownToBeTypeSpecifier(const Token &Tok) const;
 
   /// Return true if we know that we are definitely looking at a
   /// decl-specifier, and isn't part of an expression such as a function-style
   /// cast. Return false if it's no a decl-specifier, or we're not sure.
   bool isKnownToBeDeclarationSpecifier() {
     if (getLangOpts().CPlusPlus)
-      return isCXXDeclarationSpecifier() == TPResult::True;
-    return isDeclarationSpecifier(true);
+      return isCXXDeclarationSpecifier(ImplicitTypenameContext::No) == TPResult::True;
+    return isDeclarationSpecifier(ImplicitTypenameContext::No, true);
   }
 
   /// isDeclarationStatement - Disambiguates between a declaration or an
   /// expression statement, when parsing function bodies.
   /// Returns true for declaration, false for expression.
   bool isDeclarationStatement() {
     if (getLangOpts().CPlusPlus)
       return isCXXDeclarationStatement();
-    return isDeclarationSpecifier(true);
+    return isDeclarationSpecifier(ImplicitTypenameContext::No, true);
   }
 
   /// isForInitDeclaration - Disambiguates between a declaration or an
   /// expression in the context of the C 'clause-1' or the C++
   // 'for-init-statement' part of a 'for' statement.
   /// Returns true for declaration, false for expression.
   bool isForInitDeclaration() {
     if (getLangOpts().OpenMP)
       Actions.startOpenMPLoop();
     if (getLangOpts().CPlusPlus)
       return Tok.is(tok::kw_using) ||
              isCXXSimpleDeclaration(/*AllowForRangeDecl=*/true);
-    return isDeclarationSpecifier(true);
+    return isDeclarationSpecifier(ImplicitTypenameContext::No, true);
   }
 
   /// Determine whether this is a C++1z for-range-identifier.
   bool isForRangeIdentifier();
 
   /// Determine whether we are currently at the start of an Objective-C
   /// class message that appears to be missing the open bracket '['.
   bool isStartOfObjCClassMessageMissingOpenBracket();
 
   /// Starting with a scope specifier, identifier, or
   /// template-id that refers to the current class, determine whether
   /// this is a constructor declarator.
-  bool isConstructorDeclarator(bool Unqualified, bool DeductionGuide = false);
+  bool isConstructorDeclarator(bool Unqualified, bool DeductionGuide = false,
+                               bool IsFriend = false);
 
   /// Specifies the context in which type-id/expression
   /// disambiguation will occur.
   enum TentativeCXXTypeIdContext {
     TypeIdInParens,
     TypeIdUnambiguous,
     TypeIdAsTemplateArgument
   };
private:
 
   /// isCXXFunctionDeclarator - Disambiguates between a function declarator or
   /// a constructor-style initializer, when parsing declaration statements.
   /// Returns true for function declarator and false for constructor-style
   /// initializer. Sets 'IsAmbiguous' to true to indicate that this declaration
   /// might be a constructor-style initializer.
   /// If during the disambiguation process a parsing error is encountered,
   /// the function returns true to let the declaration parsing code handle it.
-  bool isCXXFunctionDeclarator(bool *IsAmbiguous = nullptr);
+  bool isCXXFunctionDeclarator(
+      bool *IsAmbiguous = nullptr,
+      ImplicitTypenameContext AllowImplicitTypename = ImplicitTypenameContext::No);
 
   struct ConditionDeclarationOrInitStatementState;
   enum class ConditionOrInitStatement {
     Expression,    ///< Disambiguated as an expression (either kind).
     ConditionDecl, ///< Disambiguated as the declaration form of condition.
     InitStmtDecl,  ///< Disambiguated as a simple-declaration init-statement.
     ForRangeDecl,  ///< Disambiguated as a for-range declaration.
     Error          ///< Can't be any of the above!
private:
   /// isCXXDeclarationSpecifier - Returns TPResult::True if it is a
   /// declaration specifier, TPResult::False if it is not,
   /// TPResult::Ambiguous if it could be either a decl-specifier or a
   /// function-style cast, and TPResult::Error if a parsing error was
   /// encountered. If it could be a braced C++11 function-style cast, returns
   /// BracedCastResult.
   /// Doesn't consume tokens.
   TPResult
-  isCXXDeclarationSpecifier(TPResult BracedCastResult = TPResult::False,
+  isCXXDeclarationSpecifier(ImplicitTypenameContext AllowImplicitTypename,
+                            TPResult BracedCastResult = TPResult::False,
                             bool *InvalidAsDeclSpec = nullptr);
 
   /// Given that isCXXDeclarationSpecifier returns \c TPResult::True or
   /// \c TPResult::Ambiguous, determine whether the decl-specifier would be
   /// a type-specifier other than a cv-qualifier.
   bool isCXXDeclarationSpecifierAType();
 
   /// Determine whether the current token sequence might be
private:
   TPResult TryParseSimpleDeclaration(bool AllowForRangeDecl);
   TPResult TryParseTypeofSpecifier();
   TPResult TryParseProtocolQualifiers();
   TPResult TryParsePtrOperatorSeq();
   TPResult TryParseOperatorId();
   TPResult TryParseInitDeclaratorList();
   TPResult TryParseDeclarator(bool mayBeAbstract, bool mayHaveIdentifier = true,
                               bool mayHaveDirectInit = false);
-  TPResult
-  TryParseParameterDeclarationClause(bool *InvalidAsDeclaration = nullptr,
-                                     bool VersusTemplateArg = false);
+  TPResult TryParseParameterDeclarationClause(
+      bool *InvalidAsDeclaration = nullptr, bool VersusTemplateArg = false,
+      ImplicitTypenameContext AllowImplicitTypename = ImplicitTypenameContext::No);
   TPResult TryParseFunctionDeclarator();
   TPResult TryParseBracketDeclarator();
   TPResult TryConsumeDeclarationSpecifier();
 
   /// Try to skip a possibly empty sequence of 'attribute-specifier's without
   /// full validation of the syntactic structure of attributes.
   bool TrySkipAttributes();
 
void InitCXXThisScopeForDeclaratorIfRelevant(
       llvm::Optional<Sema::CXXThisScopeRAII> &ThisScope);
   bool ParseRefQualifier(bool &RefQualifierIsLValueRef,
                          SourceLocation &RefQualifierLoc);
   bool isFunctionDeclaratorIdentifierList();
   void ParseFunctionDeclaratorIdentifierList(
          Declarator &D,
          SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo);
   void ParseParameterDeclarationClause(
-         DeclaratorContext DeclaratorContext,
-         ParsedAttributes &attrs,
-         SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
-         SourceLocation &EllipsisLoc);
+      Declarator &D, ParsedAttributes &attrs,
+      SourceLocation &EllipsisLoc) {
+    return ParseParameterDeclarationClause(
+        D.getContext(), attrs, ParamInfo, EllipsisLoc,
+        D.getCXXScopeSpec().isSet() &&
+            D.isFunctionDeclaratorAFunctionDeclaration());
+  }
+  void ParseParameterDeclarationClause(
+      DeclaratorContext DeclaratorContext, ParsedAttributes &attrs,
+      SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
+      SourceLocation &EllipsisLoc, bool IsACXXFunctionDeclaration = false);
+
   void ParseBracketDeclarator(Declarator &D);
   void ParseMisplacedBracketDeclarator(Declarator &D);
   bool MaybeParseTypeTransformTypeSpecifier(DeclSpec &DS);
   DeclSpec::TST TypeTransformTokToDeclSpec();
 
