[Sema] Consider unavailable functions as being unreachable

This has the effect of rejecting unavailable overrides to available
methods in a similar way as overrides that are less available than the
introduction are rejected.

lib/AST/Availability.cpp

@@ -144,8 +144,13 @@ static bool isBetterThan(const AvailableAttr *newAttr,
     return true;
 
   // If they belong to the same platform, the one that introduces later wins.
-  if (prevAttr->Platform == newAttr->Platform)
+  if (prevAttr->Platform == newAttr->Platform) {
+    if (newAttr->isUnconditionallyUnavailable())
+      return true;
+    if (prevAttr->isUnconditionallyUnavailable())
+      return false;
     return prevAttr->Introduced.getValue() < newAttr->Introduced.getValue();
+  }
 
   // If the new attribute's platform inherits from the old one, it wins.
   return inheritsAvailabilityFromPlatform(newAttr->Platform,
@@ -158,10 +163,12 @@ AvailabilityInference::annotatedAvailableRange(const Decl *D, ASTContext &Ctx) {
 
   for (auto Attr : D->getAttrs()) {
     auto *AvailAttr = dyn_cast<AvailableAttr>(Attr);
-    if (AvailAttr == nullptr || !AvailAttr->Introduced.hasValue() ||
+    if (AvailAttr == nullptr ||
         !AvailAttr->isActivePlatform(Ctx) ||
         AvailAttr->isLanguageVersionSpecific() ||
-        AvailAttr->isPackageDescriptionVersionSpecific()) {
+        AvailAttr->isPackageDescriptionVersionSpecific() ||
+        (!AvailAttr->Introduced.hasValue() &&
+         !AvailAttr->isUnconditionallyUnavailable())) {
       continue;
     }
 
@@ -172,6 +179,9 @@ AvailabilityInference::annotatedAvailableRange(const Decl *D, ASTContext &Ctx) {
   if (!bestAvailAttr)
     return None;
 
+  if (bestAvailAttr->isUnconditionallyUnavailable())
+      return AvailabilityContext(VersionRange::empty());
+
   return AvailabilityContext{
     VersionRange::allGTE(bestAvailAttr->Introduced.getValue())};
 }

test/SILGen/vtables.swift

@@ -21,7 +21,6 @@ class C : B {
 // CHECK:   #A.bar: {{.*}} : @$s7vtables1CC3bar{{[_0-9a-zA-Z]*}}F
 // CHECK:   #A.bas: {{.*}} : @$s7vtables1AC3bas{{[_0-9a-zA-Z]*}}F
 // CHECK:   #A.qux: {{.*}} : @$s7vtables1CC3qux{{[_0-9a-zA-Z]*}}F
-// CHECK:   #A.flux: {{.*}} : @$s7vtables1BC4flux{{[_0-9a-zA-Z]*}}F
 // CHECK:   #B.init!allocator: {{.*}} : @$s7vtables1CC{{[_0-9a-zA-Z]*}}fC
 // CHECK:   #B.zim: {{.*}} : @$s7vtables1BC3zim{{[_0-9a-zA-Z]*}}F
 // CHECK:   #B.zang: {{.*}} : @$s7vtables1CC4zang{{[_0-9a-zA-Z]*}}F
@@ -35,7 +34,7 @@ class A {
   func bar() {}
   func bas() {}
   func qux() {}
-  func flux() {}
+  @available(*, unavailable) func flux() {}
 }
 
 // CHECK: sil_vtable A {
@@ -54,7 +53,6 @@ class B : A {
   // bar inherited from A
   // bas inherited from A
   override func qux() {}
-  @available(*, unavailable) override func flux() {}
 
   func zim() {}
   func zang() {}
@@ -65,7 +63,6 @@ class B : A {
 // CHECK:   #A.bar: {{.*}} : @$s7vtables1AC3bar{{[_0-9a-zA-Z]*}}F
 // CHECK:   #A.bas: {{.*}} : @$s7vtables1AC3bas{{[_0-9a-zA-Z]*}}F
 // CHECK:   #A.qux: {{.*}} : @$s7vtables1BC3qux{{[_0-9a-zA-Z]*}}F
-// CHECK:   #A.flux: {{.*}} : @$s7vtables1BC4flux{{[_0-9a-zA-Z]*}}F
 // CHECK:   #B.init!allocator: {{.*}} : @$s7vtables1BC{{[_0-9a-zA-Z]*}}fC
 // CHECK:   #B.zim: {{.*}} : @$s7vtables1BC3zim{{[_0-9a-zA-Z]*}}F
 // CHECK:   #B.zang: {{.*}} : @$s7vtables1BC4zang{{[_0-9a-zA-Z]*}}F

test/Sema/availability_refinement_contexts.swift

@@ -198,6 +198,11 @@ func functionWithWhile() {
 public func olderFunction() {
 }
 
+// CHECK-NEXT: {{^}}  (decl versions=empty explicit=empty decl=unavailableFunction()
+@available(macOS, unavailable)
+public func unavailableFunction() {
+}
+
 // CHECK-NEXT: {{^}}  (decl versions=[10.10,+Inf) explicit=[10.10,+Inf) decl=inlinableFunction()
 // CHECK-NEXT: {{^}}    (condition_following_availability versions=[10.55,+Inf) explicit=[10.55,+Inf)
 // CHECK-NEXT: {{^}}    (if_then versions=[10.55,+Inf) explicit=[10.55,+Inf)

test/Sema/availability_versions.swift

@@ -788,21 +788,21 @@ class UnavailableClassExtendingUnavailableClass : ClassAvailableOn10_51 {
 // Method availability is contravariant
 
 class SuperWithAlwaysAvailableMembers {
-  func shouldAlwaysBeAvailableMethod() { // expected-note {{overridden declaration is here}}
+  func shouldAlwaysBeAvailableMethod() { // expected-note 2 {{overridden declaration is here}}
   }
 
