[Bug c++/122851] New: Ambiguous function call with an empty initializer list as an explicit template argument is accepted in favor of the last declaration

[gulackeg at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122851

            Bug ID: 122851
           Summary: Ambiguous function call with an empty initializer list
                    as an explicit template argument is accepted in favor
                    of the last declaration
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gulackeg at gmail dot com
  Target Milestone: ---

Test code:

    // https://godbolt.org/z/fKE6M4Ybq

    struct A {};
    struct B {};

    namespace test1 {
    template <A> A f();
    template <B> B f();

    // - ❌ GCC: OK
    // - ✅ Clang:
    //   - error: call to 'f' is ambiguous
    //   - note: candidate function [with $0 = A{}]
    //   - note: candidate function [with $0 = B{}]
    using T = decltype(f<{}>());
    using T = B;
    } // namespace test1

    namespace test2 {
    template <B> B f();
    template <A> A f();

    // - ❌ GCC: OK
    // - ✅ Clang:
    //   - error: call to 'f' is ambiguous
    //   - note: candidate function [with $0 = B{}]
    //   - note: candidate function [with $0 = A{}]
    using T = decltype(f<{}>());
    using T = A;
    } // namespace test2