[ModuleInterface] Allow non-mutable IBOutlet instance properties in module interfaces

If an IBOutlet property is public private(set), it's interface only has a
getter. Consuming this interface was triggering a diagnostic that IBOutlet
properties must be mutable. This patch bypasses this check for module
interfaces.

Resolves rdar://problem/49856177

Author: Nathan Hawes

lib/Sema/TypeCheckAttr.cpp

@@ -415,8 +415,13 @@ void AttributeEarlyChecker::visitIBOutletAttr(IBOutletAttr *attr) {
   if (!VD->getDeclContext()->getSelfClassDecl() || VD->isStatic())
     diagnoseAndRemoveAttr(attr, diag::invalid_iboutlet);
 
-  if (!VD->isSettable(nullptr))
-    diagnoseAndRemoveAttr(attr, diag::iboutlet_only_mutable);
+  if (!VD->isSettable(nullptr)) {
+    // Allow non-mutable IBOutlet properties in module interfaces,
+    // as they may have been private(set)
+    SourceFile *Parent = VD->getDeclContext()->getParentSourceFile();
+    if (!Parent || Parent->Kind != SourceFileKind::Interface)
+      diagnoseAndRemoveAttr(attr, diag::iboutlet_only_mutable);
+  }
 
   // Verify that the field type is valid as an outlet.
   auto type = VD->getType();

test/ParseableInterface/iboutlet-private-set.swift

@@ -0,0 +1,18 @@
+// REQUIRES: objc_interop
+
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -enable-library-evolution -disable-objc-attr-requires-foundation-module -emit-parseable-module-interface-path %t/Foo.swiftinterface %s
+// RUN: %FileCheck %s -input-file %t/Foo.swiftinterface
+// RUN: %target-swift-frontend -build-module-from-parseable-interface %t/Foo.swiftinterface -o %t/Foo.swiftmodule
+
+// Test the interface we generate for @IBOutlet private(set) properties is
+// consumable.
+
+@objc public class MyType {}
+
+open class Bar {
+	// CHECK: @objc @IBOutlet weak public var foo: MyType! {
+	// CHECK-NEXT: get
+	// CHECK-NEXT: }
+	@IBOutlet public private(set) weak var foo: MyType!
+}