protocol Integral {} extension Int: Integral {} struct Foo<T> {} extension Foo where T: Integral { func foo() { let zelf = self as! Foo<Int> print(zelf) } } let foo: Foo<Int> = Foo() foo.foo()

$ swift AlwaysFailed.swift AlwaysFailed.swift:8:25: warning: cast from 'Foo<T>' to unrelated type 'Foo<Int>' always fails let zelf = self as! Foo<Int> ~~~~ ^ ~~~~~~~~ Foo<Int>()

(edited)