[Bug c++/102367] New: Types may be defined in `decltype` or `sizeof` expressions in C++20

[Bug c++/102367] New: Types may be defined in `decltype` or `sizeof` expressions in C++20

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102367

            Bug ID: 102367
           Summary: Types may be defined in `decltype` or `sizeof`
                    expressions in C++20
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

Since C++20 lambdas are allowed in not-evaluated context. So the code as
follows must be valid:
```
using T = decltype( []{ struct S{}; return S{}; }() );

[[maybe_unused]] constexpr auto N 
          = sizeof( []{ struct S{}; return S{}; }() );
```

It is indeed accepted by MSVC and Clang, but rejected by GCC, demo:
https://gcc.godbolt.org/z/jYjxcr1eP

The error is
```
error: types may not be defined in 'decltype' expressions
    1 | using T = decltype( []{ struct S{}; return S{}; }() );
      |                                 ^
<source>: In lambda function:
<source>:4:33: error: types may not be defined in 'sizeof' expressions
    4 |           = sizeof( []{ struct S{}; return S{}; }() );
      |             
```

[Bug c++/102367] types can be defined in lambdas in unevaluated expression (decltype/sizeof) in C++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102367

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Types may be defined in     |types can be defined in
                   |`decltype` or `sizeof`      |lambdas in unevaluated
                   |expressions in C++20        |expression
                   |                            |(decltype/sizeof) in C++20
   Last reconfirmed|                            |2021-09-16
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I thought there was a dup of this bug somewhere.
Anyways confirmed.

[Bug c++/102367] types can be defined in lambdas in unevaluated expression (decltype/sizeof) in C++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102367

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> I thought there was a dup of this bug somewhere.
> Anyways confirmed.

I did, PR 101911.

*** This bug has been marked as a duplicate of bug 101911 ***