[libcxx-commits] [libcxx] [libc++][chrono] Implement LWG 4274: Allow chrono::hh_mm_ss to be constructed from unsigned durations (PR #209686)

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emmett2020 wrote:

> I dropped most of the original PR description because IMO these parts were mostly duplicate of the official description of LWG4274. Please double check the changes.
> 
> > * This may incorrectly treat a custom signed type as unsigned. `is_unsigned_v` provides a compile-time path for unsigned types, while the sign check keeps signed and custom types correct.
> 
> It's questionable whether a signed user-defined representation type `R` with `numeric_limits<R>::is_signed` being `false` is valid for `duration`. I guess it's better to rephrase this part.
> 
> `is_unsigned_v` is fine to me. No change requested for this.

Thanks! The trimmed description looks good to me. 
And also, you're right that it's unclear whether such a signed user-defined `Rep` is even a valid `duration` representation. So I've softened that paragraph to no longer assert it's handled incorrectly.
Left the impl unchanged as suggested.

https://github.com/llvm/llvm-project/pull/209686