Becca Royal-Gordon e3495f5fe7 Forbid module selectors on dependent member types ...

In code like the following:

```
protocol P { associatedtype A: Hashable }
protocol Q { associatedtype A: Comparable }

func fn<T: P & Q>(_: T) where T.A == Int { … }
```

`T.A` is actually the union of `P.A` and `Q.A`—it satisfies both associated types and has both of their constraints. This means it doesn’t actually make sense to apply a module selector to `A`—even if `P` and `Q` are in different modules, `T.A` always represents both of the declarations, not one or the other. We therefore now ban module selectors in this position, since they don’t actually jibe with the nature of a generic signature.

This justification technically doesn’t hold for *every* member type of a generic parameter—a member type can refer to a concrete typealias in a protocol extension, for instance—but in those situations, you can disambiguate (and add module selectors) by writing `P.A` or `Q.A` instead of `T.A`, so we’re not really worried about this limitation.

2025-10-24 16:23:50 -07:00

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