[Bug c++/105351] [concepts] Constraint checking does correctly match static member attributes

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105351

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-05-03
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Er, the real reason the above works is because the type auto* of dummy's
template parameter demands that the argument &T::attr2 is an ordinary pointer,
not a pointer to member, which implies that it's a static member.

We should also be able to check that attr2 is a static member via

template<auto> struct dummy;

template<typename T>
concept C = requires(T v) {
    v.attr1;
    typename dummy<T::attr2>;
    v.fun1();
    T::fun2();
};

which Clang is happy with, but GCC incorrectly emits an error during SFINAE:

<source>: In substitution of 'template<class auto:1>  requires  C<auto:1> auto
f(auto:1) [with auto:1 = Z]':
<source>:48:5:   required from here
<source>:9:14: error: invalid use of non-static data member 'Z::attr2'
    9 |     typename dummy<T::attr2>;
      |     ~~~~~~~~~^~~~~~~~~~~~~~~~
<source>:39:15: note: declared here
   39 |     const int attr2 = 0;
      |               ^~~~~

This is definitely a bug.