AST: consider extension and extended type are from the same module if valid alternative module names are equal

Framework authors may move a type along with its extensions from
a high-level framework to a low-level one. We should compare
alternative module names for them to check if they should be considered from
the same module to preserve ABI stability.

rdar://65889766

Author: nkcsgexi

lib/AST/Decl.cpp

@@ -1326,14 +1326,28 @@ bool ExtensionDecl::isConstrainedExtension() const {
 bool ExtensionDecl::isEquivalentToExtendedContext() const {
   auto decl = getExtendedNominal();
   bool extendDeclFromSameModule = false;
-  if (!decl->getAlternateModuleName().empty()) {
-    // if the extended type was defined in the same module with the extension,
-    // we should consider them as the same module to preserve ABI stability.
-    extendDeclFromSameModule = decl->getAlternateModuleName() ==
-      getParentModule()->getNameStr();
+  auto extensionAlterName = getAlternateModuleName();
+  auto typeAlterName = decl->getAlternateModuleName();
+
+  if (!extensionAlterName.empty()) {
+    if (!typeAlterName.empty()) {
+      // Case I: type and extension are both moved from somewhere else
+      extendDeclFromSameModule = typeAlterName == extensionAlterName;
+    } else {
+      // Case II: extension alone was moved from somewhere else
+      extendDeclFromSameModule = extensionAlterName ==
+        decl->getParentModule()->getNameStr();
+    }
   } else {
-    extendDeclFromSameModule = decl->getParentModule() == getParentModule();
+    if (!typeAlterName.empty()) {
+      // Case III: extended type alone was moved from somewhere else
+      extendDeclFromSameModule = typeAlterName == getParentModule()->getNameStr();
+    } else {
+      // Case IV: neither of type and extension was moved from somewhere else
+      extendDeclFromSameModule = getParentModule() == decl->getParentModule();
+    }
   }
+
   return extendDeclFromSameModule
     && !isConstrainedExtension()
     && !getDeclaredInterfaceType()->isExistentialType();

test/TBD/move_to_extension_comove.swift

@@ -0,0 +1,48 @@
+// REQUIRES: OS=macosx
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -typecheck %s -emit-tbd -emit-tbd-path %t/before_move.tbd -D BEFORE_MOVE -module-name Foo -enable-library-evolution -emit-sil -o %t/before_move.sil
+// RUN: %FileCheck %s --check-prefix=CHECK-TBD < %t/before_move.tbd
+// RUN: %FileCheck %s --check-prefix=CHECK-SIL < %t/before_move.sil
+
+// RUN: %target-swift-frontend %s -emit-module -emit-module-path %t/FooCore.swiftmodule -D AFTER_MOVE_FOO_CORE -module-name FooCore -enable-library-evolution -emit-tbd -emit-tbd-path %t/after_move.tbd -emit-sil -o %t/after_move.sil
+
+// RUN: %FileCheck %s --check-prefix=CHECK-TBD < %t/after_move.tbd
+// RUN: %FileCheck %s --check-prefix=CHECK-SIL < %t/after_move.sil
+
+// CHECK-TBD: '_$s3Foo4DateC03SubB0V4yearAESi_tcfC'
+// CHECK-TBD: '_$s3Foo4DateC03SubB0VMn'
+
+// CHECK-SIL: sil [available 10.7] @$s3Foo4DateC03SubB0V4yearAESi_tcfC : $@convention(method) (Int, @thin Date.SubDate.Type) -> @out Date.SubDate
+
+#if BEFORE_MOVE
+
+@available(OSX 10.7, *)
+public class Date {
+  public static func getCurrentYear() -> Int { return 2020 }
+}
+
+@available(OSX 10.7, *)
+extension Date {
+  public struct SubDate {
+    public init(year: Int) {}
+  }
+}
+
+#endif
+
+#if AFTER_MOVE_FOO_CORE
+
+@available(OSX 10.7, *)
+@_originallyDefinedIn(module: "Foo", OSX 10.9)
+public class Date {}
+
+@available(OSX 10.7, *)
+@_originallyDefinedIn(module: "Foo", OSX 10.9)
+extension Date {
+  public struct SubDate {
+    public init(year: Int) {}
+  }
+}
+
+#endif