* include/bits/alloc_traits.h (__is_allocator, _RequireAllocator):
New trait and alias for detecting Allocator-like types.
* include/bits/basic_string.h (basic_string): Add deduction guide
from P0433.
* include/ext/alloc_traits.h (__gnu_cxx::__alloc_traits): Add template
parameter with default template argument that causes substitution
failures for types that cannot be allocators.
* testsuite/21_strings/basic_string/cons/char/deduction.cc: New.
* testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc: New.
From-SVN: r248967
6d82c56288