Implement Casting For Extended Existentials

Implement casting to and from extended existentials. This is done by slightly generalizing the conditional conformances checking infrastructure.

Unfortunately, casts for reference types and metatypes are unsound because IRGen is peepholing all non-opaque existential conversions with a helper. I’ll disable that in a follow-up.

rdar://92197049

test/Casting/ParameterizedExistentials.swift

@@ -0,0 +1,116 @@
+// ParameterizedExistentials.swift - Tests for class constant casts w/ Obj-C
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+// -----------------------------------------------------------------------------
+///
+/// Tests for parameterized existential type conversions.
+///
+// -----------------------------------------------------------------------------
+// RUN: %empty-directory(%t)
+//
+// RUN: %target-build-swift -swift-version 5 -g -Onone -Xfrontend -disable-availability-checking -module-name a -c %s -o %t/ParameterizedExistentials.swift.Onone.o
+// RUN: %target-swiftc_driver %t/ParameterizedExistentials.swift.Onone.o -o %t/a.swift5.Onone.out
+// RUN: %target-codesign %t/a.swift5.Onone.out
+// RUN: %target-run %t/a.swift5.Onone.out
+//
+// RUN: %target-build-swift -swift-version 5 -g -O -Xfrontend -disable-availability-checking -module-name a -c %s -o %t/ParameterizedExistentials.swift.O.o
+// RUN: %target-swiftc_driver %t/ParameterizedExistentials.swift.O.o -o %t/a.swift5.O.out
+// RUN: %target-codesign %t/a.swift5.O.out
+// RUN: %target-run %t/a.swift5.O.out
+//
+// REQUIRES: executable_test
+// This test requires the new existential shape metadata accessors which are
+// not available in on-device runtimes, or in the back-deployment runtime.
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+import Swift
+
+import StdlibUnittest
+
+protocol Holder<T> {
+  associatedtype T
+  var value: T { get }
+}
+
+struct GenericHolder<T>: Holder {
+  var value: T
+
+  init(value: T) { self.value = value}
+}
+
+protocol PairType<T, U> {
+  associatedtype T
+  associatedtype U
+
+  var first: T { get }
+  var second: U { get }
+}
+
+struct Pair<T, U>: PairType {
+  var value: (T, U)
+
+  var first: T { self.value.0 }
+  var second: U { self.value.1 }
+
+  init(value: (T, U)) { self.value = value}
+}
+
+final class ReferencePair<T, U>: PairType {
+  var first: T
+  var second: U
+
+  init(value: (T, U)) { (self.first, self.second) = value }
+}
+
+let tests = TestSuite("ParameterizedExistentials")
+
+tests.test("Parameterized existential casting basics work") {
+  let a = GenericHolder(value: 5) as any Holder<Int>
+  let b = GenericHolder(value: 5) as! any Holder<Int>
+  expectEqual(a.value, b.value)
+  let c = GenericHolder(value: 5) as? any Holder<Int>
+  expectNotNil(c)
+  let d = GenericHolder(value: 5) as? any Holder<String>
+  expectNil(d)
+}
+
+tests.test("Metatype existential casting basics work")
+  .xfail(.custom({ true }, reason: "IRGen peepholes these casts"))
+  .code {
+  let a = GenericHolder<Int>.self as any Holder<Int>.Type
+  let b = GenericHolder<Int>.self as! any Holder<Int>.Type
+  expectTrue(a == b)
+  let c = GenericHolder<Int>.self as? any Holder<Int>.Type
+  expectNotNil(c)
+  let d = GenericHolder<Int>.self as? any Holder<String>.Type
+  expectNil(d)
+}
+
+tests.test("Existential box should maintain identity") {
+  let a = Pair(value: ("Hello", 42))
+  var b/*ox*/ = a as? any PairType<String, Int>
+  expectNotNil(b)
+  expectEqual(a.value, (b!.first, b!.second))
+
+  let c = ReferencePair(value: ("Goodbye", 24))
+  let d = c as? any PairType<String, Int>
+  expectNotNil(d)
+
+  b = d!
+  expectEqual(b!.first, d!.first)
+  expectEqual(b!.second, d!.second)
+
+  let e = b as? ReferencePair<String, Int>
+  expectNotNil(e)
+  expectTrue(e! === c)
+}
+
+runAllTests()