[Bug libstdc++/110593] New: The std::ratio meta arithmetic can accept non-std::ratio

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110593

            Bug ID: 110593
           Summary: The std::ratio meta arithmetic can accept
                    non-std::ratio
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

[ratio.general]: "If a template parameter is named R1 or R2, and the template
argument is not a specialization of the ratio template, the program is
ill-formed."

So for the following libstdc++ needs to be diagnosed, right?

#include <ratio>

struct Ratio { constexpr static double num = 0, den = 1; };
static_assert(std::ratio_equal<Ratio, std::ratio<0, 5>>());

https://godbolt.org/z/MhazY5ecn

Only MSVC-STL triggers the static assertion.