map on an AbstractString, if no more specific map method found, returns UTF8String always

[ScottPJones]

This affects map on UTF16String, in particular, showing up in #11460, as currently uppercase/lowercase both use map for UTF16String, and that can be solved by making more specific (and much faster) uppercase/lowercase methods for UTF16String (as is done for ASCIIString), however, the general case also needs to be fixed... map on an AbstractString should always return the same type.

[JeffBezanson]

map on an AbstractString can't always return the same type, if you map an ASCIIString to a result that has non-ASCII characters.

[ScottPJones]

OK, that makes sense, but where it is just different encodings of Unicode (as opposed to a subset, like
ASCII or Latin1), wouldn't you agree that it should stay in the same encoding (and type)?

[JeffBezanson]

Yes.