Pitch: Suppressed Associated Types With Defaults

[kavon (Kavon Farvardin)]

@xwu

Whether an associated type ought to be primary is not always obvious right away and can require a judgment call possibly based on usage experience. This has been demonstrated by the back-and-forth discussion and ultimate deferred adoption of primary associated types in parts of the standard library (as an example), which by now is not limited by lack of a language feature but lack of certainty about what's most desirable for the specific protocols in question.

I assume the standard library adoption discussions you’re referencing are here and here. It does seem like the source of uncertainty that led to the deferral for several sets of related protocols was based on predictions about future language features. That feels like a level of reluctance and forecasting that is unique to the standard library. But I do see your point about how usage experience is part of the discussion when making an associated type primary.