CHAR_TYPE is required to be a POD as per the Standard, which makes CHAR_TYPE bitwise-movable, so basic_string is bitwise-movable as long as the (template parameter) type ALLOCATOR is also bitwise-movable.
Declared in <bslstl_string.h>
operator BloombergLP::bslmf::NestedTraitDeclaration<basic_string<wchar_t>, IsBitwiseMoveable, BloombergLP::bslmf::IsBitwiseMoveable<bsl::allocator<wchar_t>>::value>() const;