[Bug c++/101004] New: SFINAE constrained conversion operator of class template parameter type does not work with built-in arithmetic operators

["namark at disroot dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101004

            Bug ID: 101004
           Summary: SFINAE constrained conversion operator of class
                    template parameter type does not work with built-in
                    arithmetic operators
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: namark at disroot dot org
  Target Milestone: ---

I believe the following code should be valid, but gcc rejects it (version
10.2.0 on my machine, but also all versions I could try on godbolt.org)

#include <type_traits>

template <typename T, std::size_t S>
class One
{
        public:
        template <std::size_t SS = S, std::enable_if_t<SS == 1>* = nullptr>
        operator T() const { return 1; }
};

int main()
{
        return 1 - One<int,1>{}; // no match for operator-
}

The implicit conversion seems to work fine in general with initialization,
assignment or user defined operators/functions, but not with the built-in
arithmetic or comparison operators. Removing the SFINAE constraint fixes it.
Changing the conversion operator type from T to int also fixes it. That is,
either of those is fine on its own, but together they cause this strange
behavior.