[Bug c++/51277] New: Feature request: C++ diagnostic for ambiguous overloads

["zeratul976 at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51277

             Bug #: 51277
           Summary: Feature request: C++ diagnostic for ambiguous
                    overloads
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zeratul976@hotmail.com


I have a feature request regarding a compiler diagnostic.

When a call of an overloaded function is ambiguous, and some of the candidates
are declared in namespaces other than the namespace of the call site (or one
of its parent namespaces), it would be helpful if the compiler helped us 
figure out why those candidates are visible in the namespace of the call site.

Specifically, it would be helpful if the compiler would say:

 - whether the candidate is visible 1) because it was imported into the 
   namespace of the call site (or one of its parent namespaces) via a
   using-declaration or a using-directive, OR 2) because it was found
   using argument-dependent lookup

 - in the first case, the location of the using-declaration or using-
   directive (if there are several, any one of them should suffice)

 - in the second case, the argument that triggered the argument-
   dependent lookup and why

Examples:

///////////////  EXAMPLE 1  ///////////////

namespace n1
{
    void foo(double);
}

using n1::foo;

void foo(float);

int main()
{
   foo(0);   
}

// Current diagnostic
test.cpp: In function 'int main()':
test.cpp:12:9: error: call of overloaded 'foo(int)' is ambiguous
test.cpp:12:9: note: candidates are:
test.cpp:3:10: note: void n1::foo(double)
test.cpp:8:6: note: void foo(float)

// What I would like to see
test.cpp: In function 'int main()':
test.cpp:12:9: error: call of overloaded 'foo(int)' is ambiguous
test.cpp:12:9: note: candidates are:
test.cpp:3:10: note: void n1::foo(double)
test.cpp:6:13: note: visible in global namespace because of using-declaration
located here
test.cpp:8:6: note: void foo(float)


///////////////  EXAMPLE 2  ///////////////

namespace n1
{
    struct Bar {};

    void foo(double, Bar);
}

void foo(float, n1::Bar);

int main()
{
   foo(n1::Bar());   
}

// Current diagnostic
test.cpp: In function 'int main()':
test.cpp:12:20: error: call of overloaded 'foo(int, n1::Bar)' is ambiguous
test.cpp:12:20: note: candidates are:
test.cpp:8:6: note: void foo(float, n1::Bar)
test.cpp:5:10: note: void n1::foo(double, n1::Bar)

// What I would like to see
test.cpp: In function 'int main()':
test.cpp:12:20: error: call of overloaded 'foo(int, n1::Bar)' is ambiguous
test.cpp:12:20: note: candidates are:
test.cpp:8:6: note: void foo(float, n1::Bar)
test.cpp:5:10: note: void n1::foo(double, n1::Bar)
test.cpp:3:14: note: found by argument-dependent lookup because second argument
is of type n1::Bar

//////////////////////////////////////////


Does this sound doable?