   //===--------------------------------------------------------------------===//
   // C++ 7: Declarations [dcl.dcl]
 
private:
 
   bool AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
                                CXXScopeSpec &SS,
                                SourceLocation TemplateKWLoc,
                                UnqualifiedId &TemplateName,
                                bool AllowTypeAnnotation = true,
                                bool TypeConstraint = false);
   void AnnotateTemplateIdTokenAsType(CXXScopeSpec &SS,
+                                     ImplicitTypenameContext AllowImplicitTypename,
                                      bool IsClassName = false);
   bool ParseTemplateArgumentList(TemplateArgList &TemplateArgs,
                                  TemplateTy Template, SourceLocation OpenLoc);
   ParsedTemplateArgument ParseTemplateTemplateArgument();
   ParsedTemplateArgument ParseTemplateArgument();
   Decl *ParseExplicitInstantiation(DeclaratorContext Context,
                                    SourceLocation ExternLoc,
                                    SourceLocation TemplateLoc,

clang/include/clang/Sema/DeclSpec.h

 #include "clang/Basic/Specifiers.h"
 #include "clang/Lex/Token.h"
 #include "clang/Sema/Ownership.h"
 #include "clang/Sema/ParsedAttr.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
+#include <memory>
 
 namespace clang {
   class ASTContext;
   class CXXRecordDecl;
   class TypeLoc;
   class LangOptions;
+  class LookupResult;
   class IdentifierInfo;
   class NamespaceAliasDecl;
   class NamespaceDecl;
   class ObjCDeclSpec;
   class Sema;
   class Declarator;
   struct TemplateIdAnnotation;
 
enum class DeclaratorContext {
   TemplateArg,         // Any template argument (in template argument list).
   TemplateTypeArg,     // Template type argument (in default argument).
   AliasDecl,           // C++11 alias-declaration.
   AliasTemplate,       // C++11 alias-declaration template.
   RequiresExpr,        // C++2a requires-expression.
   Association          // C11 _Generic selection expression association.
 };
 
+// Describes whether the current context is a context where an implicit
+// typename is allowed (C++2a [temp.res]p5]).
+enum class ImplicitTypenameContext {
+  No,
+  Yes,
+};
+
 /// Information about one declarator, including the parsed type
 /// information and the identifier.
 ///
 /// When the declarator is fully formed, this is turned into the appropriate
 /// Decl object.
 ///
 /// Declarators come in two types: normal declarators and abstract declarators.
 /// Abstract declarators are used when parsing types, and don't have an
#endif
   /// If this is the second or subsequent declarator in this declaration,
   /// the location of the comma before this declarator.
   SourceLocation CommaLoc;
 
   /// If provided, the source location of the ellipsis used to describe
   /// this declarator as a parameter pack.
   SourceLocation EllipsisLoc;
 
+  /// Lookup result of declarator, if any.
+  std::unique_ptr<LookupResult> PrevLookupResult;
+
   friend struct DeclaratorChunk;
 
 public:
   /// `DS` and `DeclarationAttrs` must outlive the `Declarator`. In particular,
   /// take care not to pass temporary objects for these parameters.
   ///
   /// `DeclarationAttrs` contains [[]] attributes from the
   /// attribute-specifier-seq at the beginning of a declaration, which appertain
   /// to the declared entity itself. Attributes with other syntax (e.g. GNU)
   /// should not be placed in this attribute list; if they occur at the
   /// beginning of a declaration, they apply to the `DeclSpec` and should be
   /// attached to that instead.
   ///
   /// Here is an example of an attribute associated with a declaration:
   ///
   ///  [[deprecated]] int x, y;
   ///
   /// This attribute appertains to all of the entities declared in the
   /// declaration, i.e. `x` and `y` in this case.
   Declarator(const DeclSpec &DS, const ParsedAttributesView &DeclarationAttrs,
-             DeclaratorContext C)
-      : DS(DS), Range(DS.getSourceRange()), Context(C),
-        InvalidType(DS.getTypeSpecType() == DeclSpec::TST_error),
-        GroupingParens(false), FunctionDefinition(static_cast<unsigned>(
-                                   FunctionDefinitionKind::Declaration)),
-        Redeclaration(false), Extension(false), ObjCIvar(false),
-        ObjCWeakProperty(false), InlineStorageUsed(false),
-        HasInitializer(false), Attrs(DS.getAttributePool().getFactory()),
-        DeclarationAttrs(DeclarationAttrs), AsmLabel(nullptr),
-        TrailingRequiresClause(nullptr),
-        InventedTemplateParameterList(nullptr) {
-    assert(llvm::all_of(DeclarationAttrs,
-                        [](const ParsedAttr &AL) {
-                          return AL.isStandardAttributeSyntax();
-                        }) &&
-           "DeclarationAttrs may only contain [[]] attributes");
-  }
+             DeclaratorContext C);
+
+  ~Declarator();
 
-  ~Declarator() {
-    clear();
-  }
   /// getDeclSpec - Return the declaration-specifier that this declarator was
   /// declared with.
   const DeclSpec &getDeclSpec() const { return DS; }
 
   /// getMutableDeclSpec - Return a non-const version of the DeclSpec.  This
   /// should be used with extreme care: declspecs can often be shared between
   /// multiple declarators, so mutating the DeclSpec affects all of the
   /// Declarators.  This should only be done when the declspec is known to not
public:
   /// redeclaration time if the decl is static.
   bool isStaticMember();
 
   /// Returns true if this declares a constructor or a destructor.
   bool isCtorOrDtor();
 
   void setRedeclaration(bool Val) { Redeclaration = Val; }
   bool isRedeclaration() const { return Redeclaration; }
+
+  void setPrevLookupResult(std::unique_ptr<LookupResult> LR);
+  LookupResult consumePrevLookupResult();
+  bool hasPrevLookupResult() const { return (bool)PrevLookupResult; }
 };
 
 /// This little struct is used to capture information about
 /// structure field declarators, which is basically just a bitfield size.
 struct FieldDeclarator {
   Declarator D;
   Expr *BitfieldSize;
   explicit FieldDeclarator(const DeclSpec &DS,

clang/include/clang/Sema/Sema.h

public:
 
