Is this overload resolution ambiguity correct?

[Andrzej Krzemieński]

Hi Everyone,
The following code fails to compile in gcc 7, in C++17 mode (call to f(s) is ambiguous):

template <class T, class U = int>
    class C
    {};

template <class T, class U>
    void f(const C<T, U>&) {}

template <class T, template <class> class CT>
    void f(CT<T>) {}

int main()
{
    C<int> s;
    f(s);
}

It does not fail in C++14, nor does it fail in clang.

I wonder if this is a bug in GCC or clang, or if the standard gives  implementers freedom here.

Regards,
&rzej;

[Tom Honermann]

This is CWG issue 150 [1] and was resolved, I believe, by P0522R0 [2].

Tom.

[1]: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#150
[2]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0522r0.html