Index: lib/Index/IndexSymbol.cpp
===================================================================
--- lib/Index/IndexSymbol.cpp
+++ lib/Index/IndexSymbol.cpp
@@ -319,6 +319,17 @@
     Info.Lang = SymbolLanguage::CXX;
 
   auto getExternalSymAttr = [](const Decl *D) -> ExternalSourceSymbolAttr* {
+    const NamedDecl *Definition = nullptr;
+    if (auto ID = dyn_cast<ObjCInterfaceDecl>(D)) {
+      Definition = ID->getDefinition();
+    } else if (auto PD = dyn_cast<ObjCProtocolDecl>(D)) {
+      Definition = PD->getDefinition();
+    } else if (auto TD = dyn_cast<TagDecl>(D)) {
+      Definition = TD->getDefinition();
+    }
+    if (Definition)
+      D = Definition;
+
     if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>())
       return attr;
     if (auto *dcd = dyn_cast<Decl>(D->getDeclContext())) {
Index: lib/Index/USRGeneration.cpp
===================================================================
--- lib/Index/USRGeneration.cpp
+++ lib/Index/USRGeneration.cpp
@@ -49,6 +49,18 @@
 static StringRef GetExternalSourceContainer(const NamedDecl *D) {
   if (!D)
     return StringRef();
+
+  const NamedDecl *Definition = nullptr;
+  if (auto ID = dyn_cast<ObjCInterfaceDecl>(D)) {
+    Definition = ID->getDefinition();
+  } else if (auto PD = dyn_cast<ObjCProtocolDecl>(D)) {
+    Definition = PD->getDefinition();
+  } else if (auto TD = dyn_cast<TagDecl>(D)) {
+    Definition = TD->getDefinition();
+  }
+  if (Definition)
+    D = Definition;
+
   if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>()) {
     return attr->getDefinedIn();
   }
Index: test/Index/Core/external-source-symbol-attr.m
===================================================================
--- test/Index/Core/external-source-symbol-attr.m
+++ test/Index/Core/external-source-symbol-attr.m
@@ -4,6 +4,10 @@
 #define GEN_DECL(mod_name) __attribute__((external_source_symbol(language="Swift", defined_in=mod_name, generated_declaration)))
 #define PUSH_GEN_DECL(mod_name) push(GEN_DECL(mod_name), apply_to=any(enum, objc_interface, objc_category, objc_protocol))
 
+// Forward declarations should not affect module namespacing below
+@class I1;
+@class I2;
+
 // This should not be indexed.
 GEN_DECL("some_module")
 @interface I1