Should this function call be ambiguous?

Question

I stumbled on this the other day and can't figure out which answer is correct, or whether both are acceptable.

Specifically, I'm referring to the call to bar(T{}) in OtherFunction. From what I've been able to test on compiler explorer, the decision seems split. msvc and icc agree that it is ambiguous while gcc and clang compile the code without issue.

The function bar inside the namespace hidden becomes visible through argument-dependent lookup. Additionally, msvc/icc consider the declaration of bar in the global namespace as a candidate while gcc/clang do not. It seems that the declaration in the global namespace should not be considered since it is declared after the call to bar(T{}) but I'm not sure whether I'm reading the rules for unqualified name lookup correctly or if the standard is ambiguous in this regard.

https://godbolt.org/z/HAS-Cv

EDIT: Looks like msvc has fixed this as long as /permissive- option is used (https://devblogs.microsoft.com/cppblog/two-phase-name-lookup-support-comes-to-msvc/)

template <typename T>
inline void OtherFunction () {
    bar(T{});
}

namespace hidden {
    struct Foo {};
    inline void bar (Foo foo) {}
}

inline void bar (hidden::Foo foo) {}

void Function () {
    OtherFunction<hidden::Foo>();
}

Answer 1

Gcc and Clang are correct. The global bar defined after the definition of OtherFunction can't be found by name lookup; while hidden::bar could be found by ADL.

(emphasis mine)

For a dependent name used in a template definition, the lookup is postponed until the template arguments are known, at which time ADL examines function declarations with external linkage (until C++11) that are visible from the template definition context as well as in the template instantiation context, while non-ADL lookup only examines function declarations with external linkage (until C++11) that are visible from the template definition context (in other words, adding a new function declaration after template definition does not make it visible except via ADL).