c++ - Dependent name lookup in function template: clang rejects, gcc accepts

Question

Consider the following fragment:

struct X { };

namespace foo {
    template <class T>
    void bar() { T{} < T{}; }

    void operator<(const X&, const X&) {}
}

int main() {
    foo::bar<X>();
}

clang rejects this code, gcc accepts it. Is this a gcc bug or is this a clang bug?

Answer 1

I believe this is a gcc bug, filed as 70099. From [temp.dep.res]:

In resolving dependent names, names from the following sources are considered:
(1.1) — Declarations that are visible at the point of definition of the template.
(1.2) — Declarations from namespaces associated with the types of the function arguments both from the instantiation context (14.6.4.1) and from the definition context.

foo::operator<() isn't visible at the point of definition of the template, and isn't in an associated namespace from the function arguments (X's associated namespace is just the global namespace ::). So I think gcc is wrong to find foo::operator< and clang is correct to reject the code.