Trouble inheriting from a protocol with an associatedtype

[Dante-Broggi]

@shawnthroop

This would be ... where Style == UIFont.TextStyle which, at the moment (swift 4.2), doesn't qualify as fully constrained?

It is fully constrained, but the compiler currently doesn't check, and thus that fact cannot affect the compilation.

The primary thread about these restrictions would probably be:

Lifting the "Self or associated type" constraint on existentials

When the first seed of Swift came out many years ago, there were technical reasons for the "self or associated type" constraint on protocol existentials: At that time, protocol witness tables did not carry associated type information, so it was impossible to re-open the dynamic type in order to dispatch methods on an existential when a protocol had associated types. This was fixed a while ago in order to allow for recursive protocol constraints, but we held on to the restriction on existentials …