Testing the covariance/contravariance behaviours of casting function types, both directly and in a generic context. There are several inconsistencies.

VarianceTestCasting.swift

protocol Foo { }
protocol Bar { }
protocol Baz { }

struct FooStruct: Foo, Bar {}

/// Test covariance of return type

let block: () -> FooStruct = { FooStruct() }

block is () -> FooStruct // true
block is () -> Foo // false, but expected true
block is () -> Bar // false, but expected true
block is () -> Baz // false

block as? () -> FooStruct // non-nil
block as? () -> Foo // non-nil
block as? () -> Bar // non-nil
block as? () -> Baz // nil

func testIs<T,U>(_ returnType: U.Type, _ block: @escaping ()->T) -> Bool {
    return block is () -> U
}

testIs(FooStruct.self, block) // true
testIs(Foo.self, block) // false, but expected true
testIs(Bar.self, block) // false, but expected true
testIs(Baz.self, block) // false

func testAs<T,U>(_ returnType: U.Type, _ block: @escaping ()->T) -> ( () -> U )? {
    return block as? () -> U
}

testAs(FooStruct.self, block) // non-nil
testAs(Foo.self, block) // nil, but expected non-nil
testAs(Bar.self, block) // nil, but expected non-nil
testAs(Baz.self, block) // nil


func testRun<T,U>(_ returnType: U.Type, _ block: @escaping ()->T) -> Bool {
    return block() is U
}

testRun(FooStruct.self, block) // true
testRun(Foo.self, block) // true
testRun(Bar.self, block) // true
testRun(Baz.self, block) // false


/// Test contravariance of parameter types

let block2: (Foo) -> () = { _ in return }

block2 is (Foo) -> () // true
block2 is (FooStruct) -> () // false, but expected true

block2 as? (Foo) -> () // non-nil
block2 as? (FooStruct) -> () // non-nil

func testIs2<T,U>(_ paramType: U.Type, _ block: @escaping (T)->()) -> Bool {
    return block is (U) -> ()
}

testIs2(Foo.self, block2) // true
testIs2(FooStruct.self, block2) // false, but expected true

func testAs2<T,U>(_ paramType: U.Type, _ block: @escaping (T)->()) -> ( (U) -> () )? {
    return block as? (U) -> ()
}

testAs2(Foo.self, block2) // non-nil
testAs2(FooStruct.self, block2) // nil, but expected non-nil