Index: lib/Sema/SemaInit.cpp
===================================================================
--- lib/Sema/SemaInit.cpp
+++ lib/Sema/SemaInit.cpp
@@ -1632,8 +1632,26 @@
           CheckEmptyInitializable(
               InitializedEntity::InitializeMember(*Field, &Entity),
               IList->getLocEnd());
-        else
+        else {
+          FieldDecl *CurrentField =
+            StructuredList->getInitializedFieldInUnion();
+          if (CurrentField && *Field != CurrentField) {
+            assert(StructuredList->getNumInits() == 1
+                   && "A union should never have more than one initializer!");
+
+            // FIXME: emit warning because we're overriding the initializer.
+            Expr *ExistingInit = StructuredList->getInit(0);
+            SemaRef.Diag(ExistingInit->getLocStart(),
+                         diag::note_previous_initializer)
+              << /*FIXME:has side effects=*/0
+              << ExistingInit->getSourceRange();
+
+            // remove existing initializer
+            StructuredList->resizeInits(SemaRef.Context, 0);
+            StructuredList->setInitializedFieldInUnion(nullptr);
+          }
           StructuredList->setInitializedFieldInUnion(*Field);
+        }
         break;
       }
     }