x.swift:5:46: error: 'R' was defined in extension of protocol 'Collection' and cannot be referenced from a 'where' clause
func foo<U: Collection, T>(_ arg: U) where U.R == T {}
                                             ^
x.swift:5:25: error: generic parameter 'T' is not used in function signature
func foo<U: Collection, T>(_ arg: U) where U.R == T {}
                        ^
x.swift:13:37: error: 'Element' was defined in extension of protocol 'P' and cannot be referenced from a 'where' clause
func foo<U: P, T>(_ arg: U) where U.Element == T {} // Ok
                                    ^
x.swift:11:25: note: consider moving 'Element' into the definition of protocol 'P'
extension P { typealias Element = Foo.Element }
                        ^
x.swift:13:16: error: generic parameter 'T' is not used in function signature
func foo<U: P, T>(_ arg: U) where U.Element == T {} // Ok