[4.2][Sema] Restrict new optional-to-archetype casting behaviour to Swift 5 mode

lib/Sema/CSApply.cpp

@@ -3509,8 +3509,11 @@ namespace {
       // If the destination value type is 'AnyObject' when performing a
       // bridging operation, or if the destination value type could dynamically
       // be an optional type, leave any extra optionals on the source in place.
-      if (isBridgeToAnyObject || destValueType->canDynamicallyBeOptionalType(
-                                              /* includeExistential */ false)) {
+      // Only apply the latter condition in Swift 5 mode to best preserve
+      // compatibility with Swift 4.1's casting behaviour.
+      if (isBridgeToAnyObject || (tc.Context.isSwiftVersionAtLeast(5) &&
+                                  destValueType->canDynamicallyBeOptionalType(
+                                      /*includeExistential*/ false))) {
         auto destOptionalsCount = destOptionals.size() - destExtraOptionals;
         if (srcOptionals.size() > destOptionalsCount) {
           srcType = srcOptionals[destOptionalsCount];

test/SILGen/generic_casts.swift

@@ -1,5 +1,5 @@
 
-// RUN: %target-swift-frontend -module-name generic_casts -Xllvm -sil-full-demangle -emit-silgen -enable-sil-ownership %s | %FileCheck --check-prefix=CHECK --check-prefix=CHECK-%target-runtime %s
+// RUN: %target-swift-frontend -swift-version 5 -module-name generic_casts -Xllvm -sil-full-demangle -emit-silgen -enable-sil-ownership %s | %FileCheck --check-prefix=CHECK --check-prefix=CHECK-%target-runtime %s
 
 protocol ClassBound : class {}
 protocol NotClassBound {}

test/SILGen/generic_casts_swift4.swift

@@ -0,0 +1,64 @@
+
+// RUN: %target-swift-frontend -swift-version 4 -module-name generic_casts -Xllvm -sil-full-demangle -emit-silgen -enable-sil-ownership %s | %FileCheck %s
+
+// The below tests are to ensure we maintain compatibility with Swift 4.1's
+// behaviour when casting to an archetype – the compiler assumes a non-optional
+// archetype is non-optional, and therefore can unwrap the source.
+
+// CHECK-LABEL: sil hidden @$S13generic_casts32optional_any_to_opaque_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_to_opaque_archetype<T>(_ x: Any?) -> T {
+  return x as! T
+  // CHECK: bb0([[RET:%.*]] : @trivial $*T, {{%.*}} : @trivial $*Optional<Any>):
+  // CHECK: unconditional_checked_cast_addr Any in {{%.*}} : $*Any to T in [[RET]] : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts46optional_any_conditionally_to_opaque_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_conditionally_to_opaque_archetype<T>(_ x: Any?) -> T? {
+  return x as? T
+  // CHECK: checked_cast_addr_br take_always Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts32optional_any_is_opaque_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_is_opaque_archetype<T>(_ x: Any?, _: T) -> Bool {
+  return x is T
+  // CHECK: checked_cast_addr_br take_always Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts016optional_any_to_C17_opaque_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_to_optional_opaque_archetype<T>(_ x: Any?) -> T? {
+  return x as! T?
+  // CHECK: unconditional_checked_cast_addr Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts030optional_any_conditionally_to_C17_opaque_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_conditionally_to_optional_opaque_archetype<T>(_ x: Any?) -> T?? {
+  return x as? T?
+  // CHECK: checked_cast_addr_br take_always Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts016optional_any_is_C17_opaque_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_is_optional_opaque_archetype<T>(_ x: Any?, _: T) -> Bool {
+  return x is T?
+  //   Because the levels of optional are the same, 'is' doesn't transform into an 'as?',
+  //   so we just cast directly without digging into the optional operand.
+  // CHECK: checked_cast_addr_br take_always Optional<Any> in {{%.*}} : $*Optional<Any> to Optional<T> in {{%.*}} : $*Optional<T>
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts31optional_any_to_class_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_to_class_archetype<T : AnyObject>(_ x: Any?) -> T {
+  return x as! T
+  // CHECK: unconditional_checked_cast_addr Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts45optional_any_conditionally_to_class_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_conditionally_to_class_archetype<T : AnyObject>(_ x: Any?) -> T? {
+  return x as? T
+  // CHECK: checked_cast_addr_br take_always Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+
+// CHECK-LABEL: sil hidden @$S13generic_casts31optional_any_is_class_archetype{{[_0-9a-zA-Z]*}}F
+func optional_any_is_class_archetype<T : AnyObject>(_ x: Any?, _: T) -> Bool {
+  return x is T
+  // CHECK: checked_cast_addr_br take_always Any in {{%.*}} : $*Any to T in {{%.*}} : $*T
+}
+