diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -8707,6 +8707,12 @@
     if (entity.getKind() == InitializedEntity::EK_Result)
       S.EmitRelatedResultTypeNoteForReturn(destType);
   }
+  QualType fromType = op->getType();
+  auto *fromDecl = fromType.getTypePtr()->getPointeeCXXRecordDecl();
+  if (fromDecl && !fromDecl->hasDefinition() && !fromDecl->isInvalidDecl() &&
+      fromDecl->getDeclKind() == Decl::CXXRecord)
+    S.Diag(fromDecl->getLocation(), diag::note_forward_declaration)
+        << S.getASTContext().getTagDeclType(fromDecl);
 }
 
 static void diagnoseListInit(Sema &S, const InitializedEntity &Entity,
diff --git a/clang/test/Modules/namespaces.cpp b/clang/test/Modules/namespaces.cpp
--- a/clang/test/Modules/namespaces.cpp
+++ b/clang/test/Modules/namespaces.cpp
@@ -78,7 +78,8 @@
 
 // expected-note@Inputs/namespaces-right.h:60 {{passing argument to parameter here}}
 // expected-note@Inputs/namespaces-right.h:67 {{passing argument to parameter here}}
-
+// expected-note@Inputs/namespaces-left.h:63 {{forward declaration of 'N11::(anonymous namespace)::Foo'}}
+// expected-note@Inputs/namespaces-left.h:70 {{forward declaration of 'N12::(anonymous namespace)::Foo'}}
 // Test that bringing in one name from an overload set does not hide the rest.
 void testPartialImportOfOverloadSet() {
   void (*p)() = N13::p;
diff --git a/clang/test/SemaCXX/elaborated-type-specifier.cpp b/clang/test/SemaCXX/elaborated-type-specifier.cpp
--- a/clang/test/SemaCXX/elaborated-type-specifier.cpp
+++ b/clang/test/SemaCXX/elaborated-type-specifier.cpp
@@ -26,7 +26,7 @@
 }
 
 void test_X_elab(NS::X x) {
-  struct S4 *s4 = 0;
+  struct S4 *s4 = 0; // expected-note{{forward declaration of 'S4'}}
   x.test_elab2(s4); // expected-error{{cannot initialize a parameter of type 'NS::S4 *' with an lvalue of type 'struct S4 *'}}
 }