Nested types in protocols (and nesting protocols in types)

[Danny]

Why is it allowed to shadow the `Self` type?

in Swift 6, it became illegal to shadow generic parameters from outer scopes: struct S<T> { func f<T>() -> T // ~ // Generic parameter 'T' shadows generic parameter from outer scope with the same name { let t:T = { fatalError() }() return t } } however, it appears it is still legal to shadow the Self type: struct S<T> { func f<Self>() -> Self { let t:Self = { fatalError() }() return t } } why?