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

["fchelnokov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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{}; }() );
      |             
```