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>
2d55d94e5d