[Bug c++/99160] New: A wrong accessible check when using a using-declaration that introduces the names of type and function

[xmh970252187 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99160

            Bug ID: 99160
           Summary: A wrong accessible check when using a
                    using-declaration that introduces the names of type
                    and function
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xmh970252187 at gmail dot com
  Target Milestone: ---

````cpp
#include <iostream>
struct A{
    struct C{
        using type = int;
    };
    void C(){}  //#1 if comment out this function definition, GCC will compile 
                //   this  example  
};
struct B:private A{
    using A::C;
};
int main(){
    B::C::type c;  //#2
}
````
For this example, GCC will report an error, which says "'struct A::C' is
inaccessible within this context". The result of this example is
https://godbolt.org/z/ddfrPM. If we comment out `void C(){}`, GCC will compile
the example.  

According to [namespace.udecl]#19. 
> A synonym created by a using-declaration has the usual accessibility for a member-declaration.   

That means the name `C` is accessible in the  nested-name-specifier at #2(the
name would be found in the scope of `B` in which the using-declaration will
create the synonym name for that name, the created name has the same
accessibility as the member-declaration)

Clang can compile this example even if we do not comment out the function
definition at #1