diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9898,4 +9898,7 @@
   "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
 def err_bit_cast_type_size_mismatch : Error<
   "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
+
+def warn_union_zero_init : Warning<
+  "zero-initializing union '%0' whose first non-static named data member '%1' is not the largest in the union will leave uninitialized bits in the rest of the union">;
 } // end of sema component.
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
@@ -2041,8 +2041,16 @@
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
-        if (StructuredList)
+        if (StructuredList) {
           StructuredList->setInitializedFieldInUnion(*Field);
+
+          ASTContext &Ctx = SemaRef.Context;
+          unsigned UnionSize = Ctx.getTypeSize(Ctx.getRecordType(RD));
+          unsigned FieldSize = Ctx.getTypeSize(Field->getType());
+          if (UnionSize > FieldSize)
+            SemaRef.Diag(IList->getLBraceLoc(), diag::warn_union_zero_init)
+              << RD->getName() << Field->getName();
+        }
         break;
       }
     }