[libc++][format] Formatting range with m range-type is incorrect

[JMazurkiewicz]

[format.range.formatter], Table 80:

(m option) Indicates that the opening bracket should be "{", the closing bracket should be "}", the separator should be ", ", and each range element should be formatted as if m were specified for its tuple-type.

Current implementation does not format range elements as if m tuple-type was specified, see for example https://godbolt.org/z/nzenfobqW:

#include <format>
#include <print>
#include <vector>
#include <utility>

int main() {
    std::vector<std::pair<int, int>> v = {{1, 2}, {3, 4}, {5, 6}};
    std::println("{:m}", v);
}

Expected:

{1: 2, 3: 4, 5: 6}

Got:

{(1, 2), (3, 4), (5, 6)}

[frederick-vs-ja]

It seems that we should add

   __underlying_.set_separator(_LIBCPP_STATICALLY_WIDEN(_CharT, ": "));
   __underlying_.set_brackets({}, {});

to

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

Lines 216 to 220 in 431be86

	if constexpr (__fmt_pair_like<_Tp>) {
	set_brackets(_LIBCPP_STATICALLY_WIDEN(_CharT, "{"), _LIBCPP_STATICALLY_WIDEN(_CharT, "}"));
	set_separator(_LIBCPP_STATICALLY_WIDEN(_CharT, ", "));
	++__begin;
	} else

[mordante]

Thanks for the report. I thought I had addressed this issue. I'll try to look at it soon.

[JMazurkiewicz]

It looks like someone tried to address this issue: #70616.