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2d55d94e5d
The recursively defined constraints on _Variadic_union's user-defined destructor (used for maintaining trivial destructibility of the variant iff all of its alternatives are) turn out to require a template instantiation depth of 3x the number of variants in C++20 mode, with the instantiation stack looking like ... _Variadic_union<B, C, ...> std::is_trivially_destructible_v<_Variadic_union<B, C, ...>> _Variadic_union<A, B, C, ...>::~_Variadic_union() _Variadic_union<A, B, C, ...> ... Ideally the template depth should be ~equal to the number of variants (plus a constant). Luckily it seems we don't need to compute trivial destructibility of the alternatives at all from _Variadic_union, since its only user _Variant_storage already has that information. To that end this patch removes these recursive constraints and instead passes this information down from _Variant_storage. After this patch, the template instantiation depth for 87619.cc in C++20 mode is ~270 instead of ~780. libstdc++-v3/ChangeLog: * include/std/variant (__detail::__variant::_Variadic_union): Add bool __trivially_destructible template parameter. (__detail::__variant::_Variadic_union::~_Variadic_union): Use __trivially_destructible in constraints instead. (__detail::__variant::_Variant_storage): Pass __trivially_destructible value to _Variadic_union. Reviewed-by: Jonathan Wakely <jwakely@redhat.com>