[Bug c++/93740] New: Template base classes parametrized by pointer-to-member are amibiguous

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93740

            Bug ID: 93740
           Summary: Template base classes parametrized by
                    pointer-to-member are amibiguous
           Product: gcc
           Version: 9.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.cencora at gmail dot com
  Target Milestone: ---

Following code does not compile on any gcc version regardless of chosen C++
standard version, but compiles correctly on clang, msvc and icc.

struct A
{
    virtual void foo() = 0;
    virtual void bar() = 0;
};

template <typename T, T val>
struct const_val{};

template <int N, typename T>
struct indexed_elem{};

using mem_fun_A_foo = const_val<decltype(&A::foo), &A::foo>;
using mem_fun_A_bar = const_val<decltype(&A::bar), &A::bar>;

struct A_indexed_member_funcs
  : indexed_elem<0, mem_fun_A_foo>,
    indexed_elem<1, mem_fun_A_bar>
{};

template <typename T, int N>
constexpr int index_of(indexed_elem<N, T>)
{
    return N;
}

void test()
{
    static_assert(index_of<mem_fun_A_foo>(A_indexed_member_funcs{}) == 0, "");
}