Regression: custom ostream formatting doesn't work with namespaced types after v7

[alabuzhev]

Version: latest

operator<< in global namespace should be discoverable as long as it is defined before fmt facilities.
And it worked like that at least until v7 (godbolt).

However, at some point after v7, presumably after 16cac46, it stopped working (godbolt).

The latest trunk works if the offending code is commented (godbolt).

[LukeMauldin]

I have experienced this issue as well.

[vitaut]

16cac46 is a fix for #1766. Unfortunately I don't know how to make both cases work. Considering that defining operator<< in a different namespace from the type is not well-supported anyway (depends on the order of inclusion), I recommend moving it to the appropriate namespace where it can be found by ADL (https://godbolt.org/z/q3MPM6):

namespace ns
{
    class test {};

    std::ostream& operator<<(std::ostream& Stream, test const&)
    {
        Stream << "ns::test\n";
        return Stream;
    }
}

If someone knows a solution that doesn't break #1766, a PR would be welcome.

[vitaut]

Clarified the limitation in the docs: d281018.

[alabuzhev]

I recommend moving it to the appropriate namespace where it can be found by ADL

Unfortunately that's UB for std types :(

[vitaut]

You can also provide a formatter specialization that delegates to operator<<:

template <>
struct fmt::formatter<ns::test> : formatter<std::string> {
  auto format(const ns::test& t, format_context& ctx) {
    std::ostringstream os;
    os << t;
    return formatter<std::string>::format(os.str(), ctx);
  }
};

We could make it simpler, something like:

template <> struct fmt::formatter<ns::test> : fmt::ostream_formatter<ns::test> {};

and this won't have the include order problem.

[alabuzhev]

Thank you - a formatter specialization works as expected.
I've seen this API before, but it seemed quite cumbersome and I avoided it as long as << worked.