Why can't GCC disambiguate multiple inherited functions (yet clang can)? [duplicate]

Answer 1

Trying to understand the actual text in the standard (§10.2) isn't easy, but there is an example which makes it clear: name lookup for a name x in the derived class fails if the name isn't present in the derived class, but it is present in more than one base class, and it isn't hidden. (Hidden isn't relevant here, as it only intervenes when virtual inheritance is present.) As far as I can tell, this is the case regardless of the name of the member; I can find no exception if the member happens to have the special name operator(). Overload resolution doesn't come into play, because their is failure in the name lookup, before the overload set is completely built. I'm pretty sure that both snippets of code are illegal, and that there is a bug in clang.

You can use using declarations to inject the names into the derived class, or you can explicitly define forwarding operators in the derived class. Once the name is found in the derived class, the compiler stops, and does not look in the base classes.

Answer 2

I think it has something to do with hiding the function(s) in the base classes, and the GCC's error message doesn't seem to help much, even if you use struct instead of enum : In fact, the error message is misleading, because now Ea and Eb are two different classes, with no implicit conversion from Ea to Eb, the ambiguity shouldn't arise, but GCC seems to disagree with me : http://ideone.com/cvzLW (see the modification also).

Anyway, if you bring the functions in the class scope, explicitly by writing using as:

struct Sbroken : Sa, Sb 
{
   using Sa::operator();
   using Sb::operator();
};

then it works : http://ideone.com/LBZgC

Same with other example as well:

struct Sbroken : Sa, Sb 
{
   using Sa::f;
   using Sb::f;
};

Code : http://ideone.com/3hojd