libstdc++: Optimize std::tuple_element and std::tuple_size_v
This reduces the number of class template instantiations needed for code
using tuples, by reusing _Nth_type in tuple_element and specializing
tuple_size_v for tuple, pair and array (and const-qualified versions of
them).
Also define the _Nth_type primary template as a complete type (but with
no nested 'type' member). This avoids "invalid use of incomplete type"
errors for out-of-range specializations of tuple_element. Those errors
would probably be confusing and unhelpful for users. We already have
a user-friendly static assert in tuple_element itself.
Also ensure that tuple_size_v is available whenever tuple_size is (as
proposed by LWG 3387). We already do that for tuple_element_t.
libstdc++-v3/ChangeLog:
* include/bits/stl_pair.h (tuple_size_v): Define partial
specializations for std::pair.
* include/bits/utility.h (_Nth_type): Move definition here
and define primary template.
(tuple_size_v): Move definition here.
* include/std/array (tuple_size_v): Define partial
specializations for std::array.
* include/std/tuple (tuple_size_v): Move primary template to
<bits/utility.h>. Define partial specializations for
std::tuple.
(tuple_element): Change definition to use _Nth_type.
* include/std/variant (_Nth_type): Move to <bits/utility.h>.
(variant_alternative, variant): Adjust qualification of
_Nth_type.
* testsuite/20_util/tuple/element_access/get_neg.cc: Prune
additional errors from _Nth_type.