Lifting the "Self or associated type" constraint on existentials

[xwu (Xiaodi Wu)]

Here is a good example of a genuine use case where our current restriction would have saved some confusion if the protocol in question had Self or associated type requirements:

Pattern matching Errors

I ran into some strange behavior while testing pattern matching in catch blocks: protocol FooError : Error { func whatWentWrong() -> String } extension Array : FooError, Error where Element: FooError { func whatWentWrong() -> String { return "idk i'm an array" } } struct MyError : FooError { func whatWentWrong() -> String { return "something happened" } } func test() throws { throw [MyError()] } do { try test() } catch let err where err is [FooError] { // Why is this n…