Pitch: Suppressed Default Conformances on Associated Types

[xwu (Xiaodi Wu)]

[...] a protocol extension of Queue always introduces a default Self: Copyable requirement; that is:

extension Queue /* where Self: Copyable */ { ... }

[...but] this defaulting behavior does not extend to associated types with sup[p]ressed conformances. In particular, no implicit Self.Element: Copyable requirement is introduced above, by either extension. Instead, a protocol extension for queue types where both the queue itself and the element type are Copyable takes the following form:

extension Queue where Self.Element: Copyable { ... }

This is discussed further in Source Compatibility below.

Can you discuss whether you've considered—and if so, the reasons for not adopting—a rule analogous to the conservative one we adopted in SE-0446 for conditional conformances?

Namely, instead of having users learn a new exception about implicit Copyable, we could require stating copyability or non-copyability explicitly.

In other words, for the example given above where the protocol has a ~Copyable associated type, we could say that it is always necessary to write either extension Queue where Self: Copyable, Self.Element: Copyable or extension Queue where Self: ~Copyable, Self.Element: Copyable.

Is that workable?