`<format>`: The definition of `__fmt_pair_like` is wrong

[hewillk]

llvm-project/libcxx/include/__format/concepts.h

Lines 67 to 70 in 9d53c86

	template <class _Tp>
	concept __fmt_pair_like = __is_specialization_v<_Tp, pair> ||
	// Use a requires since tuple_size_v may fail to instantiate,
	(__is_specialization_v<_Tp, tuple> && requires { tuple_size_v<_Tp> == 2; });

requires { tuple_size_v<_Tp> == 2; } should only require the expression to be valid and not actually evaluate it, right?

https://godbolt.org/z/c6aMq4xzd

#include <iostream>
#include <vector>
#include <format>

int main() {
  std::vector<std::tuple<>> v(3);
  std::cout << std::format("{:nm}", v); // (), (), ()
}

[hewillk]

Also, I wonder why std::format("{:m}", std::vector{std::pair{0, "hello"}}) outputs {(0, hello)}, shouldn't it be {0: hello}? Is the implementation not yet completed?
(I haven't followed the progress of libc++'s <format> closely, please correct me if I'm missing something)

[mordante]

Thanks for the report. The implementation should be complete and I'll have a closer look. (Note that the wording in the WP is not without its own flaws, but I'll have a closer look.)

[mordante]

I believe the second example is correct. The :m is for the vector and not the pair, Some more examples https://godbolt.org/z/ox8ePG6cc.

Note that the escaping of strings is inconsistent, see https://cplusplus.github.io/LWG/issue3892

[mordante]

I've posted a fix https://reviews.llvm.org/D145309

[timsong-cpp]

I believe the second example is correct. The :m is for the vector and not the pair, Some more examples https://godbolt.org/z/ox8ePG6cc.

m on the range also causes each range element should be formatted as if m were specified for its tuple-type.

[mordante]

I believe the second example is correct. The :m is for the vector and not the pair, Some more examples https://godbolt.org/z/ox8ePG6cc.

m on the range also causes each range element should be formatted as if m were specified for its tuple-type.

Thanks I overlooked that.

I now wonder what std::format("{:m:n}", std::vector{std::pair{0, "hello"}}); should do.

The original review landed, I will keep this open for the second issue.