[cfe-dev] Feature request: C++ diagnostic for ambiguous overloads

[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:12:4: error: call to 'foo' is ambiguous
   foo(0);
   ^~~
test.cpp:3:10: note: candidate function
    void foo(double);
         ^
test.cpp:8:6: note: candidate function
void foo(float);
     ^

// What I would like to see
test.cpp:12:4: error: call to 'foo' is ambiguous
   foo(0);
   ^~~
test.cpp:3:10: note: candidate function
    void foo(double);
         ^
test.cpp:6:13: note: found due to using-declaration here
using n1::foo;
      ^        
test.cpp:8:6: note: candidate function
void foo(float);
     ^

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

namespace n1
{
    struct Bar {};

    void foo(double, Bar);
}

void foo(float, n1::Bar);

int main()
{
   foo(0, n1::Bar());   
}

// Current diagnostic
test.cpp:12:4: error: call to 'foo' is ambiguous
   foo(0, n1::Bar());
   ^~~
test.cpp:5:10: note: candidate function
    void foo(double, Bar);
         ^
test.cpp:8:6: note: candidate function
void foo(float, n1::Bar);
     ^

// What I would like to see
test.cpp:12:4: error: call to 'foo' is ambiguous
   foo(0, n1::Bar());
   ^~~
test.cpp:5:10: note: candidate function
    void foo(double, Bar);
         ^
test.cpp:12:11: note: found by argument-dependent lookup because 'n1' is an associated namespace of argument 2 of type 'n1::Bar'
   foo(0, n1::Bar());
          ^~~~~~~~~
test.cpp:8:6: note: candidate function
void foo(float, n1::Bar);
     ^

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

Does this sound doable? Should I file a bugzilla entry?

Thanks,
Nate