This was changed by LWG 3857.
libstdc++-v3/ChangeLog:
* include/std/string_view (basic_string_view(R&&)): Remove
constraint that traits_type must be the same, as per LWG 3857.
* testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc:
Explicit conversion between different specializations should be
allowed.
* testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc:
Likewise.