custom ostream formatter vs namespace vs clang

[anordal]

I have basically #952 (comment) with a namespace:

#include <fmt/ostream.h>
#include <iostream>

namespace something{ struct Custom {}; }

std::ostream& operator<<(std::ostream& os, const something::Custom&)
{
	return os << "Ok";
}

int main()
{
	// Compiles with Gcc and Clang
	std::cout << something::Custom() << '\n';

	// Compiles with Gcc, but not Clang
	fmt::print("{}\n", something::Custom());
}

I don't know if this is a compiler bug, but it seems that clang + libfmt = operator<< not found.

The absurd thing is that it compiles (with both compilers, also the non-libfmt example) if I move operator<< into the namespace. That is not supposed to compile (if my understanding of argument dependent lookup is correct) – I learned this from the exact opposite experience.

Libfmt tested:

• libfmt 6.1.2-1.2 (OpenSuse package)
• latest git (dc22360)

Compilers tested:

• g++ 9.2.1
• g++ 7.4.1
• clang++ 9.0.0
• clang++ 6.0.1

[vitaut]

I think clang is correct here because operator<< should be defined in the something namespace for ADL to work.

[anordal]

I agree, after finding back my "exact opposite experience". That was with a type alias in the namespace. I forgot that distinction.