   ParsedType getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
                          Scope *S, CXXScopeSpec *SS = nullptr,
                          bool isClassName = false, bool HasTrailingDot = false,
                          ParsedType ObjectType = nullptr,
                          bool IsCtorOrDtorName = false,
                          bool WantNontrivialTypeSourceInfo = false,
                          bool IsClassTemplateDeductionContext = true,
+                         ImplicitTypenameContext AllowImplicitTypename =
+                             ImplicitTypenameContext::No,
                          IdentifierInfo **CorrectedII = nullptr);
   TypeSpecifierType isTagName(IdentifierInfo &II, Scope *S);
   bool isMicrosoftMissingTypename(const CXXScopeSpec *SS, Scope *S);
   void DiagnoseUnknownTypeName(IdentifierInfo *&II,
                                SourceLocation IILoc,
                                Scope *S,
                                CXXScopeSpec *SS,
                                ParsedType &SuggestedType,
public:
   /// definition, assuming we don't care about analyzing its body or emitting
   /// code for that function.
   ///
   /// This will be \c false only if we may need the body of the function in
   /// order to parse the rest of the program (for instance, if it is
   /// \c constexpr in C++11 or has an 'auto' return type in C++14).
   bool canSkipFunctionBody(Decl *D);
 
+  /// Determine whether \param D is function like (function or function
+  /// template) for parsing.
+  bool isDeclaratorFunctionLike(Declarator &D);
+
   void computeNRVO(Stmt *Body, sema::FunctionScopeInfo *Scope);
   Decl *ActOnFinishFunctionBody(Decl *Decl, Stmt *Body);
   Decl *ActOnFinishFunctionBody(Decl *Decl, Stmt *Body, bool IsInstantiation);
   Decl *ActOnSkippedFunctionBody(Decl *Decl);
   void ActOnFinishInlineFunctionDef(FunctionDecl *D);
 
   /// ActOnFinishDelayedAttribute - Invoked when we have finished parsing an
   /// attribute for which parsing is delayed.
QualType CheckTemplateIdType(TemplateName Template,
                                SourceLocation TemplateLoc,
                               TemplateArgumentListInfo &TemplateArgs);
 
   TypeResult
   ActOnTemplateIdType(Scope *S, CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
                       TemplateTy Template, IdentifierInfo *TemplateII,
                       SourceLocation TemplateIILoc, SourceLocation LAngleLoc,
                       ASTTemplateArgsPtr TemplateArgs, SourceLocation RAngleLoc,
-                      bool IsCtorOrDtorName = false, bool IsClassName = false);
+                      bool IsCtorOrDtorName = false, bool IsClassName = false,
+                      ImplicitTypenameContext AllowImplicitTypename =
+                          ImplicitTypenameContext::No);
 
   /// Parsed an elaborated-type-specifier that refers to a template-id,
   /// such as \c class T::template apply<U>.
   TypeResult ActOnTagTemplateIdType(TagUseKind TUK,
                                     TypeSpecifierType TagSpec,
                                     SourceLocation TagLoc,
                                     CXXScopeSpec &SS,
                                     SourceLocation TemplateKWLoc,

   /// Called when the parser has parsed a C++ typename
   /// specifier, e.g., "typename T::type".
   ///
   /// \param S The scope in which this typename type occurs.
   /// \param TypenameLoc the location of the 'typename' keyword
   /// \param SS the nested-name-specifier following the typename (e.g., 'T::').
   /// \param II the identifier we're retrieving (e.g., 'type' in the example).
   /// \param IdLoc the location of the identifier.
-  TypeResult
-  ActOnTypenameType(Scope *S, SourceLocation TypenameLoc,
+  /// \param IsImplicitTypename context where T::type refers to a type.
+  TypeResult ActOnTypenameType(Scope *S, SourceLocation TypenameLoc,
                                const CXXScopeSpec &SS, const IdentifierInfo &II,
-                    SourceLocation IdLoc);
+                               SourceLocation IdLoc,
+                               ImplicitTypenameContext IsImplicitTypename =
+                                   ImplicitTypenameContext::No);
 
   /// Called when the parser has parsed a C++ typename
   /// specifier that ends in a template-id, e.g.,
   /// "typename MetaFun::template apply<T1, T2>".
   ///
   /// \param S The scope in which this typename type occurs.
   /// \param TypenameLoc the location of the 'typename' keyword
   /// \param SS the nested-name-specifier following the typename (e.g., 'T::').

clang/lib/Parse/ParseDecl.cpp

if (!isDeclarationAfterDeclarator()) {
             DS.ClearStorageClassSpecs();
           }
 
           Decl *TheDecl = ParseFunctionDefinition(D, ParsedTemplateInfo(),
                                                   &LateParsedAttrs);
           return Actions.ConvertDeclToDeclGroup(TheDecl);
         }
 
-        if (isDeclarationSpecifier()) {
+        if (isDeclarationSpecifier(ImplicitTypenameContext::No)) {
           // If there is an invalid declaration specifier right after the
           // function prototype, then we must be in a missing semicolon case
           // where this isn't actually a body.  Just fall through into the code
           // that handles it as a prototype, and let the top-level code handle
           // the erroneous declspec where it would otherwise expect a comma or
           // semicolon.
         } else {
           Diag(Tok, diag::err_expected_fn_body);
Parser::DeclGroupPtrTy Parser::ParseDeclGroup(ParsingDeclSpec &DS,
 
   if (ExpectSemi && ExpectAndConsumeSemi(
                         Context == DeclaratorContext::File
                             ? diag::err_invalid_token_after_toplevel_declarator
                             : diag::err_expected_semi_declaration)) {
     // Okay, there was no semicolon and one was expected.  If we see a
     // declaration specifier, just assume it was missing and continue parsing.
     // Otherwise things are very confused and we skip to recover.
-    if (!isDeclarationSpecifier()) {
+    if (!isDeclarationSpecifier(ImplicitTypenameContext::No)) {
       SkipUntil(tok::r_brace, StopAtSemi | StopBeforeMatch);
       TryConsumeToken(tok::semi);
     }
   }
 
   return Actions.FinalizeDeclaratorGroup(getCurScope(), DS, DeclsInGroup);
 }
 

 }
 
 /// ParseSpecifierQualifierList
 ///        specifier-qualifier-list:
 ///          type-specifier specifier-qualifier-list[opt]
 ///          type-qualifier specifier-qualifier-list[opt]
 /// [GNU]    attributes     specifier-qualifier-list[opt]
 ///
-void Parser::ParseSpecifierQualifierList(DeclSpec &DS, AccessSpecifier AS,
-                                         DeclSpecContext DSC) {
+void Parser::ParseSpecifierQualifierList(
+    DeclSpec &DS, ImplicitTypenameContext AllowImplicitTypename,
+    AccessSpecifier AS, DeclSpecContext DSC) {
   /// specifier-qualifier-list is a subset of declaration-specifiers.  Just
   /// parse declaration-specifiers and complain about extra stuff.
   /// TODO: diagnose attribute-specifiers and alignment-specifiers.
-  ParseDeclarationSpecifiers(DS, ParsedTemplateInfo(), AS, DSC);
+  ParseDeclarationSpecifiers(DS, ParsedTemplateInfo(), AS, DSC, nullptr,
+                             AllowImplicitTypename);
 
