[Bug c++/90897] New: user defined conversion operators ambiguous in GCC8, but not GCC7 and GCC9

[h2+bugs at fsfe dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90897

            Bug ID: 90897
           Summary: user defined conversion operators ambiguous in GCC8,
                    but not GCC7 and GCC9
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: h2+bugs at fsfe dot org
  Target Milestone: ---

The rather simple example below fails to build on GCC8 in C++17 mode, but
passes on GCC7, GCC9 and trunk in C++17 mode. It fails on all compilers in
C++14 mode.

Tested versions:
g++7 (FreeBSD Ports Collection) 7.4.0
g++8 (FreeBSD Ports Collection) 8.3.0
g++9 (FreeBSD Ports Collection) 9.1.0
g++10 (FreeBSD Ports Collection) 10.0.0 20190602 (experimental)

Apparently the code is correct for C++17, but GCC8 was not updated accordingly?

Thanks for your help!

struct S2
{
    constexpr S2() = default;
    constexpr S2(S2 const &) = default;
    constexpr S2(S2 &&) = default;
    constexpr S2& operator=(S2 const &) = default;
    constexpr S2& operator=(S2 &&) = default;

    constexpr S2 (int const &) {}
};

struct S
{
    operator int()
    {
        return 3;
    }

    operator S2()
    {
        return S2{};
    }
};

int main()
{
    S2 s = static_cast<S2>(S{});
}