diagnostic feature request

[Nathan Ridge]

Hi,

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? Should I open a ticket for this feature request?

Thanks,
Nate