Short-circuit some variant
constraints
#4966
Merged
+41
−10
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variant
's converting constructor and assignment operator templates are constrained to reject arguments of thevariant
's type. In such a case, the templates instantiated to check the constructibility constraint might be ill-formed outside the immediate context of template instantiation causing a hard error. We should split the constraints into multipleenable_if_t
s to enable short-circuiting of later constraints when the earlier constraints fail.Fixes #4959.