Prevent shadowing of unavailable member impls

Typically, access control denies access to member implementations, so the imported interface decl will be used instead. However, in contexts that permit direct access to stored properties—such as accessors, inits, and deinits—their member implementations are accessible; the compiler then relies on a shadowing rule favoring Swift decls over ObjC decls to eliminate the imported interface decl.

However, there are many rules that are higher-priority than the Swift vs. ObjC decls one. In particular, a recent change to availability checking in #77886 caused a higher-priority rule to begin eliminating member implementations which belonged to unavailable extensions. This caused regressions in projects using `@objc @implementation` with classes that are unavailable in Mac Catalyst.

Introduce a fairly high-priority shadowing rule that favors a member implementation over its interface when both are present (i.e. when direct access to storage is permitted).

Fixes rdar://143582383.

Author: beccadax

lib/AST/NameLookup.cpp

@@ -393,6 +393,16 @@ static ConstructorComparison compareConstructors(ConstructorDecl *ctor1,
   return ConstructorComparison::Same;
 }
 
+static bool isMemberImplementation(ValueDecl *VD) {
+  return VD->isObjCMemberImplementation();
+}
+static bool isMemberImplementation(OperatorDecl *VD) {
+  return false;
+}
+static bool isMemberImplementation(PrecedenceGroupDecl *VD) {
+  return false;
+}
+
 /// Given a set of declarations whose names and interface types have matched,
 /// figure out which of these declarations have been shadowed by others.
 template <typename T>
@@ -412,7 +422,8 @@ static void recordShadowedDeclsAfterTypeMatch(
   for (unsigned firstIdx : indices(decls)) {
     auto firstDecl = decls[firstIdx];
     auto firstModule = firstDecl->getModuleContext();
-    bool firstTopLevel = firstDecl->getDeclContext()->isModuleScopeContext();
+    auto firstDC = firstDecl->getDeclContext();
+    bool firstTopLevel = firstDC->isModuleScopeContext();
 
     auto name = firstDecl->getBaseName();
 
@@ -454,7 +465,8 @@ static void recordShadowedDeclsAfterTypeMatch(
       // Determine whether one module takes precedence over another.
       auto secondDecl = decls[secondIdx];
       auto secondModule = secondDecl->getModuleContext();
-      bool secondTopLevel = secondDecl->getDeclContext()->isModuleScopeContext();
+      auto secondDC = secondDecl->getDeclContext();
+      bool secondTopLevel = secondDC->isModuleScopeContext();
       bool secondPrivate = isPrivateImport(secondModule);
 
       // For member types, we skip most of the below rules. Instead, we allow
@@ -538,6 +550,22 @@ static void recordShadowedDeclsAfterTypeMatch(
         }
       }
 
+      // Member implementations are usually filtered out by access control.
+      // They're sometimes visible in contexts that can directly access storage,
+      // though, and there they should shadow the matching imported declaration.
+      if (firstDC != secondDC
+            && firstDC->getImplementedObjCContext() ==
+                 secondDC->getImplementedObjCContext()) {
+        if (isMemberImplementation(firstDecl) && secondDecl->hasClangNode()) {
+          shadowed.insert(secondDecl);
+          continue;
+        }
+        if (isMemberImplementation(secondDecl) && firstDecl->hasClangNode()) {
+          shadowed.insert(firstDecl);
+          continue;
+        }
+      }
+
       // Swift 4 compatibility hack: Don't shadow properties defined in
       // extensions of generic types with properties defined elsewhere.
       // This is due to the fact that in Swift 4, we only gave custom overload

test/decl/ext/Inputs/objc_implementation_internal.h

@@ -6,3 +6,13 @@
 
 @end
 
+@interface ObjCPropertyTest : NSObject
+
+@property (readonly) int prop1;
+@property (readwrite) int prop2;
+
+- (instancetype)init;
+
+- (void)doSomething;
+
+@end

test/decl/ext/objc_implementation_direct_to_storage.swift

@@ -0,0 +1,30 @@
+// RUN: %target-typecheck-verify-swift -Xcc -fmodule-map-file=%S/Inputs/objc_implementation_private.modulemap -enable-experimental-feature ObjCImplementation -target %target-stable-abi-triple -debug-diagnostic-names
+// REQUIRES: objc_interop
+// REQUIRES: swift_feature_ObjCImplementation
+
+import objc_implementation_internal
+
+@available(*, unavailable)
+@objc @implementation extension ObjCPropertyTest {
+  // FIXME: Shouldn't this be on the `@available` above?
+  // expected-note@+1 {{'prop1' has been explicitly marked unavailable here}}
+  let prop1: Int32
+
+  // expected-note@+1 2 {{'prop2' has been explicitly marked unavailable here}}
+  var prop2: Int32 {
+    didSet {
+      _ = prop2 // expected-error {{'prop2' is unavailable}}
+    }
+  }
+
+  override init() {
+    self.prop1 = 1 // expected-error {{'prop1' is unavailable}}
+    self.prop2 = 2 // expected-error {{'prop2' is unavailable}}
+    super.init()
+  }
+
+  func doSomething() {
+    _ = self.prop1
+    _ = self.prop2
+  }
+}