Index: lib/Index/IndexSymbol.cpp
===================================================================
--- lib/Index/IndexSymbol.cpp
+++ lib/Index/IndexSymbol.cpp
@@ -319,8 +319,10 @@
     Info.Lang = SymbolLanguage::CXX;
 
   auto getExternalSymAttr = [](const Decl *D) -> ExternalSourceSymbolAttr* {
-    if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>())
-      return attr;
+    for (auto *Redecl: D->redecls()) {
+      if (auto *attr = Redecl->getAttr<ExternalSourceSymbolAttr>())
+        return attr;
+    }
     if (auto *dcd = dyn_cast<Decl>(D->getDeclContext())) {
       if (auto *attr = dcd->getAttr<ExternalSourceSymbolAttr>())
         return attr;
Index: lib/Index/USRGeneration.cpp
===================================================================
--- lib/Index/USRGeneration.cpp
+++ lib/Index/USRGeneration.cpp
@@ -49,8 +49,10 @@
 static StringRef GetExternalSourceContainer(const NamedDecl *D) {
   if (!D)
     return StringRef();
-  if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>()) {
-    return attr->getDefinedIn();
+  for (auto *Redecl: D->redecls()) {
+    if (auto *attr = Redecl->getAttr<ExternalSourceSymbolAttr>()) {
+      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