<chrono>: Overhaul zoned_time to use concepts instead of SFINAE
#5907
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I'm phrasing the constraints slightly differently from the Standard to perma-workaround EDG bugs, but I believe they're morally equivalent, and in practice everyone uses `const time_zone*` anyways.
In theory this repeats a bit of code, but IMO it clarifies what's happening (because zoned_time has so many constructors). This also avoids repeatedly evaluating constraints.
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I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
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Works towards #602.
LWG-4067 "Inconsistency and potential infinity meta-recursion in
std::chrono::zoned_time's constructors" is relevant, but it's P3 according to LWG so it probably won't be resolved any time soon. I believe that converting our constraints to concepts is an improvement, even if the Standard is kind of confused right now.type_identity_tfor consistency.zoned_timeoverhaul: Convert SFINAE to concepts.const time_zone*anyways.zoned_time: Avoid delegating constructors.zoned_timehas so many constructors). This also avoids repeatedly evaluating constraints.