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