BloombergLP::bslstl::Variant_ConvertIndex

This component-private metafunction computes the index of the alternative in (template parameter) t_VARIANT that is the "unique" best match for conversion from std::declval<t_TYPE>(), or bsl::variant_npos if there is no such alternative. An alternative that occurs multiple times in t_VARIANT (possibly with varying cv-qualifications) is considered to occur only once; thus, if all alternatives that are tied for the best match are the same type (possibly with varying cv-qualifications), the result is the lowest index at which that type (with any cv-qualification) occurs.