Currently the heterogeneous lookup in associative container are enabled by presence of is_transparent nested type in the comparator type ([associative.reqmts]). This complicates the definition of call wrapper types that want to define is_transparent if they wrap a callable type that defines is_transparent, and requires the target to be a complete type in cases where an incomplete type would otherwise be ok.

Another problem is that users cannot add the is_transparent member to a third-party comparison type that they do not control, even if they know it supports heterogeneous comparisons.

If the associative containers used a trait instead of checking for an is_transparent member type then it would avoid the requirement for complete types, and would allow customization of the trait without modifying the comparator type. This would also be consistent with the traits is_placeholder and is_bind_expression.

For backward compatibility with the existing design, the default implementation of the is_transparent trait could depend on the presence of the is_transparent nested type.