Sema: Reject @available on observing accessors.

Marking an observer unavailable (or potentially unavailable) has no effect
since the compiler emits calls to them unconditionally.

Resolves rdar://80337141.

Author: tshortli

lib/Sema/TypeCheckAttr.cpp

@@ -4932,6 +4932,12 @@ TypeChecker::diagnosticIfDeclCannotBePotentiallyUnavailable(const Decl *D) {
   if (auto *DD = dyn_cast<DestructorDecl>(D))
     return Diagnostic(diag::availability_decl_no_potential, D);
 
+  // Observing accessors are always called implicitly.
+  if (auto *AD = dyn_cast<AccessorDecl>(D)) {
+    if (AD->isObservingAccessor())
+      return Diagnostic(diag::availability_decl_no_potential, D);
+  }
+
   if (auto *VD = dyn_cast<VarDecl>(D)) {
     if (!VD->hasStorageOrWrapsStorage())
       return std::nullopt;
@@ -4987,6 +4993,12 @@ TypeChecker::diagnosticIfDeclCannotBeUnavailable(const Decl *D) {
   if (auto *AT = dyn_cast<AssociatedTypeDecl>(D))
     return Diagnostic(diag::availability_decl_no_unavailable, D);
 
+  // Observing accessors are always called implicitly.
+  if (auto *AD = dyn_cast<AccessorDecl>(D)) {
+    if (AD->isObservingAccessor())
+      return Diagnostic(diag::availability_decl_no_unavailable, D);
+  }
+
   if (auto *VD = dyn_cast<VarDecl>(D)) {
     if (!VD->hasStorageOrWrapsStorage())
       return std::nullopt;

test/Sema/availability_accessors.swift

@@ -98,7 +98,6 @@ struct SubscriptHelper {
     @available(*, unavailable)
     set { } // expected-note {{setter for 'subscript(unavailableGetterAndSetter:)' has been explicitly marked unavailable here}}
   }
-
 }
 
 @discardableResult func takesInOut<T>(_ t: inout T) -> T {
@@ -553,3 +552,22 @@ struct TestPatternBindingInitExprs {
   var unavailableGetterAndSetter_0 = global.unavailableGetterAndSetter[0] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
   var unavailableGetterAndSetter_0_b = global.unavailableGetterAndSetter[0].b // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
 }
+
+struct BadAccessorAvailability<T> {
+  var alwaysUnavailableObservers: T  {
+    @available(*, unavailable) // expected-error {{willSet observer for property cannot be marked unavailable with '@available'}}
+    willSet { }
+
+    @available(*, unavailable) // expected-error {{didSet observer for property cannot be marked unavailable with '@available'}}
+    didSet { }
+  }
+
+  var observersUnavailableBeforeSwift99: T  {
+    @available(swift, introduced: 99) // expected-error {{willSet observer for property cannot be marked unavailable with '@available'}}
+    willSet { }
+
+    @available(swift, introduced: 99) // expected-error {{didSet observer for property cannot be marked unavailable with '@available'}}
+    didSet { }
+  }
+
+}

test/Sema/availability_versions.swift

@@ -1801,3 +1801,21 @@ class StoredPropertiesWithAvailabilityInClosures {
     return 0
   }()
 }
+
+struct PropertyObservers {
+  var hasPotentiallyUnavailableObservers: Int {
+    @available(macOS 51, *) // expected-error {{willSet observer for property cannot be marked potentially unavailable with '@available'}}
+    willSet { }
+
+    @available(macOS 51, *) // expected-error {{didSet observer for property cannot be marked potentially unavailable with '@available'}}
+    didSet { }
+  }
+
+  var hasObsoletedObservers: Int {
+    @available(macOS, obsoleted: 10.9) // expected-error {{willSet observer for property cannot be marked unavailable with '@available'}}
+    willSet { }
+
+    @available(macOS, obsoleted: 10.9) // expected-error {{didSet observer for property cannot be marked unavailable with '@available'}}
+    didSet { }
+  }
+}