I wrote the following program that compiles with msvc but rejected by clang and gcc. It uses explicit object member function. Demo.

#include <iostream>

struct C{
   int f(this int);
};

decltype(C::f) func;
  
int main()
{ 
     func(3);    //msvc accepts but clang and gcc rejects
}
int func(int j)
{
   std::cout << "called with: " << j << std::endl;
   return 5;
}

As you can see msvc accepts this program but gcc says:

<source>:7:13: error: invalid use of non-static member function 'int C::f(this int)'
    7 | decltype(C::f) func;
      |             ^
<source>:7:13: error: invalid use of non-static member function 'int C::f(this int)'
<source>:7:13: error: invalid use of non-static member function 'int C::f(this int)'
<source>: In function 'int main()':
<source>:11:10: error: 'func' cannot be used as a function
   11 |      func(3);    //msvc accepts but clang and gcc rejects
      |      ~~~~^~~
<source>: At global scope:
<source>:13:15: error: 'int func(int)' redeclared as different kind of entity
   13 | int func(int j)
      |               ^
<source>:7:16: note: previous declaration 'int func'
    7 | decltype(C::f) func;
      |                ^~~~

I want to know if this is well-formed or ill-formed etc as per the standard c++.