[Bug c++/102721] New: out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

["nickhuang99 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

            Bug ID: 102721
           Summary: out-of-line member function definition fails to allow
                    lambda in unevaluated-context of  new feature in c++20
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nickhuang99 at hotmail dot com
  Target Milestone: ---

The following snippet code exposes that GCC parser doesn't comply with c++20
standard to allow lambda in unevaluated context. It seems this out-of-line
definition creates a new lambda type where lambda as operand of "decltype"
should be treated as in unevaluated context. So, it shouldn't create a new
lambda type in out-of-line definition.

Both clang and MSVC++ (https://www.godbolt.org/z/EMbb44fd8) parse this
correctly.
(with "-std=c++20")

template<class T>
struct A
{    
    void foo(const T){}
};

template<>
void A<decltype(+[]{})[3]>::foo(const decltype(+[]{})[3])
{}




<source>:9:6: error: ambiguating new declaration of 'void A<void (*
[3])()>::foo(void (* const*)())'
    9 | void A<decltype(+[]{})[3]>::foo(const decltype(+[]{})[3])
      |      ^~~~~~~~~~~~~~~~~~~~~
<source>:5:10: note: old declaration 'void A<T>::foo(T) [with T = void (*
[3])()]'
    5 |     void foo(const T){}
      |          ^~~