Index: include/clang/Index/IndexSymbol.h
===================================================================
--- include/clang/Index/IndexSymbol.h
+++ include/clang/Index/IndexSymbol.h
@@ -16,6 +16,7 @@
 
 namespace clang {
   class Decl;
+  class ExternalSourceSymbolAttr;
   class LangOptions;
 
 namespace index {
@@ -132,6 +133,8 @@
 
 bool isFunctionLocalSymbol(const Decl *D);
 
+ExternalSourceSymbolAttr *getExternalSourceSymbolAttr(const Decl *D);
+
 void applyForEachSymbolRole(SymbolRoleSet Roles,
                             llvm::function_ref<void(SymbolRole)> Fn);
 bool applyForEachSymbolRoleInterruptible(SymbolRoleSet Roles,
Index: lib/Index/IndexSymbol.cpp
===================================================================
--- lib/Index/IndexSymbol.cpp
+++ lib/Index/IndexSymbol.cpp
@@ -80,6 +80,30 @@
   return true;
 }
 
+ExternalSourceSymbolAttr *index::getExternalSourceSymbolAttr(const Decl *D) {
+  if (!D)
+    return nullptr;
+
+  const Decl *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)
+    Definition = D;
+
+  if (auto *attr = Definition->getAttr<ExternalSourceSymbolAttr>())
+    return attr;
+  if (auto *dcd = dyn_cast<Decl>(D->getDeclContext())) {
+    return dcd->getAttr<ExternalSourceSymbolAttr>();
+  }
+
+  return nullptr;
+}
+
 SymbolInfo index::getSymbolInfo(const Decl *D) {
   assert(D);
   SymbolInfo Info;
@@ -318,16 +342,7 @@
   if (Info.Properties & (unsigned)SymbolProperty::Generic)
     Info.Lang = SymbolLanguage::CXX;
 
-  auto getExternalSymAttr = [](const Decl *D) -> ExternalSourceSymbolAttr* {
-    if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>())
-      return attr;
-    if (auto *dcd = dyn_cast<Decl>(D->getDeclContext())) {
-      if (auto *attr = dcd->getAttr<ExternalSourceSymbolAttr>())
-        return attr;
-    }
-    return nullptr;
-  };
-  if (auto *attr = getExternalSymAttr(D)) {
+  if (auto *attr = getExternalSourceSymbolAttr(D)) {
     if (attr->getLanguage() == "Swift")
       Info.Lang = SymbolLanguage::Swift;
   }
Index: lib/Index/USRGeneration.cpp
===================================================================
--- lib/Index/USRGeneration.cpp
+++ lib/Index/USRGeneration.cpp
@@ -11,6 +11,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclVisitor.h"
+#include "clang/Index/IndexSymbol.h"
 #include "clang/Lex/PreprocessingRecord.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
@@ -47,9 +48,7 @@
 }
 
 static StringRef GetExternalSourceContainer(const NamedDecl *D) {
-  if (!D)
-    return StringRef();
-  if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>()) {
+  if (auto *attr = getExternalSourceSymbolAttr(D)) {
     return attr->getDefinedIn();
   }
   return StringRef();
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