using declaration: one more bug of gcc and clang?

Question

Why do gcc HEAD 10.0.0 20190 and Clang HEAD 9.0.0 both reject this program?

#include <iostream>

void g( int x )
{
    std::cout << "Hello g( " << x  << " )\n";
}

template <int N>
void g()
{
    std::cout << "Hello g<N>( " << N  << " )\n";
}

namespace N
{
    using ::g;
}

void g( int x = 20 );

template <int N = 10>
void g();

int main()
{
    N::g();
    N::g<>();
}

For example gcc issues the error

prog.cc: In function 'int main()':
prog.cc:27:11: error: no matching function for call to 'g()'
   27 |    N::g<>();
      |           ^
prog.cc:9:6: note: candidate: 'template<int N> void g()'
    9 | void g()
      |      ^
prog.cc:9:6: note:   template argument deduction/substitution failed:
prog.cc:27:11: note:   couldn't deduce template parameter 'N'
   27 |    N::g<>();
      |           ^

though according to the C++ 20 (and 17) Standard (9.8 The using declaration)

11 [Note: For a using-declaration whose nested-name-specifier names a namespace, members added to the namespace after the using-declaration are not in the set of introduced declarations, so they are not considered when a use of the name is made. Thus, additional overloads added after the using-declaration are ignored, but default function arguments (9.2.3.6), default template arguments (13.1), and template specializations (13.6.5, 13.8.3) are considered. — end note]

Based on my reading of the standard, I believe this is a bug.

Answer 1

This issue is the subject of Core issue 1907. The current direction is to treat such cases as ill-formed, no diagnostic required.

As it turns out, some implementations track default arguments on a "per-entity" basis (so it would be difficult to not consider default arguments added in a later redeclaration), while others track them on a "per-declaration" basis (so it would difficult to make them consider such default arguments). CWG decided to accommodate both implementation strategies by classifying code that depends on such things IFNDR.