[Bug c++/71954] template partial specialization for constexpr error

[suratovvlad at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71954

--- Comment #2 from Vladislav Suratov <suratovvlad at gmail dot com> ---
Confirmed with 9.3, 10.1 and current trunk (11.0). 
Tested with --std=c++17, meanwhile clang 10.0 (and trunk) and MSVC 19.24 and
MSVC 19.25 successfully compile it.
The following example is what I used for testing:

#include <ratio>

struct Test1
{
    template <typename _Ty>
    struct Helper1{
      static constexpr bool value = false;
    };

    template <intmax_t _R1, intmax_t _R2>
    struct Helper1<std::ratio<_R1, _R2>>{
      static constexpr bool value = true;
    };
};

struct Test2
{
    template <typename _Ty>
    static constexpr bool Helper1 = false;

    template <intmax_t _R1, intmax_t _R2>
    static constexpr bool Helper1<std::ratio<_R1, _R2>> = true;
};

static_assert(Test1::Helper1<std::ratio<1, 1000>>::value, "Check");
static_assert(Test2::Helper1<std::ratio<1, 1000>>, "Check");