[Bug c++/65695] New: [4.9/5 Regression] calling constexpr constructor with pointer-to-member is not a constant expression

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65695

            Bug ID: 65695
           Summary: [4.9/5 Regression] calling constexpr constructor with
                    pointer-to-member is not a constant expression
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

From https://gcc.gnu.org/ml/gcc-help/2015-04/msg00034.html

template <typename T>
struct Bar
{
    using MemberFuncT = int (T::*)();

    MemberFuncT h_;
    constexpr Bar(MemberFuncT h) : h_{h}
    {
    }
};

struct Foo
{
    int test()
    {
        return -1;
    }

    // this causes the brace-enclosed initializer error message:
    static constexpr Bar<Foo> bar = Bar<Foo>(&Foo::test);

    // but this line does not:
    // static constexpr Bar<Foo> bar = Bar<Foo>(nullptr);

    // this line also causes the error message, unless you remove the
    // explict constructor in Bar.
    // static constexpr Bar<Foo> bar = {&Foo::test};
};

constexpr Bar<Foo> Foo::bar;

// the line below does not cause any problems, either:
// static constexpr Bar<Foo> bar = Bar<Foo>(&Foo::test);

int main(void)
{
    Foo f;
    return (f.*(Foo::bar.h_))();
}

c.cc:20:56: error: âBar<Foo>{&Foo::test}â is not a constant expression
     static constexpr Bar<Foo> bar = Bar<Foo>(&Foo::test);
                                                        ^

Clang and EDG accept the code.