bsl::basic_string::operator BloombergLP::bslmf::NestedTraitDeclaration<basic_string, IsBitwiseMoveable, BloombergLP::bslmf::IsBitwiseMoveable<ALLOCATOR>::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.