   // Validate declspec for type-name.
   unsigned Specs = DS.getParsedSpecifiers();
   if (isTypeSpecifier(DSC) && !DS.hasTypeSpecifier()) {
     Diag(Tok, diag::err_expected_type);
     DS.SetTypeSpecError();
   } else if (Specs == DeclSpec::PQ_None && !DS.hasAttributes()) {
     Diag(Tok, diag::err_typename_requires_specqual);
if (!isTypeSpecifier(DSC) && (!SS || DSC == DeclSpecContext::DSC_top_level ||
     case tok::comma:
     case tok::equal:
     case tok::kw_asm:
     case tok::l_brace:
     case tok::l_square:
     case tok::semi:
       // This looks like a variable or function declaration. The type is
       // probably missing. We're done parsing decl-specifiers.
       // But only if we are not in a function prototype scope.
       if (getCurScope()->isFunctionPrototypeScope())
         break;
       if (SS)
         AnnotateScopeToken(*SS, /*IsNewAnnotation*/false);
       return false;
 
     default:
       // This is probably supposed to be a type. This includes cases like:
       //   int f(itn);
       //   struct S { unsigned : 4; };

 
 /// Determine the declaration specifier context from the declarator
 /// context.
 ///
 /// \param Context the declarator context, which is one of the
 /// DeclaratorContext enumerator values.
 Parser::DeclSpecContext
 Parser::getDeclSpecContextFromDeclaratorContext(DeclaratorContext Context) {
-  if (Context == DeclaratorContext::Member)
+  switch (Context) {
+  case DeclaratorContext::Member:
     return DeclSpecContext::DSC_class;
-  if (Context == DeclaratorContext::File)
+  case DeclaratorContext::File:
     return DeclSpecContext::DSC_top_level;
-  if (Context == DeclaratorContext::TemplateParam)
+  case DeclaratorContext::TemplateParam:
     return DeclSpecContext::DSC_template_param;
-  if (Context == DeclaratorContext::TemplateArg ||
-      Context == DeclaratorContext::TemplateTypeArg)
+  case DeclaratorContext::TemplateArg:
+    return DeclSpecContext::DSC_template_arg;
+  case DeclaratorContext::TemplateTypeArg:
     return DeclSpecContext::DSC_template_type_arg;
-  if (Context == DeclaratorContext::TrailingReturn ||
-      Context == DeclaratorContext::TrailingReturnVar)
+  case DeclaratorContext::TrailingReturn:
+  case DeclaratorContext::TrailingReturnVar:
     return DeclSpecContext::DSC_trailing;
-  if (Context == DeclaratorContext::AliasDecl ||
-      Context == DeclaratorContext::AliasTemplate)
+  case DeclaratorContext::AliasDecl:
+  case DeclaratorContext::AliasTemplate:
     return DeclSpecContext::DSC_alias_declaration;
-  if (Context == DeclaratorContext::Association)
+  case DeclaratorContext::Association:
     return DeclSpecContext::DSC_association;
+  case DeclaratorContext::TypeName:
+    return DeclSpecContext::DSC_type_specifier;
+  case DeclaratorContext::Condition:
+    return DeclSpecContext::DSC_condition;
+  case DeclaratorContext::ConversionId:
+    return DeclSpecContext::DSC_conv_operator;
+  case DeclaratorContext::Prototype:
+  case DeclaratorContext::ObjCResult:
+  case DeclaratorContext::ObjCParameter:
+  case DeclaratorContext::KNRTypeList:
+  case DeclaratorContext::FunctionalCast:
+  case DeclaratorContext::Block:
+  case DeclaratorContext::ForInit:
+  case DeclaratorContext::SelectionInit:
+  case DeclaratorContext::CXXNew:
+  case DeclaratorContext::CXXCatch:
+  case DeclaratorContext::ObjCCatch:
+  case DeclaratorContext::BlockLiteral:
+  case DeclaratorContext::LambdaExpr:
+  case DeclaratorContext::LambdaExprParameter:
+  case DeclaratorContext::RequiresExpr:
     return DeclSpecContext::DSC_normal;
   }
 
+  llvm_unreachable("Missing DeclaratorContext case");
+}
+
 /// ParseAlignArgument - Parse the argument to an alignment-specifier.
 ///
 /// FIXME: Simply returns an alignof() expression if the argument is a
 /// type. Ideally, the type should be propagated directly into Sema.
 ///
 /// [C11]   type-id
 /// [C11]   constant-expression
 /// [C++0x] type-id ...[opt]

 /// [GNU]   '__thread'
 ///       function-specifier: [C99 6.7.4]
 /// [C99]   'inline'
 /// [C++]   'virtual'
 /// [C++]   'explicit'
 /// [OpenCL] '__kernel'
 ///       'friend': [C++ dcl.friend]
 ///       'constexpr': [C++0x dcl.constexpr]
-void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
-                                        const ParsedTemplateInfo &TemplateInfo,
-                                        AccessSpecifier AS,
-                                        DeclSpecContext DSContext,
-                                        LateParsedAttrList *LateAttrs) {
+void Parser::ParseDeclarationSpecifiers(
+    DeclSpec &DS, const ParsedTemplateInfo &TemplateInfo, AccessSpecifier AS,
+    DeclSpecContext DSContext, LateParsedAttrList *LateAttrs,
+    ImplicitTypenameContext AllowImplicitTypename) {
   if (DS.getSourceRange().isInvalid()) {
     // Start the range at the current token but make the end of the range
     // invalid.  This will make the entire range invalid unless we successfully
     // consume a token.
     DS.SetRangeStart(Tok.getLocation());
     DS.SetRangeEnd(SourceLocation());
   }
 
+  // If we are in a operator context, convert it back into a type specifier
+  // context for better error handling later on.
+  if (DSContext == DeclSpecContext::DSC_conv_operator) {
+    // No implicit typename here.
+    AllowImplicitTypename = ImplicitTypenameContext::No;
+    DSContext = DeclSpecContext::DSC_type_specifier;
+  }
+
   bool EnteringContext = (DSContext == DeclSpecContext::DSC_class ||
                           DSContext == DeclSpecContext::DSC_top_level);
   bool AttrsLastTime = false;
   ParsedAttributes attrs(AttrFactory);
   // We use Sema's policy to get bool macros right.
   PrintingPolicy Policy = Actions.getPrintingPolicy();
   while (true) {
     bool isInvalid = false;
case tok::annot_cxxscope: {
         // per C++ [class.qual]p2.
         //
         // To improve diagnostics for this case, parse the declaration as a
         // constructor (and reject the extra template arguments later).
         if ((DSContext == DeclSpecContext::DSC_top_level ||
              DSContext == DeclSpecContext::DSC_class) &&
             TemplateId->Name &&
             Actions.isCurrentClassName(*TemplateId->Name, getCurScope(), &SS) &&
-            isConstructorDeclarator(/*Unqualified=*/false)) {
+            isConstructorDeclarator(/*Unqualified=*/false,
+                                    /*DeductionGuide=*/false,
+                                    DS.isFriendSpecified())) {
           // The user meant this to be an out-of-line constructor
           // definition, but template arguments are not allowed
           // there.  Just allow this as a constructor; we'll
           // complain about it later.
           goto DoneWithDeclSpec;
         }
 
