[Bug libstdc++/81076] New: __byte_operand is not SFINAE-friendly

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81076

            Bug ID: 81076
           Summary: __byte_operand<cv T> is not SFINAE-friendly
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

Because the primary template is left undefined, the cv specializations of
__byte_operand causes a hard error for nonintegral types:

#include <cstddef>

template<class> void to_integer(...);

using t = decltype(to_integer<void* const>(std::byte{}));
using t = void;