-  var shouldAlwaysBeAvailableProperty: Int { // expected-note {{overridden declaration is here}}
+  var shouldAlwaysBeAvailableProperty: Int { // expected-note 2 {{overridden declaration is here}}
     get { return 9 }
     set(newVal) {}
   }
 
   var setterShouldAlwaysBeAvailableProperty: Int {
     get { return 9 }
-    set(newVal) {} // expected-note {{overridden declaration is here}}
+    set(newVal) {} // expected-note 2 {{overridden declaration is here}}
   }
 
   var getterShouldAlwaysBeAvailableProperty: Int {
-    get { return 9 } // expected-note {{overridden declaration is here}}
+    get { return 9 } // expected-note 2 {{overridden declaration is here}}
     set(newVal) {}
   }
 }
@@ -832,6 +832,31 @@ class SubWithLimitedMemberAvailability : SuperWithAlwaysAvailableMembers {
   }
 }
 
+class SubWithUnavailableMembers : SuperWithAlwaysAvailableMembers {
+  @available(OSX, unavailable)
+  override func shouldAlwaysBeAvailableMethod() { // expected-error {{overriding 'shouldAlwaysBeAvailableMethod' must be as available as declaration it overrides}}
+  }
+
+  @available(OSX, unavailable)
+  override var shouldAlwaysBeAvailableProperty: Int { // expected-error {{overriding 'shouldAlwaysBeAvailableProperty' must be as available as declaration it overrides}}
+    get { return 10 }
+    set(newVal) {}
+  }
+
+  override var setterShouldAlwaysBeAvailableProperty: Int {
+    get { return 9 }
+    @available(OSX, unavailable)
+    set(newVal) {} // expected-error {{overriding setter for 'setterShouldAlwaysBeAvailableProperty' must be as available as declaration it overrides}}
+    // This is a terrible diagnostic. rdar://problem/20427938
+  }
+
+  override var getterShouldAlwaysBeAvailableProperty: Int {
+    @available(OSX, unavailable)
+    get { return 9 } // expected-error {{overriding getter for 'getterShouldAlwaysBeAvailableProperty' must be as available as declaration it overrides}}
+    set(newVal) {}
+  }
+}
+
 class SuperWithLimitedMemberAvailability {
   @available(OSX, introduced: 10.51)
   func someMethod() {

test/attr/attr_availability_maccatalyst.swift

@@ -1,6 +1,6 @@
 // RUN: %swift -typecheck -verify -parse-stdlib -target x86_64-apple-ios51.0-macabi %s
 
-// REQUIRES: OS=maccatalyst
+// REQUIRES: VENDOR=apple
 
 @available(macCatalyst, introduced: 1.0, deprecated: 2.0, obsoleted: 9.0,
            message: "you don't want to do that anyway")
@@ -52,6 +52,16 @@ func introducedLaterOnMacCatalyst() {
 func introducedLaterOnIOS() {
 }
 
+@available(iOS, introduced: 1.0)
+@available(macCatalyst, unavailable)
+func unavailableOnMacCatalyst() { // expected-note 3 {{'unavailableOnMacCatalyst()' has been explicitly marked unavailable here}}
+}
+
+@available(iOS, unavailable)
+@available(macCatalyst, introduced: 1.0)
+func unavailableOnIOS() {
+}
+
 // expected-note@+1 *{{add @available attribute to enclosing global function}}
 func testPoundAvailable() {
 
@@ -60,6 +70,9 @@ func testPoundAvailable() {
     // expected-note@-1 {{add 'if #available' version check}}
     introducedLaterOnIOS() // expected-error {{'introducedLaterOnIOS()' is only available in Mac Catalyst 56.0 or newer}}
     // expected-note@-1 {{add 'if #available' version check}}
+
+    unavailableOnMacCatalyst() // expected-error {{'unavailableOnMacCatalyst()' is unavailable in Mac Catalyst}}
+    unavailableOnIOS()
   }
 
   // macCatalyst should win over iOS when present
@@ -69,6 +82,9 @@ func testPoundAvailable() {
     // expected-note@-1 {{add 'if #available' version check}}
     introducedLaterOnIOS() // expected-error {{'introducedLaterOnIOS()' is only available in Mac Catalyst 56.0 or newer}}
     // expected-note@-1 {{add 'if #available' version check}}
+
+    unavailableOnMacCatalyst() // expected-error {{'unavailableOnMacCatalyst()' is unavailable in Mac Catalyst}}
+    unavailableOnIOS()
   }
 
   if #available(iOS 55.0, macCatalyst 56.0, *) {
@@ -88,6 +104,9 @@ func testPoundAvailable() {
     // expected-note@-1 {{add 'if #available' version check}}
     introducedLaterOnIOS() // expected-error {{'introducedLaterOnIOS()' is only available in Mac Catalyst 56.0 or newer}}
     // expected-note@-1 {{add 'if #available' version check}}
+
+    unavailableOnMacCatalyst() // expected-error {{'unavailableOnMacCatalyst()' is unavailable in Mac Catalyst}}
+    unavailableOnIOS()
   }
 
   if #available(iOS 56.0, *) {