Relax @retroactive check to allow same-package declarations

Author: Harlan Haskins

lib/AST/ProtocolConformance.cpp

@@ -185,16 +185,22 @@ usesDefaultDefinition(AssociatedTypeDecl *requirement) const {
 bool ProtocolConformance::isRetroactive() const {
   auto extensionModule = getDeclContext()->getParentModule();
   auto protocolModule = getProtocol()->getParentModule();
-  if (extensionModule->isSameModuleLookingThroughOverlays(protocolModule)) {
+  
+  auto isSameRetroactiveContext = 
+    [](ModuleDecl *moduleA, ModuleDecl *moduleB) -> bool {
+      return moduleA->isSameModuleLookingThroughOverlays(moduleB)) ||
+        moduleA->inSamePackage(moduleB));
+    };
+  
+  if (isSameRetroactiveContext(extensionModule, protocolModule)) {
     return false;
   }
 
   auto conformingTypeDecl =
       ConformingType->getNominalOrBoundGenericNominal();
   if (conformingTypeDecl) {
     auto conformingTypeModule = conformingTypeDecl->getParentModule();
-    if (extensionModule->
-        isSameModuleLookingThroughOverlays(conformingTypeModule)) {
+    if (isSameRetroactiveContext(extensionModule, conformingTypeModule)) {
       return false;
     }
   }

test/Sema/extension_retroactive_conformances_package.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -swift-version 5 %t/OtherLibrary.swift -package-name Library -emit-module -module-name OtherLibrary -package-name Library -o %t
+// RUN: %target-swift-frontend -typecheck %t/Library.swift -module-name Library -package-name Library -verify -swift-version 5 -import-underlying-module -I %t
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -module-name Client -package-name Library -verify -swift-version 5 -I %t
+
+//--- Library.swift
+
+public class LibraryClass {
+}
+
+public protocol LibraryProtocol {
+}
+
+//--- OtherLibrary.swift
+public class OtherLibraryClass {}
+public class OtherLibraryProtocol {}
+
+//--- Client.swift
+
+import Library
+import OtherLibrary
+
+// These are all fine because all 3 of these libraries are in the same package.
+
+extension LibraryClass: LibraryProtocol {}
+extension OtherLibraryClass: LibraryProtocol {}
+extension LibraryClass: OtherLibraryProtocol {}