Fix layering of cross-import and clang overlays

If a clang module declares a cross-import overlay, but it also has a traditional overlay, we want the cross-import overlay to be registered with the SourceFile as sitting atop the traditional overlay. Otherwise module-qualified name lookups will bypass the cross-import overlay.

Fixes rdar://62139656.

Author: beccadax

lib/Sema/ImportResolution.cpp

@@ -803,9 +803,17 @@ void ImportResolver::crossImport(ModuleDecl *M, UnboundImport &I) {
   if (!SF.shouldCrossImport())
     return;
 
-  if (I.getUnderlyingModule())
+  if (I.getUnderlyingModule()) {
+    auto underlying = I.getUnderlyingModule().get();
+
+    // If this is a clang module, and it has a clang overlay, we want the
+    // separately-imported overlay to sit on top of the clang overlay.
+    if (underlying->isNonSwiftModule())
+      underlying = underlying->getTopLevelModule(true);
+
     // FIXME: Should we warn if M doesn't reexport underlyingModule?
-    SF.addSeparatelyImportedOverlay(M, I.getUnderlyingModule().get());
+    SF.addSeparatelyImportedOverlay(M, underlying);
+  }
 
   auto newImports = crossImportableModules.getArrayRef()
                         .drop_front(nextModuleToCrossImport);

test/CrossImport/Inputs/lib-templates/Frameworks/ClangFramework.framework/Modules/ClangFramework.swiftcrossimport/BystandingLibrary.swiftoverlay

@@ -0,0 +1,5 @@
+%YAML 1.2
+---
+version: 1
+modules:
+  - name: _ClangFramework_BystandingLibrary

test/CrossImport/Inputs/lib-templates/Frameworks/OverlaidClangFramework.framework/Modules/OverlaidClangFramework.swiftcrossimport/BystandingLibrary.swiftoverlay

@@ -0,0 +1,5 @@
+%YAML 1.2
+---
+version: 1
+modules:
+  - name: _OverlaidClangFramework_BystandingLibrary

test/CrossImport/Inputs/lib-templates/Frameworks/SwiftFramework.framework/Modules/SwiftFramework.swiftcrossimport/BystandingLibrary.swiftoverlay

@@ -0,0 +1,5 @@
+%YAML 1.2
+---
+version: 1
+modules:
+  - name: _SwiftFramework_BystandingLibrary

test/CrossImport/Inputs/lib-templates/Frameworks/_ClangFramework_BystandingLibrary.framework/Modules/_ClangFramework_BystandingLibrary.swiftmodule/module-triple-here.swiftinterface

@@ -0,0 +1,7 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -swift-version 5 -enable-library-evolution -module-name _ClangFramework_BystandingLibrary
+
+import Swift
+@_exported import ClangFramework
+
+public func fromClangFrameworkCrossImport()

test/CrossImport/Inputs/lib-templates/Frameworks/_OverlaidClangFramework_BystandingLibrary.framework/Modules/_OverlaidClangFramework_BystandingLibrary.swiftmodule/module-triple-here.swiftinterface

@@ -0,0 +1,7 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -swift-version 5 -enable-library-evolution -module-name _OverlaidClangFramework_BystandingLibrary
+
+import Swift
+@_exported import OverlaidClangFramework
+
+public func fromOverlaidClangFrameworkCrossImport()

test/CrossImport/Inputs/lib-templates/Frameworks/_SwiftFramework_BystandingLibrary.framework/Modules/_SwiftFramework_BystandingLibrary.swiftmodule/module-triple-here.swiftinterface

@@ -0,0 +1,7 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -swift-version 5 -enable-library-evolution -module-name _SwiftFramework_BystandingLibrary
+
+import Swift
+@_exported import SwiftFramework
+
+public func fromSwiftFrameworkCrossImport()

test/CrossImport/Inputs/lib-templates/lib/swift/OverlaidClangFramework.swiftmodule/module-triple-here.swiftinterface

@@ -3,3 +3,5 @@
 
 import Swift
 @_exported import OverlaidClangFramework
+
+public func fromOverlaidClangFrameworkOverlay()

test/CrossImport/common-case.swift

@@ -0,0 +1,40 @@
+// This explicitly tests "common" cases with well-constructed cross-import
+// overlays. Some behaviors here are poorly covered by other tests.
+
+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/lib-templates/* %t/
+// RUN: %{python} %S/Inputs/rewrite-module-triples.py %t %module-target-triple
+
+// RUN: %target-typecheck-verify-swift -enable-cross-import-overlays -I %t/include -I %t/lib/swift -F %t/Frameworks
+
+// Each framework has a cross-import overlay with this library:
+import BystandingLibrary
+
+// 1. A Swift framework
+
+import SwiftFramework
+
+fromSwiftFramework()
+fromSwiftFrameworkCrossImport()
+SwiftFramework.fromSwiftFramework()
+SwiftFramework.fromSwiftFrameworkCrossImport()
+
+// 2. A Clang framework
+
+import ClangFramework
+
+fromClangFramework()
+fromClangFrameworkCrossImport()
+ClangFramework.fromClangFramework()
+ClangFramework.fromClangFrameworkCrossImport()
+
+// 3. A Swift-overlaid Clang framework
+
+import OverlaidClangFramework
+
+fromOverlaidClangFramework()
+fromOverlaidClangFrameworkOverlay()
+fromOverlaidClangFrameworkCrossImport()
+OverlaidClangFramework.fromOverlaidClangFramework()
+OverlaidClangFramework.fromOverlaidClangFrameworkOverlay()
+OverlaidClangFramework.fromOverlaidClangFrameworkCrossImport()