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
@@ -6441,6 +6441,12 @@
   "ordered comparison between pointer and zero (%0 and %1)">;
 def err_typecheck_three_way_comparison_of_pointer_and_zero : Error<
   "three-way comparison between pointer and zero">;
+def ext_typecheck_compare_complete_incomplete_pointers : Extension<
+  "ordered comparison of complete and incomplete pointers (%0 and %1)">,
+  InGroup<C99>;
+def warn_typecheck_compare_complete_incomplete_pointers : ExtWarn<
+  "ordered comparison of complete and incomplete pointers (%0 and %1)">,
+  InGroup<C99>;
 def ext_typecheck_ordered_comparison_of_function_pointers : ExtWarn<
   "ordered comparison of function pointers (%0 and %1)">,
   InGroup<DiagGroup<"ordered-compare-function-pointers">>;
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -11422,11 +11422,23 @@
     // C99 6.5.9p2 and C99 6.5.8p2
     if (Context.typesAreCompatible(LCanPointeeTy.getUnqualifiedType(),
                                    RCanPointeeTy.getUnqualifiedType())) {
-      // Valid unless a relational comparison of function pointers
-      if (IsRelational && LCanPointeeTy->isFunctionType()) {
-        Diag(Loc, diag::ext_typecheck_ordered_comparison_of_function_pointers)
-          << LHSType << RHSType << LHS.get()->getSourceRange()
-          << RHS.get()->getSourceRange();
+      if (IsRelational) {
+        // Pointers both need to point to complete or incomplete types
+        if (LCanPointeeTy->isIncompleteType() !=
+            RCanPointeeTy->isIncompleteType()) {
+          Diag(Loc,
+               getLangOpts().C11
+                   ? diag::ext_typecheck_compare_complete_incomplete_pointers
+                   : diag::warn_typecheck_compare_complete_incomplete_pointers)
+              << LHSType << RHSType << LHS.get()->getSourceRange()
+              << RHS.get()->getSourceRange();
+        }
+        if (LCanPointeeTy->isFunctionType()) {
+          // Valid unless a relational comparison of function pointers
+          Diag(Loc, diag::ext_typecheck_ordered_comparison_of_function_pointers)
+              << LHSType << RHSType << LHS.get()->getSourceRange()
+              << RHS.get()->getSourceRange();
+        }
       }
     } else if (!IsRelational &&
                (LCanPointeeTy->isVoidType() || RCanPointeeTy->isVoidType())) {
diff --git a/clang/test/Sema/c89.c b/clang/test/Sema/c89.c
--- a/clang/test/Sema/c89.c
+++ b/clang/test/Sema/c89.c
@@ -127,3 +127,11 @@
   struct Test17 t1 = test17_aux(); /* this is allowed */
 }
 
+int incomplete[]; /* expected-warning {{tentative array definition assumed to have one element}} */
+int complete[5];
+
+void test18() {
+  if (&incomplete < &complete) { /* expected-warning {{ordered comparison of complete and incomplete pointers}} */
+    return;
+  }
+}