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
@@ -6446,6 +6446,12 @@
   "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<
+  "pointer comparisons before C11 "
+  "need to be between two complete or two incomplete types; "
+  "%0 is %select{|in}2complete and "
+  "%1 is %select{|in}3complete">,
+  InGroup<C11>;
 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
@@ -11584,11 +11584,22 @@
     // 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()) &&
+            !getLangOpts().C11) {
+          Diag(Loc, diag::ext_typecheck_compare_complete_incomplete_pointers)
+              << LHS.get()->getSourceRange() << RHS.get()->getSourceRange()
+              << LHSType << RHSType << LCanPointeeTy->isIncompleteType()
+              << RCanPointeeTy->isIncompleteType();
+        }
+        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/complete-incomplete-pointer-relational-c99.c b/clang/test/Sema/complete-incomplete-pointer-relational-c99.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/complete-incomplete-pointer-relational-c99.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c99 -Wc11-extensions %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c89 -Wc11-extensions %s
+
+int incomplete[]; // expected-warning {{tentative array definition assumed to have one element}}
+int complete[6];
+
+int test_comparison_between_incomplete_and_complete_pointer() {
+  return (&incomplete < &complete) &&  // expected-warning {{pointer comparisons before C11 need to be between two complete or two incomplete types; 'int (*)[]' is incomplete and 'int (*)[6]' is complete}}
+         (&incomplete <= &complete) && // expected-warning {{pointer comparisons before C11 need to be between two complete or two incomplete types; 'int (*)[]' is incomplete and 'int (*)[6]' is complete}}
+         (&incomplete > &complete) &&  // expected-warning {{pointer comparisons before C11 need to be between two complete or two incomplete types; 'int (*)[]' is incomplete and 'int (*)[6]' is complete}}
+         (&incomplete >= &complete) && // expected-warning {{pointer comparisons before C11 need to be between two complete or two incomplete types; 'int (*)[]' is incomplete and 'int (*)[6]' is complete}}
+         (&incomplete == &complete) &&
+         (&incomplete != &complete);
+}