What's the intended behavior here (associated type with default type, conditional conformances)?

[Jens]

@CTMacUser

Shouldn’t a (value) type’s binary layout be solely determined by the primary definition, including any protocols declared directly in that primary definition? Shouldn’t an error occurr if an extension is needed to complete the binary layout of a type, or would change the layout?

The answer to this came up in another thread, so I'm quoting it here:

@anthonylatsis

By the way, as long as the extension resides in the same module, this should be alright.
You can treat non-conditional extensions like these as part of the primary definition.

struct S {
    let v: A
    func roo() {foo()}
}
extension S {
    typealias A = Int
    func foo() {}
}