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