bsl::basic_string<char8_t>::operator BloombergLP::bslmf::NestedTraitDeclaration<basic_string<char8_t>, IsBitwiseMoveable, BloombergLP::bslmf::IsBitwiseMoveable<bsl::allocator<char8_t>>::value>

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.