[Modules] Fix the inconsistency of which Decl should be serialized for an identifier.

clang/lib/Serialization/ASTWriter.cpp

@@ -5274,9 +5274,10 @@ void ASTWriter::PrepareWritingSpecialDecls(Sema &SemaRef) {
     }
     // Sort the identifiers to visit based on their name.
     llvm::sort(IIs, llvm::deref<std::less<>>());
+    const LangOptions &LangOpts = getLangOpts();
     for (const IdentifierInfo *II : IIs)
-      for (const Decl *D : SemaRef.IdResolver.decls(II))
-        GetDeclRef(D);
+      for (NamedDecl *D : SemaRef.IdResolver.decls(II))
+        GetDeclRef(getDeclForLocalLookup(LangOpts, D));
   }
 
   // Write all of the DeclsToCheckForDeferredDiags.

clang/test/Modules/non-modular-decl-use.c

@@ -0,0 +1,100 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -fsyntax-only -I %t/include %t/test.c \
+// RUN:            -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache
+
+// Test when a decl is present in multiple modules through an inclusion of
+// a non-modular header. Make sure such decl is serialized correctly and can be
+// used after deserialization.
+
+//--- include/non-modular.h
+#ifndef NON_MODULAR_H
+#define NON_MODULAR_H
+
+union TestUnion {
+  int x;
+  float y;
+};
+
+struct ReferenceUnion1 {
+  union TestUnion name;
+  unsigned versionMajor;
+};
+struct ReferenceUnion2 {
+  union TestUnion name;
+  unsigned versionMinor;
+};
+
+// Test another kind of RecordDecl.
+struct TestStruct {
+  int p;
+  float q;
+};
+
+struct ReferenceStruct1 {
+  unsigned fieldA;
+  struct TestStruct fieldB;
+};
+
+struct ReferenceStruct2 {
+  unsigned field1;
+  struct TestStruct field2;
+};
+
+#endif
+
+//--- include/piecewise1-empty.h
+//--- include/piecewise1-initially-hidden.h
+#include <non-modular.h>
+
+//--- include/piecewise2-empty.h
+//--- include/piecewise2-initially-hidden.h
+#include <non-modular.h>
+
+//--- include/with-multiple-decls.h
+#include <piecewise1-empty.h>
+// Include the non-modular header and resolve a name duplication between decl
+// in non-modular.h and in piecewise1-initially-hidden.h, create a
+// redeclaration chain.
+#include <non-modular.h>
+// Include a decl with a duplicate name again to add more to IdentifierResolver.
+#include <piecewise2-empty.h>
+
+//--- include/module.modulemap
+module Piecewise1 {
+  module Empty {
+    header "piecewise1-empty.h"
+  }
+  module InitiallyHidden {
+    header "piecewise1-initially-hidden.h"
+    export *
+  }
+}
+
+module Piecewise2 {
+  module Empty {
+    header "piecewise2-empty.h"
+  }
+  module InitiallyHidden {
+    header "piecewise2-initially-hidden.h"
+    export *
+  }
+}
+
+module WithMultipleDecls {
+  header "with-multiple-decls.h"
+  export *
+}
+
+//--- test.c
+#include <with-multiple-decls.h>
+
+struct Test {
+  int x;
+  union TestUnion name;
+};
+
+struct Test2 {
+  struct TestStruct name;
+  float y;
+};