In our code base, we have the following (code linked below):
- A class template for “handles” to objects of assorted types,
Handle<T, Policy>; T is the actual element type
- Handle contains a member operator < (and others). It uses the
“address” of the object to do the comparison.
- There is a special class where we want to customize operator <
so it looks into the associated object, so we have a templated
overload free function of operator< on Handle<Special, T2>.
I’ve got my doubts as to the sanity of this design in some ways, but I don’t think I can reasonably change it.
This compiles with GCC , and with MSVC , but Clang gives an error about the two operator<s being ambiguous, starting with 3.5 .
Perhaps importantly, the operator< member of Handle has this “signature”:
template <typename T, typename Policy>
bool Handle<T, Policy>::operator< (RHS_Handle const &)
and the free function has this signature:
typename LHS_T, typename LHS_Policy,
typename RHS_T, typename RHS_Policy>
bool operator< (
Handle<LHS_T, LHS_Policy> lhs,
Handle<RHS_T, RHS_Policy> rhs);
At least if we ignore the references and cv-quals, my understanding is that the latter function should have priority over the former because it’s more specialized – the second parameter is Handle<RHS_T, RHS_Policy> rather than a generic RHS_Handle type.
So if the function is called with
Handle<MyType, SomePolicy> h1, h2;
h1 < h2;
then it should not be ambiguous, and select the free overload. However, perhaps the cv-qualification matters, and GCC/MSVC get it wrong?
Credit to OmegaNaughtEquals1 on Reddit who pointed me to some standards text that seems to support the position that it should work (maybe… it’s too late to really tell what should be happening with the cv/ref stuff) .
If I change the free function to take ‘lhs’ by reference, then Clang compiles it .
I’m happy to report this to the bug tracker if that’s what I should do – I just wasn’t sure if I should with something that I’m not sure is a bug.
 GCC compiles it: https://godbolt.org/g/9EaoYh
 MSVC does too: https://godbolt.org/g/ubXeC3
 Clang’s error: https://godbolt.org/g/MhNEky
 Reddit thread: https://www.reddit.com/r/cpp_questions/comments/7ir4j5/-/
 Modified copy with ref parameter: https://godbolt.org/g/v9HtuH