The following program compiled with gcc 9.3
 
#include <iostream>
 
void g( int x, int y = 20 )
{
    std::cout << "x = " << x << ", y = " << y << '\n';
}
 
int main() 
{
    void g( int, int );
    using ::g;
//    void g( int, int );    
    g( 10 );
    
  return 0;
}
 
issues error message
 
error: too few arguments to function ‘void g(int, int)’
  g( 10 );
        ^
 
However this program
 
#include <iostream>
 
void g( int x, int y = 20 )
{
    std::cout << "x = " << x << ", y = " << y << '\n';
}
 
int main()
{
// void g( int, int );
    using ::g;
    void g( int, int );
    g( 10 );
    
    return 0;
}
 
runs successfully.
 
I did not found in the section 9.9 The using declaration whether such a program is ill-formed or not.
 
The only text in the C++ 20 Standard that seems refers to this case is the quote in the section 12.2 Overloadable declarations
 
1 Not all function declarations can be overloaded. Those that cannot be overloaded are specified here. A program is ill-formed if it contains two such non-overloadable declarations in the same scope. [Note: This restriction applies to explicit declarations in a scope, and between such declarations and declarations made through a using-declaration (9.9). It does not apply to sets of functions fabricated as a result of name lookup (e.g., because of using-directives) or overload resolution (e.g., for operator functions). — end note]
 
Though the bold text is not clear enough and whether it indeed refers to the pointed case.
 
So a question arises are the both above programs ill-formed? And if so then why is there nothing said about this in the section 9.9 if I am not mistaken?
 
With best regards
(Vlad from Moscow)
 
 
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