[Bug c++/123878] New: GCC rejects valid program involving lambda evaluation inside decltype with deleted parameterized ctor

[jlame646 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123878

            Bug ID: 123878
           Summary: GCC rejects valid program involving lambda evaluation
                    inside decltype with deleted parameterized ctor
           Product: gcc
           Version: 15.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following valid program is rejected by gcc and clang but accepted by msvc.
Demo: https://godbolt.org/z/KnzqaM1Ev

```

struct C
{
  C(int) = delete;
  C(){};
};

decltype([b = C(3)](){ return 4;}()) var; //msvc OK but gcc and clang rejects

decltype([C(3)](){ return 4;}()) var2; //accepted by all compilers
int main()
{

} 
```
This is well-formed as explained in the following stackoverflow thread:
https://stackoverflow.com/q/79878786/12002570