[Bug c++/102728] New: requires statement fails to recognize lambda in unevaluated context

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102728

            Bug ID: 102728
           Summary: requires statement fails to recognize lambda in
                    unevaluated context
           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: ---

Consider that this snippet of code gives error of no matching specialization
which is not rooted from lambda in function parameter, instead it is rooted
from "requires" statement violation of lambda in unevaluated-context. The
warning about no-return-statement in lambda within requirement exposes this is
"requires" issue.

#include <concepts>
template<class T>
concept IsLambda=requires(T t){
    {t}->std::convertible_to<decltype(+[]{})>; 
};

template<IsLambda T> // OK if replace IsLambda with "class"
void foo(T t){ t();}

template<>
void foo<decltype(+[]{})>(decltype(+[]{}) t){
    t();
}

Also, a proof that this is not lambda-in-function-parameter issue is that you
can replace template parameter "IsLambda" in template function declaration with
"class" and GCC works fine.

Both clang and MSVC++ accepts above code. (https://www.godbolt.org/z/xhW7Tofao)





<source>: In substitution of 'template<class T>  requires  IsLambda<T> void
foo(T) [with T = void (*)()]':
<source>:12:44:   required from here
<source>:5:40: warning: no return statement in function returning non-void
[-Wreturn-type]
    5 |     {t}->std::convertible_to<decltype(+[]{})>;
      |                                        ^~~~
<source>:12:6: error: template-id 'foo<void (*)()>' for 'void foo(void (*)())'
does not match any template declaration
   12 | void foo<decltype(+[]{})>(decltype(+[]{}) t){
      |      ^~~~~~~~~~~~~~~~~~~~
<source>:9:6: note: candidate is: 'template<class T>  requires  IsLambda<T>
void foo(T)'
    9 | void foo(T t){ t();}
      |      ^~~