         DS.getTypeSpecScope() = SS;
         ConsumeAnnotationToken(); // The C++ scope.
         assert(Tok.is(tok::annot_template_id) &&
                "ParseOptionalCXXScopeSpecifier not working");
-        AnnotateTemplateIdTokenAsType(SS);
+        AnnotateTemplateIdTokenAsType(SS, AllowImplicitTypename);
         continue;
       }
 
       if (TemplateId && TemplateId->Kind == TNK_Concept_template &&
           GetLookAheadToken(2).isOneOf(tok::kw_auto, tok::kw_decltype)) {
         DS.getTypeSpecScope() = SS;
         // This is a qualified placeholder-specifier, e.g., ::C<int> auto ...
         // Consume the scope annotation and continue to consume the template-id
case tok::annot_cxxscope: {
                                        Tok.getAnnotationEndLoc(),
                                        PrevSpec, DiagID, T, Policy);
         if (isInvalid)
           break;
         DS.SetRangeEnd(Tok.getAnnotationEndLoc());
         ConsumeAnnotationToken(); // The typename
       }
 
+      if (AllowImplicitTypename == ImplicitTypenameContext::Yes &&
+          Next.is(tok::annot_template_id) &&
+          static_cast<TemplateIdAnnotation *>(Next.getAnnotationValue())
+                  ->Kind == TNK_Dependent_template_name) {
+        DS.getTypeSpecScope() = SS;
+        ConsumeAnnotationToken(); // The C++ scope.
+        AnnotateTemplateIdTokenAsType(SS, AllowImplicitTypename);
+        continue;
+      }
+
       if (Next.isNot(tok::identifier))
         goto DoneWithDeclSpec;
 
       // Check whether this is a constructor declaration. If we're in a
       // context where the identifier could be a class name, and it has the
       // shape of a constructor declaration, process it as one.
       if ((DSContext == DeclSpecContext::DSC_top_level ||
            DSContext == DeclSpecContext::DSC_class) &&
           Actions.isCurrentClassName(*Next.getIdentifierInfo(), getCurScope(),
                                      &SS) &&
-          isConstructorDeclarator(/*Unqualified*/ false))
+          isConstructorDeclarator(/*Unqualified*/ false,
+                                  /*DeductionGuide*/ false,
+                                  DS.isFriendSpecified()))
         goto DoneWithDeclSpec;
 
       // C++20 [temp.spec] 13.9/6.
       // This disables the access checking rules for function template explicit
       // instantiation and explicit specialization:
       // - `return type`.
       SuppressAccessChecks SAC(*this, IsTemplateSpecOrInst);
 
       ParsedType TypeRep =
           Actions.getTypeName(*Next.getIdentifierInfo(), Next.getLocation(),
                               getCurScope(), &SS, false, false, nullptr,
                               /*IsCtorOrDtorName=*/false,
                               /*WantNontrivialTypeSourceInfo=*/true,
-                              isClassTemplateDeductionContext(DSContext));
+                              isClassTemplateDeductionContext(DSContext),
+                              AllowImplicitTypename);
 
       if (IsTemplateSpecOrInst)
         SAC.done();
 
       // If the referenced identifier is not a type, then this declspec is
       // erroneous: We already checked about that it has no type specifier, and
       // C++ doesn't have implicit int.  Diagnose it as a typo w.r.t. to the
       // typename.
ParseIdentifier: {
         ConsumeToken();
         continue;
       }
 
       // If we're in a context where the identifier could be a class name,
       // check whether this is a constructor declaration.
       if (getLangOpts().CPlusPlus && DSContext == DeclSpecContext::DSC_class &&
           Actions.isCurrentClassName(*Tok.getIdentifierInfo(), getCurScope()) &&
-          isConstructorDeclarator(/*Unqualified*/true))
+          isConstructorDeclarator(/*Unqualified=*/ true,
+                                  /*DeductionGuide=*/ false,
+                                  DS.isFriendSpecified()))
         goto DoneWithDeclSpec;
 
       ParsedType TypeRep = Actions.getTypeName(
           *Tok.getIdentifierInfo(), Tok.getLocation(), getCurScope(), nullptr,
           false, false, nullptr, false, false,
           isClassTemplateDeductionContext(DSContext));
 
       // If this is not a typedef name, don't parse it as part of the declspec,
case tok::annot_template_id: {
         goto DoneWithDeclSpec;
       }
 
       // If we're in a context where the template-id could be a
       // constructor name or specialization, check whether this is a
       // constructor declaration.
       if (getLangOpts().CPlusPlus && DSContext == DeclSpecContext::DSC_class &&
           Actions.isCurrentClassName(*TemplateId->Name, getCurScope()) &&
-          isConstructorDeclarator(/*Unqualified=*/true))
+          isConstructorDeclarator(/*Unqualified=*/true,
+                                  /*DeductionGuide=*/false,
+                                  DS.isFriendSpecified()))
         goto DoneWithDeclSpec;
 
       // Turn the template-id annotation token into a type annotation
       // token, then try again to parse it as a type-specifier.
       CXXScopeSpec SS;
-      AnnotateTemplateIdTokenAsType(SS);
+      AnnotateTemplateIdTokenAsType(SS, AllowImplicitTypename);
       continue;
     }
 
     // Attributes support.
     case tok::kw___attribute:
     case tok::kw___declspec:
       ParseAttributes(PAKM_GNU | PAKM_Declspec, DS.getAttributes(), LateAttrs);
       continue;
if (shouldDelayDiagsInTag)
     diagsFromTag.done();
 
   TypeResult BaseType;
   SourceRange BaseRange;
 
   bool CanBeBitfield =
       getCurScope()->isClassScope() && ScopedEnumKWLoc.isInvalid() && Name;
 
   // Parse the fixed underlying type.
   if (Tok.is(tok::colon)) {
     // This might be an enum-base or part of some unrelated enclosing context.
     //
     // 'enum E : base' is permitted in two circumstances:
     //
     // 1) As a defining-type-specifier, when followed by '{'.
     // 2) As the sole constituent of a complete declaration -- when DS is empty
     //    and the next token is ';'.
     //
if (Tok.is(tok::colon)) {
     } else if (CanHaveEnumBase || !ColonIsSacred) {
       SourceLocation ColonLoc = ConsumeToken();
 
