[libcxx-commits] [libcxx] [libc++] Mark vector<bool>::at() as constexpr to conform to C++20 standard (PR #121848)

[A. Jiang via libcxx-commits]

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frederick-vs-ja wrote:

Previously, Test coverage for `vector<bool, A>::at` seemed completely missing. I think we should add an `at.pass.cpp` which tests `at` functions in all modes.

https://github.com/llvm/llvm-project/pull/121848