It seems that gcc HEAD 10.0.0 20190 has one more bug relative to the using declaration.

The following program does not compile.

#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<>();
}

The compiler issues the error

prog.cc:Infunction'int main()':
prog.cc:27:11: error:no matching functionfor 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]