       // Parse a type-specifier-seq as a type. We can't just ParseTypeName here,
       // because under -fms-extensions,
       //   enum E : int *p;
       // declares 'enum E : int; E *p;' not 'enum E : int*; E p;'.
       DeclSpec DS(AttrFactory);
-      ParseSpecifierQualifierList(DS, AS, DeclSpecContext::DSC_type_specifier);
+      // enum-base is not assumed to be a type and therefore requires the
+      // typename keyword [p0634r3].
+      ParseSpecifierQualifierList(DS, ImplicitTypenameContext::No, AS,
+                                  DeclSpecContext::DSC_type_specifier);
       Declarator DeclaratorInfo(DS, ParsedAttributesView::none(),
                                 DeclaratorContext::TypeName);
       BaseType = Actions.ActOnTypeName(getCurScope(), DeclaratorInfo);
 
       BaseRange = SourceRange(ColonLoc, DeclaratorInfo.getSourceRange().getEnd());
 
       if (!getLangOpts().ObjC) {
         if (getLangOpts().CPlusPlus11)
#include "clang/Basic/OpenCLImageTypes.def"
   case tok::kw__Atomic:
     return true;
   }
 }
 
 /// isDeclarationSpecifier() - Return true if the current token is part of a
 /// declaration specifier.
 ///
+/// \param AllowImplicitTypename whether this is a context where T::type [T
+/// dependent] can appear.
 /// \param DisambiguatingWithExpression True to indicate that the purpose of
 /// this check is to disambiguate between an expression and a declaration.
-bool Parser::isDeclarationSpecifier(bool DisambiguatingWithExpression) {
+bool Parser::isDeclarationSpecifier(
+    ImplicitTypenameContext AllowImplicitTypename,
+    bool DisambiguatingWithExpression) {
   switch (Tok.getKind()) {
   default: return false;
 
   // OpenCL 2.0 and later define this keyword.
   case tok::kw_pipe:
     return getLangOpts().OpenCL &&
            getLangOpts().getOpenCLCompatibleVersion() >= 200;
 
   case tok::identifier:   // foo::bar
     // Unfortunate hack to support "Class.factoryMethod" notation.
     if (getLangOpts().ObjC && NextToken().is(tok::period))
       return false;
     if (TryAltiVecVectorToken())
       return true;
     [[fallthrough]];
   case tok::kw_decltype: // decltype(T())::type
   case tok::kw_typename: // typename T::type
     // Annotate typenames and C++ scope specifiers.  If we get one, just
     // recurse to handle whatever we get.
-    if (TryAnnotateTypeOrScopeToken())
+    if (TryAnnotateTypeOrScopeToken(AllowImplicitTypename))
       return true;
     if (TryAnnotateTypeConstraint())
       return true;
     if (Tok.is(tok::identifier))
       return false;
 
     // If we're in Objective-C and we have an Objective-C class type followed
     // by an identifier and then either ':' or ']', in a place where an
     // expression is permitted, then this is probably a class message send
     // missing the initial '['. In this case, we won't consider this to be
     // the start of a declaration.
     if (DisambiguatingWithExpression &&
         isStartOfObjCClassMessageMissingOpenBracket())
       return false;
 
-    return isDeclarationSpecifier();
+    return isDeclarationSpecifier(AllowImplicitTypename);
 
   case tok::coloncolon:   // ::foo::bar
     if (NextToken().is(tok::kw_new) ||    // ::new
         NextToken().is(tok::kw_delete))   // ::delete
       return false;
 
     // Annotate typenames and C++ scope specifiers.  If we get one, just
     // recurse to handle whatever we get.
     if (TryAnnotateTypeOrScopeToken())
       return true;
-    return isDeclarationSpecifier();
+    return isDeclarationSpecifier(ImplicitTypenameContext::No);
 
     // storage-class-specifier
   case tok::kw_typedef:
   case tok::kw_extern:
   case tok::kw___private_extern__:
   case tok::kw_static:
   case tok::kw_auto:
   case tok::kw___auto_type:
#include "clang/Basic/OpenCLImageTypes.def"
 
     return true;
 
   case tok::kw_private:
     return getLangOpts().OpenCL;
   }
 }
 
-bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide) {
+bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
+                                     bool IsFriend) {
   TentativeParsingAction TPA(*this);
 
   // Parse the C++ scope specifier.
   CXXScopeSpec SS;
   if (ParseOptionalCXXScopeSpecifier(SS, /*ObjectType=*/nullptr,
                                      /*ObjectHasErrors=*/false,
                                      /*EnteringContext=*/true)) {
     TPA.Revert();
bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
 
   // Optionally skip Microsoft attributes.
   ParsedAttributes Attrs(AttrFactory);
   MaybeParseMicrosoftAttributes(Attrs);
 
   // Check whether the next token(s) are part of a declaration
   // specifier, in which case we have the start of a parameter and,
   // therefore, we know that this is a constructor.
+  // Due to an ambiguity with implicit typename, the above is not enough.
+  // Additionally, check to see if we are a friend.
   bool IsConstructor = false;
-  if (isDeclarationSpecifier())
+  if (isDeclarationSpecifier(IsFriend ? ImplicitTypenameContext::No
+                                      : ImplicitTypenameContext::Yes))
     IsConstructor = true;
   else if (Tok.is(tok::identifier) ||
            (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier))) {
     // We've seen "C ( X" or "C ( X::Y", but "X" / "X::Y" is not a type.
     // This might be a parenthesized member name, but is more likely to
     // be a constructor declaration with an invalid argument type. Keep
     // looking.
     if (Tok.is(tok::annot_cxxscope))
       ConsumeAnnotationToken();
if (Tok.is(tok::l_paren)) {
                                 (IsFunctionDeclaration
                                    ? Scope::FunctionDeclarationScope : 0));
 
       // The paren may be part of a C++ direct initializer, eg. "int x(1);".
       // In such a case, check if we actually have a function declarator; if it
       // is not, the declarator has been fully parsed.
       bool IsAmbiguous = false;
       if (getLangOpts().CPlusPlus && D.mayBeFollowedByCXXDirectInit()) {
+        // C++2a [temp.res]p5
+        // A qualified-id is assumed to name a type if
+        //   - [...]
+        //   - it is a decl-specifier of the decl-specifier-seq of a
+        //     - [...]
+        //     - parameter-declaration in a member-declaration [...]
+        //     - parameter-declaration in a declarator of a function or function
+        //       template declaration whose declarator-id is qualified [...]
+        auto AllowImplicitTypename = ImplicitTypenameContext::No;
+        if (D.getCXXScopeSpec().isSet())
+          AllowImplicitTypename =
+              (ImplicitTypenameContext)Actions.isDeclaratorFunctionLike(D);
+        else if (D.getContext() == DeclaratorContext::Member) {
+          AllowImplicitTypename = ImplicitTypenameContext::Yes;
+        }
+
         // The name of the declarator, if any, is tentatively declared within
         // a possible direct initializer.
         TentativelyDeclaredIdentifiers.push_back(D.getIdentifier());
-        bool IsFunctionDecl = isCXXFunctionDeclarator(&IsAmbiguous);
+        bool IsFunctionDecl =
+            isCXXFunctionDeclarator(&IsAmbiguous, AllowImplicitTypename);
         TentativelyDeclaredIdentifiers.pop_back();
         if (!IsFunctionDecl)
           break;
       }
       ParsedAttributes attrs(AttrFactory);
       BalancedDelimiterTracker T(*this, tok::l_paren);
       T.consumeOpen();
       if (IsFunctionDeclaration)
void Parser::ParseParenDeclarator(Declarator &D) {
   // grouping parens. However, if this could be an abstract-declarator, then
   // this could also be the start of function arguments (consider 'void()').
   bool isGrouping;
 
