[derivation] Do we need to clarify "all types specified in the C++ standard library"? LWG4252

[frederick-vs-ja]

Does [derivation]/4 (added via LWG2514) cover only standard library classes (and possibly some internal exposition-only classes whose synopses are specified), or ever more classes (observable by the user code) provided by the implementation?

E.g. are some implementation-defined or unspecified iterator types, or return types or std::bind etc. allowed to be final?

[jwakely]

Everything about unspecified types that isn't stated explicitly is unspecified, including whether they're final. We don't need to say anything for them, because unspecified types are not covered by that paragraph. For exposition-only types where we do specify a class definition, we might want to clarify whether those can or can't be final.

[frederick-vs-ja]

LWG4252