[Serialization] Handle re-export of error enums harder (#21880)

A refinement of Doug's e3207f7 from a year ago to account for
trying to reference an import-as-member error enum /through/ an
overlay when it's in a Clang module using 'export_as'. The problem is
that importing error enums synthesizes a corresponding struct, and
that struct doesn't have a Clang node and so when it isn't found in
the right module the compiler didn't know to allow it anyway.

I know, right?

(Looking at the source change might be simpler than trying to read
the above paragraph.)

I'm still not 100% sure on how this occurred in the original
(Apple-internal) projects hitting this, but I think it's just that we
handle all import-as-member types at once, and the clients were using
something else from a type that contained the error enum, and then
that resulted in the error struct's conformances getting written into
a serialized swiftmodule.

rdar://problem/47152185

Author: jrose-apple

lib/Serialization/Deserialization.cpp

@@ -1140,21 +1140,20 @@ getActualCtorInitializerKind(uint8_t raw) {
   return None;
 }
 
-/// Determine whether the two modules are re-exported to the same module.
-static bool reExportedToSameModule(const ModuleDecl *fromModule,
-                                   const ModuleDecl *toModule) {
+static bool isReExportedToModule(const ValueDecl *value,
+                                 const ModuleDecl *expectedModule) {
+  const DeclContext *valueDC = value->getDeclContext();
   auto fromClangModule
-    = dyn_cast<ClangModuleUnit>(fromModule->getFiles().front());
+      = dyn_cast<ClangModuleUnit>(valueDC->getModuleScopeContext());
   if (!fromClangModule)
     return false;
+  std::string exportedName = fromClangModule->getExportedModuleName();
 
   auto toClangModule
-  = dyn_cast<ClangModuleUnit>(toModule->getFiles().front());
-  if (!toClangModule)
-    return false;
-
-  return fromClangModule->getExportedModuleName() ==
-    toClangModule->getExportedModuleName();
+      = dyn_cast<ClangModuleUnit>(expectedModule->getFiles().front());
+  if (toClangModule)
+    return exportedName == toClangModule->getExportedModuleName();
+  return exportedName == expectedModule->getName().str();
 }
 
 /// Remove values from \p values that don't match the expected type or module.
@@ -1197,7 +1196,7 @@ static void filterValues(Type expectedTy, ModuleDecl *expectedModule,
     // module to the original definition in a base module.
     if (expectedModule && !value->hasClangNode() &&
         value->getModuleContext() != expectedModule &&
-        !reExportedToSameModule(value->getModuleContext(), expectedModule))
+        !isReExportedToModule(value, expectedModule))
       return true;
 
     // If we're expecting a member within a constrained extension with a
@@ -1357,6 +1356,7 @@ ModuleFile::resolveCrossReference(ModuleID MID, uint32_t pathLen) {
       if (entry.Kind != llvm::BitstreamEntry::Record)
         return Identifier();
 
+      scratch.clear();
       unsigned recordID = DeclTypeCursor.readRecord(entry.ID, scratch,
                                                     &blobData);
       switch (recordID) {

test/ClangImporter/private_frameworks.swift

@@ -12,7 +12,7 @@
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withprivate -swift-version 4 %s -import-objc-header %S/Inputs/privateframeworks/bridging-somekitcore.h -verify
 
 // Use the overlay without private frameworks.
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withoutprivate -I %t -swift-version 4 -import-objc-header %S/Inputs/privateframeworks/bridging-somekit.h %s
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withoutprivate -swift-version 4 -import-objc-header %S/Inputs/privateframeworks/bridging-somekit.h %s
 
 // Build the overlay with public frameworks.
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-module -F %S/Inputs/privateframeworks/withoutprivate -o %t %S/Inputs/privateframeworks/overlay/SomeKit.swift
@@ -21,7 +21,18 @@
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withprivate -swift-version 4 %s -import-objc-header %S/Inputs/privateframeworks/bridging-somekitcore.h -verify
 
 // Use the overlay without private frameworks.
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withoutprivate -I %t -swift-version 4 %s -import-objc-header %S/Inputs/privateframeworks/bridging-somekit.h 
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withoutprivate -swift-version 4 %s -import-objc-header %S/Inputs/privateframeworks/bridging-somekit.h 
+
+// Use something that uses the overlay.
+// RUN: echo 'import private_frameworks; testErrorConformance()' > %t/main.swift
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-module -o %t -F %S/Inputs/privateframeworks/withprivate -swift-version 4 %s -import-objc-header %S/Inputs/privateframeworks/bridging-somekitcore.h
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withprivate %t/main.swift -verify
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withoutprivate %t/main.swift -verify
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-module -o %t -F %S/Inputs/privateframeworks/withoutprivate -swift-version 4 %s -import-objc-header %S/Inputs/privateframeworks/bridging-somekit.h
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withprivate %t/main.swift -verify
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -emit-sil -o /dev/null -F %S/Inputs/privateframeworks/withoutprivate %t/main.swift -verify
 
 // REQUIRES: objc_interop
 
@@ -50,3 +61,6 @@ func testGlobals() {
   SomeKit.someKitOtherGlobalFunc()
   someKitOtherGlobalFunc()
 }
+
+public struct ErrorsOnly<T: Error> {}
+public func testErrorConformance(_ code: ErrorsOnly<SKWidget.Error>? = nil) {}