[Bug c++/78173] Hard error subtracting pointers to incomplete type in SFINAE context

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78173

Casey Carter <Casey at Carter dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Casey at Carter dot net
             Blocks|                            |67491

--- Comment #1 from Casey Carter <Casey at Carter dot net> ---
This still reproduces on trunk, and in concepts land. This program fragment:

  template <class T>
  concept bool CanDifference = requires(T const& x, T const& y) {
      x - y;
  };

  static_assert(!CanDifference<void*>);

produces diagnostics when compiled with "g++ -std=c++2a -fconcepts"
(https://godbolt.org/g/e36eFK):

  <source>:3:7: error: invalid use of 'void'
       x - y;
       ~~^~~


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues