Add test cases for when a property/subscript is redeclared readwrite

And improve the comment around the FIXME for this, which is tricky to
implement.

Author: jrose-apple

lib/Sema/TypeCheckAccess.cpp

@@ -1712,7 +1712,10 @@ class ExportabilityChecker : public DeclVisitor<ExportabilityChecker> {
     }
 
     // FIXME: Check storage decls where the setter is in a separate module from
-    // the getter, which is a thing Objective-C can do.
+    // the getter, which is a thing Objective-C can do. The ClangImporter
+    // doesn't make this easy, though, because it just gives the setter the same
+    // DeclContext as the property or subscript, which means we've lost the
+    // information about whether its module was implementation-only imported.
   }
 
   void visit(Decl *D) {

test/Sema/Inputs/implementation-only-override/FooKit.h

@@ -3,8 +3,16 @@
 
 @interface Parent : Base
 - (nonnull instancetype)init __attribute__((objc_designated_initializer));
+
+// The SECRET is on a non-secret property here because we need to enforce that
+// it's not printed.
+@property (readonly, strong, nullable) Parent *redefinedPropSECRET;
 @end
 
 @interface GenericParent<T: Base *> : Base
 - (nonnull instancetype)init __attribute__((objc_designated_initializer));
 @end
+
+@interface SubscriptParent : Base
+- (nullable Parent *)objectAtIndexedSubscript:(int)index;
+@end

test/Sema/Inputs/implementation-only-override/FooKit_SECRET.h

@@ -13,6 +13,8 @@
 @property (readwrite, strong, nullable) Parent *rwPropSECRET;
 
 - (nullable Parent *)objectAtIndexedSubscript:(int)index;
+
+@property (readwrite, strong, nullable) Parent *redefinedPropSECRET;
 @end
 
 @protocol MandatorySecrets
@@ -27,3 +29,7 @@
 @property (readonly, strong, nullable) T roPropSECRET;
 - (nullable Parent *)objectAtIndexedSubscript:(int)index;
 @end
+
+@interface SubscriptParent ()
+- (void)setObject:(nullable Parent *)object atIndexedSubscript:(int)index;
+@end

test/Sema/implementation-only-override.swift

@@ -31,6 +31,10 @@ public class GoodChild: Parent {
     set {}
   }
   @_implementationOnly public override subscript(_ index: Int32) -> Parent? { nil } // expected-note {{overridden declaration is here}}
+  @_implementationOnly public override var redefinedPropSECRET: Parent? { // expected-note {{overridden declaration is here}}
+    get { nil }
+    set {}
+  }
 }
 
 // CHECK-LABEL: class QuietChild : FooKit.Parent {
@@ -76,6 +80,10 @@ internal class PrivateChild: Parent {
     set {}
   }
   override subscript(_ index: Int32) -> Parent? { nil }
+  override var redefinedPropSECRET: Parent? {
+    get { nil }
+    set {}
+  }
 }
 
 internal class PrivateGrandchild: GoodChild {
@@ -87,6 +95,20 @@ internal class PrivateGrandchild: GoodChild {
     set {}
   }
   override subscript(_ index: Int32) -> Parent? { nil }
+  override var redefinedPropSECRET: Parent? {
+    get { nil }
+    set {}
+  }
+}
+
+// CHECK-LABEL: class SubscriptChild : FooKit.SubscriptParent {
+//  CHECK-NEXT:   @objc deinit
+//  CHECK-NEXT: }
+public class SubscriptChild: SubscriptParent {
+  @_implementationOnly public override subscript(_ index: Int32) -> Parent? {
+    get { nil }
+    set {}
+  }
 }
 
 #if ERRORS
@@ -107,6 +129,10 @@ public class NaughtyChild: Parent {
     set {}
   }
   public override subscript(_ index: Int32) -> Parent? { nil } // expected-error {{override of subscript imported as implementation-only must be declared '@_implementationOnly'}} {{3-3=@_implementationOnly }}
+  public override var redefinedPropSECRET: Parent? { // FIXME: the setter here is from the implementation-only import, so we'd like to complain about this too.
+    get { nil }
+    set {}
+  }
 }
 
 public class NaughtyChildByType: Parent {
@@ -128,6 +154,10 @@ public class NaughtyGrandchild: GoodChild {
     set {}
   }
   public override subscript(_ index: Int32) -> Parent? { nil } // expected-error {{override of '@_implementationOnly' subscript should also be declared '@_implementationOnly'}} {{3-3=@_implementationOnly }}
+  public override var redefinedPropSECRET: Parent? { // expected-error {{override of '@_implementationOnly' property should also be declared '@_implementationOnly'}} {{3-3=@_implementationOnly }}
+    get { nil }
+    set {}
+  }
 }
 
 #endif // ERRORS