[ClangImporter] Resolve forward declarations before importing names.

This allows a previously-working case of Objective-C forward-declaring
a type in a /different/ Swift module to continue working, as long as
the Swift context being compiled manages to import the other module
properly (including its generated header). This isn't really our
recommended pattern---that would be to @import the module in the
bridging header and forego the forward declaration---but it doesn't
cost much to keep it working. It's also a place where textual and
precompiled bridging headers behaved differently, because precompiled
ones are processed much earlier.

https://bugs.swift.org/browse/SR-3798

Author: jrose-apple

lib/ClangImporter/ImportDecl.cpp

@@ -4030,18 +4030,8 @@ namespace {
     }
 
     Decl *VisitObjCInterfaceDecl(const clang::ObjCInterfaceDecl *decl) {
-      Optional<ImportedName> correctSwiftName;
-      auto importedName = importFullName(decl, correctSwiftName);
-      if (!importedName) return nullptr;
-
-      // If we've been asked to produce a Swift 2 stub, handle it via a
-      // typealias.
-      if (correctSwiftName)
-        return importSwift2TypeAlias(decl, importedName, *correctSwiftName);
-
-      auto name = importedName.getDeclName().getBaseName();
-
-      auto createRootClass = [=](DeclContext *dc = nullptr) -> ClassDecl * {
+      auto createRootClass = [=](Identifier name,
+                                 DeclContext *dc = nullptr) -> ClassDecl * {
         if (!dc) {
           dc = Impl.getClangModuleForDecl(decl->getCanonicalDecl(),
                                           /*allowForwardDeclaration=*/true);
@@ -4076,11 +4066,26 @@ namespace {
         const ClassDecl *nsObjectDecl =
           nsObjectTy->getClassOrBoundGenericClass();
 
-        auto result = createRootClass(nsObjectDecl->getDeclContext());
+        auto result = createRootClass(Impl.SwiftContext.Id_Protocol,
+                                      nsObjectDecl->getDeclContext());
         result->setForeignClassKind(ClassDecl::ForeignKind::RuntimeOnly);
         return result;
       }
 
+      if (auto *definition = decl->getDefinition())
+        decl = definition;
+
+      Optional<ImportedName> correctSwiftName;
+      auto importedName = importFullName(decl, correctSwiftName);
+      if (!importedName) return nullptr;
+
+      // If we've been asked to produce a Swift 2 stub, handle it via a
+      // typealias.
+      if (correctSwiftName)
+        return importSwift2TypeAlias(decl, importedName, *correctSwiftName);
+
+      auto name = importedName.getDeclName().getBaseName();
+
       if (!decl->hasDefinition()) {
         // Check if this class is implemented in its adapter.
         if (auto clangModule = Impl.getClangModuleForDecl(decl, true)) {
@@ -4092,7 +4097,7 @@ namespace {
 
         if (Impl.ImportForwardDeclarations) {
           // Fake it by making an unavailable opaque @objc root class.
-          auto result = createRootClass();
+          auto result = createRootClass(name);
           result->setImplicit();
           auto attr = AvailableAttr::createPlatformAgnostic(Impl.SwiftContext,
               "This Objective-C class has only been forward-declared; "
@@ -4105,9 +4110,6 @@ namespace {
         return nullptr;
       }
 
-      decl = decl->getDefinition();
-      assert(decl);
-
       auto dc =
           Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
       if (!dc)

test/ClangImporter/MixedSource/Inputs/import-mixed-framework-with-forward.h

@@ -0,0 +1,6 @@
+@class SwiftClass, SwiftClassWithCustomName;
+
+@interface BridgingHeader
++ (void)takeForward:(SwiftClass *)class;
++ (void)takeRenamedForward:(SwiftClassWithCustomName *)class;
+@end

test/ClangImporter/MixedSource/Inputs/mixed-framework/Mixed.framework/Headers/Mixed.h

@@ -1,3 +1,5 @@
+// Manual PrintAsObjC for testing purposes.
+
 struct PureClangType {
   int x;
   int y;
@@ -16,7 +18,7 @@ struct PureClangType {
 #  define SWIFT_PROTOCOL(SWIFT_NAME) SWIFT_PROTOCOL_EXTRA
 #endif
 
-#ifndef SWIFT_EXTENSION(X)
+#ifndef SWIFT_EXTENSION
 #  define SWIFT_EXTENSION(X) X##__LINE__
 #endif
 

test/ClangImporter/MixedSource/Inputs/resolve-cross-language/Base.swift

@@ -32,3 +32,9 @@ extension BaseClass {
   @objc public func getSwiftEnum() -> SwiftEnum { return .Quux }
   public init() {}
 }
+
+@objc(RenamedClass) public class SwiftClass {}
+
+public func getSwiftClass() -> SwiftClass {
+  return SwiftClass()
+}

test/ClangImporter/MixedSource/import-mixed-framework-with-forward.swift

@@ -0,0 +1,20 @@
+// RUN: rm -rf %t && mkdir -p %t
+// RUN: cp -r %S/Inputs/mixed-framework/Mixed.framework %t
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module -o %t/Mixed.framework/Modules/Mixed.swiftmodule/%target-swiftmodule-name %S/Inputs/mixed-framework/Mixed.swift -import-underlying-module -F %t -module-name Mixed -disable-objc-attr-requires-foundation-module
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -F %t -import-objc-header %S/Inputs/import-mixed-framework-with-forward.h %s -verify
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-pch -F %t %S/Inputs/import-mixed-framework-with-forward.h -o %t/bridge.pch
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -F %t -import-objc-header %t/bridge.pch %s -verify
+
+
+// REQUIRES: objc_interop
+
+import Mixed
+
+BridgingHeader.takeForward(SwiftClass(x: 42))
+BridgingHeader.takeRenamedForward(CustomNameClass())
+
+// Check that we're compiling at all.
+BridgingHeader.takeRenamedForward(SwiftClass(x: 42)) // expected-error {{cannot convert value of type 'SwiftClass' to expected argument type 'CustomNameClass!'}}