Defend sequence container constructors from bad CTAD #4254
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Found by libcxx testing (Iter, Iter, BadAlloc).
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Related to #4060 (but not exactly that issue). |
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I'm speculatively mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
strega-nil-ms
approved these changes
Dec 14, 2023
CaseyCarter
approved these changes
Dec 14, 2023
It's not guaranteed but IMO it should be. The Standard tells us to do so explicitly for The Standard has wording for deduction guides but not for constructors. This is slightly confusing since pre-Standard language in WG21 used "implicit" and "explicit" deduction guides to distinguish between the function templates derived from constructors for use in CTAD and those derived from deduction guides. We ought to file an LWG issue. |
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Found by the upcoming libcxx update, where they test CTAD for
(Iter, Iter, BadAlloc)and expect it to SFINAE away.(The explicit deduction guides are already constrained to take only things that qualify as allocators. The issue is that every constructor generates an implicit guide, and those can lead to the container being instantiated with a BadAlloc unless we guard against that.)
It's actually unclear to me whether this is guaranteed by the Standard, but the defense is simple and we already do this in
basic_string.