[libc++][format] Should `formatter<vec-bool-ref>` be default constructible without `<format>` include?

[JMazurkiewicz]

According to [vector.syn], formatter specialization for vector<bool>::reference should be available in <vector> header and (since it satisfies BasicFormatter) it should be also default constructible.

Repro:

#include <concepts>
#include <vector>

using R = std::vector<bool>::reference;
using F = std::formatter<R>;
static_assert(std::default_initializable<F>);

Expected: correct compilation
Got: failed static assertion, because <format> include is missing
Compiler explorer: https://godbolt.org/z/sW8dM8cvd

[llvmbot]

@llvm/issue-subscribers-clang-format

[mordante]

Thanks for the report, this is indeed a bug. I noticed there's no explicit test to verify this works.

[mordante]

Fixing this is not trivial, <format> depends on <vector> and to fix it <vector> will depend on <format>. I've some refactoring patches to solve the cycle. However this may take a bit of time to land. After that fixing the bug should be trivial.

[mordante]

This has been addressed in https://reviews.llvm.org/D149543.