   if (!D.mayOmitIdentifier()) {
     // If this can't be an abstract-declarator, this *must* be a grouping
     // paren, because we haven't seen the identifier yet.
     isGrouping = true;
-  } else if (Tok.is(tok::r_paren) ||           // 'int()' is a function.
+  } else if (Tok.is(tok::r_paren) ||                // 'int()' is a function.
              (getLangOpts().CPlusPlus && Tok.is(tok::ellipsis) &&
-              NextToken().is(tok::r_paren)) || // C++ int(...)
-             isDeclarationSpecifier() ||       // 'int(int)' is a function.
-             isCXX11AttributeSpecifier()) {    // 'int([[]]int)' is a function.
+              NextToken().is(tok::r_paren)) ||      // C++ int(...)
+             isDeclarationSpecifier(
+                 ImplicitTypenameContext::No) || // 'int(int)' is a function.
+             isCXX11AttributeSpecifier()) {         // 'int([[]]int)' is a function.
     // This handles C99 6.7.5.3p11: in "typedef int X; void foo(X)", X is
     // considered to be a type, not a K&R identifier-list.
     isGrouping = false;
   } else {
     // Otherwise, this is a grouping paren, e.g. 'int (*X)' or 'int(X)'.
     isGrouping = true;
   }
 
if (isFunctionDeclaratorIdentifierList()) {
     EndLoc = RParenLoc;
 
     // If there are attributes following the identifier list, parse them and
     // prohibit them.
     MaybeParseCXX11Attributes(FnAttrs);
     ProhibitAttributes(FnAttrs);
   } else {
     if (Tok.isNot(tok::r_paren))
-      ParseParameterDeclarationClause(D.getContext(), FirstArgAttrs, ParamInfo,
+      ParseParameterDeclarationClause(D, FirstArgAttrs, ParamInfo,
                                       EllipsisLoc);
     else if (RequiresArg)
       Diag(Tok, diag::err_argument_required_after_attribute);
 
     // OpenCL disallows functions without a prototype, but it doesn't enforce
     // strict prototypes as in C2x because it allows a function definition to
     // have an identifier list. See OpenCL 3.0 6.11/g for more details.
     HasProto = ParamInfo.size() || getLangOpts().requiresStrictPrototypes() ||

 /// [C++]   declaration-specifiers abstract-declarator[opt]
 ///           '=' assignment-expression
 /// [GNU]   declaration-specifiers abstract-declarator[opt] attributes
 /// [C++11] attribute-specifier-seq parameter-declaration
 ///
 void Parser::ParseParameterDeclarationClause(
     DeclaratorContext DeclaratorCtx, ParsedAttributes &FirstArgAttrs,
     SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
-    SourceLocation &EllipsisLoc) {
+    SourceLocation &EllipsisLoc, bool IsACXXFunctionDeclaration) {
 
   // Avoid exceeding the maximum function scope depth.
   // See https://bugs.llvm.org/show_bug.cgi?id=19607
   // Note Sema::ActOnParamDeclarator calls ParmVarDecl::setScopeInfo with
   // getFunctionPrototypeDepth() - 1.
   if (getCurScope()->getFunctionPrototypeDepth() - 1 >
       ParmVarDecl::getMaxFunctionScopeDepth()) {
     Diag(Tok.getLocation(), diag::err_function_scope_depth_exceeded)
         << ParmVarDecl::getMaxFunctionScopeDepth();
     cutOffParsing();
     return;
   }
 
+  // C++2a [temp.res]p5
+  // A qualified-id is assumed to name a type if
+  //   - [...]
+  //   - it is a decl-specifier of the decl-specifier-seq of a
+  //     - [...]
+  //     - parameter-declaration in a member-declaration [...]
+  //     - parameter-declaration in a declarator of a function or function
+  //       template declaration whose declarator-id is qualified [...]
+  //     - parameter-declaration in a lambda-declarator [...]
+  auto AllowImplicitTypename = ImplicitTypenameContext::No;
+  if (DeclaratorCtx == DeclaratorContext::Member ||
+      DeclaratorCtx == DeclaratorContext::LambdaExpr ||
+      IsACXXFunctionDeclaration) {
+      AllowImplicitTypename = ImplicitTypenameContext::Yes;
+    }
+
   do {
     // FIXME: Issue a diagnostic if we parsed an attribute-specifier-seq
     // before deciding this was a parameter-declaration-clause.
     if (TryConsumeToken(tok::ellipsis, EllipsisLoc))
       break;
 
     // Parse the declaration-specifiers.
     // Just use the ParsingDeclaration "scope" of the declarator.
if (FirstArgAttrs.Range.isValid()) {
       MaybeParseCXX11Attributes(ArgDeclAttrs);
 
       // Skip any Microsoft attributes before a param.
       MaybeParseMicrosoftAttributes(ArgDeclSpecAttrs);
     }
 
     SourceLocation DSStart = Tok.getLocation();
 
-    ParseDeclarationSpecifiers(DS);
+    ParseDeclarationSpecifiers(DS, /*TemplateInfo=*/ParsedTemplateInfo(),
+                               AS_none, DeclSpecContext::DSC_normal,
+                               /*LateAttrs=*/nullptr, AllowImplicitTypename);
     DS.takeAttributesFrom(ArgDeclSpecAttrs);
 
     // Parse the declarator.  This is "PrototypeContext" or
     // "LambdaExprParameterContext", because we must accept either
     // 'declarator' or 'abstract-declarator' here.
     Declarator ParmDeclarator(DS, ArgDeclAttrs,
                               DeclaratorCtx == DeclaratorContext::RequiresExpr
                                   ? DeclaratorContext::RequiresExpr

clang/lib/Parse/ParseDeclCXX.cpp

Declarator DeclaratorInfo(DS, ParsedAttributesView::none(),
                               DeclaratorContext::TypeName);
     return Actions.ActOnTypeName(getCurScope(), DeclaratorInfo);
   }
 
   // Check whether we have a template-id that names a type.
   if (Tok.is(tok::annot_template_id)) {
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     if (TemplateId->mightBeType()) {
-      AnnotateTemplateIdTokenAsType(SS, /*IsClassName*/ true);
+      AnnotateTemplateIdTokenAsType(SS, ImplicitTypenameContext::No,
+      /*IsClassName=*/ true);
 
       assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
       TypeResult Type = getTypeAnnotation(Tok);
       EndLocation = Tok.getAnnotationEndLoc();
       ConsumeAnnotationToken();
       return Type;
     }
 
if (Tok.is(tok::less)) {
     TemplateName.setIdentifier(Id, IdLoc);
 
