Unconditional evaluation of `char_traits<_CharT>::length(__str)` is problematic, because it causes
UB when `__str` points to a non-null-terminated array. We should only call `length` (currently, in
`basic_string_view`'s constructor) when `__n == npos` per [bitset.cons]/8.
Drive-by change: Reduction of conditional compilation, given that
- both `basic_string_view<_CharT>::size_type` and `basic_string<_CharT>::size_type` must be
`size_t`, and thus
- both `basic_string_view<_CharT>::npos` and `basic_string<_CharT>::npos` must be `size_t(-1)`.
For the type sameness in the standard wording, see:
- [string.view.template.general]
- [basic.string.general]
- [allocator.traits.types]/6
- [default.allocator.general]/1
Fixes#143684
bba4ded3c2