[Bug c++/85806] New: [concepts] Hard error for "invalid use of non-static data member" in a requires expression

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85806

            Bug ID: 85806
           Summary: [concepts] Hard error for "invalid use of non-static
                    data member" in a requires expression
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

Compiling this program fragment with g++ -std=c++17 -fconcepts
(https://godbolt.org/g/L1b6TS):

  template<int> using helper = void;

  template<typename T>
  concept bool HasCount = requires {
      typename ::helper<T::count>;
  };

  struct S {
      int count = 42;
  };
  static_assert(!HasCount<S>);

produces a diagnostic:

  <source>:11:18: error: invalid use of non-static data member 'S::count'
     static_assert(!HasCount<S>);
                    ^~~~~~~~~~~
  <source>:9:19: note: declared here
         int count = 42;
                     ^~

rather than the expected successful and silent compile.