     // Parse the full template-id, then turn it into a type.
     if (AnnotateTemplateIdToken(Template, TNK, SS, SourceLocation(),
                                 TemplateName))
       return true;
     if (Tok.is(tok::annot_template_id) &&
         takeTemplateIdAnnotation(Tok)->mightBeType())
-      AnnotateTemplateIdTokenAsType(SS, /*IsClassName*/ true);
+      AnnotateTemplateIdTokenAsType(SS, ImplicitTypenameContext::No,
+                                    /*IsClassName=*/true);
 
     // If we didn't end up with a typename token, there's nothing more we
     // can do.
     if (Tok.isNot(tok::annot_typename))
       return true;
 
     // Retrieve the type from the annotation token, consume that token, and
     // return.
     EndLocation = Tok.getAnnotationEndLoc();
     TypeResult Type = getTypeAnnotation(Tok);
     ConsumeAnnotationToken();
     return Type;
   }
 
   // We have an identifier; check whether it is actually a type.
   IdentifierInfo *CorrectedII = nullptr;
   ParsedType Type = Actions.getTypeName(
       *Id, IdLoc, getCurScope(), &SS, /*isClassName=*/true, false, nullptr,
       /*IsCtorOrDtorName=*/false,
       /*WantNontrivialTypeSourceInfo=*/true,
-      /*IsClassTemplateDeductionContext*/ false, &CorrectedII);
+      /*IsClassTemplateDeductionContext=*/false, ImplicitTypenameContext::No,
+      &CorrectedII);
   if (!Type) {
     Diag(IdLoc, diag::err_expected_class_name);
     return true;
   }
 
   // Consume the identifier.
   EndLocation = IdLoc;
 
if (Tok.is(tok::identifier)) {
     // ParseOptionalCXXScopeSpecifier at this point.
     // FIXME: Can we get here with a scope specifier?
     ParseDecltypeSpecifier(DS);
   } else {
     TemplateIdAnnotation *TemplateId = Tok.is(tok::annot_template_id)
                                            ? takeTemplateIdAnnotation(Tok)
                                            : nullptr;
     if (TemplateId && TemplateId->mightBeType()) {
-      AnnotateTemplateIdTokenAsType(SS, /*IsClassName*/ true);
+      AnnotateTemplateIdTokenAsType(SS, ImplicitTypenameContext::No,
+      /*IsClassName=*/true);
       assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
       TemplateTypeTy = getTypeAnnotation(Tok);
       ConsumeAnnotationToken();
     } else {
       Diag(Tok, diag::err_expected_member_or_base_name);
       return true;
     }
   }

clang/lib/Parse/ParseExpr.cpp

if (Next.is(tok::annot_template_id)) {
       if (TemplateId->Kind == TNK_Type_template) {
         // We have a qualified template-id that we know refers to a
         // type, translate it into a type and continue parsing as a
         // cast expression.
         CXXScopeSpec SS;
         ParseOptionalCXXScopeSpecifier(SS, /*ObjectType=*/nullptr,
                                        /*ObjectHasErrors=*/false,
                                        /*EnteringContext=*/false);
-        AnnotateTemplateIdTokenAsType(SS);
+        AnnotateTemplateIdTokenAsType(SS, ImplicitTypenameContext::Yes);
         return ParseCastExpression(ParseKind, isAddressOfOperand, NotCastExpr,
                                    isTypeCast, isVectorLiteral,
                                    NotPrimaryExpression);
       }
     }
 
     // Parse as an id-expression.
     Res = ParseCXXIdExpression(isAddressOfOperand);
     break;
   }
 
   case tok::annot_template_id: { // [C++]          template-id
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     if (TemplateId->Kind == TNK_Type_template) {
       // We have a template-id that we know refers to a type,
       // translate it into a type and continue parsing as a cast
       // expression.
       CXXScopeSpec SS;
-      AnnotateTemplateIdTokenAsType(SS);
+      AnnotateTemplateIdTokenAsType(SS, ImplicitTypenameContext::Yes);
       return ParseCastExpression(ParseKind, isAddressOfOperand,
                                  NotCastExpr, isTypeCast, isVectorLiteral,
                                  NotPrimaryExpression);
     }
 
     // Fall through to treat the template-id as an id-expression.
     [[fallthrough]];
   }

clang/lib/Parse/ParseExprCXX.cpp

if (Tok.is(tok::l_paren)) {
     // Parse parameter-declaration-clause.
     SmallVector<DeclaratorChunk::ParamInfo, 16> ParamInfo;
     SourceLocation EllipsisLoc;
 
     if (Tok.isNot(tok::r_paren)) {
       Actions.RecordParsingTemplateParameterDepth(
           CurTemplateDepthTracker.getOriginalDepth());
 
-      ParseParameterDeclarationClause(D.getContext(), Attr, ParamInfo,
+      ParseParameterDeclarationClause(D, 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);
if (Next.is(tok::colon) && areTokensAdjacent(Tok, Next))
       FixDigraph(*this, PP, Tok, Next, Kind, /*AtDigraph*/true);
   }
 
   if (ExpectAndConsume(tok::less, diag::err_expected_less_after, CastName))
     return ExprError();
 
   // Parse the common declaration-specifiers piece.
   DeclSpec DS(AttrFactory);
-  ParseSpecifierQualifierList(DS);
+  ParseSpecifierQualifierList(DS, /*AccessSpecifier=*/AS_none,
+                              DeclSpecContext::DSC_type_specifier);
 
   // Parse the abstract-declarator, if present.
   Declarator DeclaratorInfo(DS, ParsedAttributesView::none(),
                             DeclaratorContext::TypeName);
   ParseDeclarator(DeclaratorInfo);
 
   SourceLocation RAngleBracketLoc = Tok.getLocation();
 

 /// by parsing the type-specifier-seq, because these sequences are
 /// typically followed by some form of declarator. Returns true and
 /// emits diagnostics if this is not a type-specifier-seq, false
 /// otherwise.
 ///
 ///   type-specifier-seq: [C++ 8.1]
 ///     type-specifier type-specifier-seq[opt]
 ///
-bool Parser::ParseCXXTypeSpecifierSeq(DeclSpec &DS) {
-  ParseSpecifierQualifierList(DS, AS_none, DeclSpecContext::DSC_type_specifier);
+bool Parser::ParseCXXTypeSpecifierSeq(DeclSpec &DS, DeclaratorContext Context) {
+  ParseSpecifierQualifierList(DS, AS_none,
+                              getDeclSpecContextFromDeclaratorContext(Context));
   DS.Finish(Actions, Actions.getASTContext().getPrintingPolicy());