diff --git a/include/clang/Basic/DiagnosticASTKinds.td b/include/clang/Basic/DiagnosticASTKinds.td
index 9fe22ad..1cdbcb9 100644
--- a/include/clang/Basic/DiagnosticASTKinds.td
+++ b/include/clang/Basic/DiagnosticASTKinds.td
@@ -53,6 +53,8 @@
 def note_constexpr_typeid_polymorphic : Note<
   "typeid applied to expression of polymorphic type %0 is "
   "not allowed in a constant expression">;
+def note_constexpr_void_comparison : Note<
+  "comparison between unequal pointers to void has unspecified result">;
 def note_constexpr_temporary_here : Note<"temporary created here">;
 def note_constexpr_literal_here : Note<"literal written here">;
 def note_constexpr_depth_limit_exceeded : Note<
diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index c038e3f..5a385e6 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -4331,6 +4331,18 @@
 
       const CharUnits &LHSOffset = LHSValue.getLValueOffset();
       const CharUnits &RHSOffset = RHSValue.getLValueOffset();
+
+      // C++11 [expr.rel]p3:
+      //   Pointers to void (after pointer conversions) can be compared, with a
+      //   result defined as follows: If both pointers represent the same
+      //   address or are both the null pointer value, the result is true if the
+      //   operator is <= or >= and false otherwise; otherwise the result is
+      //   unspecified.
+      // We interpret this as applying to pointers to *cv* void.
+      if (LHSTy->isVoidPointerType() && LHSOffset != RHSOffset &&
+          E->getOpcode() != BO_EQ && E->getOpcode() != BO_NE)
+        CCEDiag(E, diag::note_constexpr_void_comparison);
+
       switch (E->getOpcode()) {
       default: llvm_unreachable("missing comparison operator");
       case BO_LT: return Success(LHSOffset < RHSOffset, E);
diff --git a/test/CXX/expr/expr.const/p2-0x.cpp b/test/CXX/expr/expr.const/p2-0x.cpp
index a39de4d..617edc5 100644
--- a/test/CXX/expr/expr.const/p2-0x.cpp
+++ b/test/CXX/expr/expr.const/p2-0x.cpp
@@ -376,10 +376,21 @@
   // If two pointers point to non-static data members of the same object with
   // different access control, the result is unspecified.
 
-  // FIXME:
   // [expr.rel]p3: Pointers to void can be compared [...] if both pointers
   // represent the same address or are both the null pointer [...]; otherwise
   // the result is unspecified.
+  struct S { int a, b; } s;
+  constexpr void *null = 0;
+  constexpr void *pv = (void*)&s.a;
+  constexpr void *qv = (void*)&s.b;
+  constexpr bool v1 = null < 0;
+  constexpr bool v2 = null < pv; // expected-error {{constant expression}}
+  constexpr bool v3 = null == pv; // ok
+  constexpr bool v4 = qv == pv; // ok
+  constexpr bool v5 = qv >= pv; // expected-error {{constant expression}} expected-note {{unequal pointers to void}}
+  constexpr bool v6 = qv > null; // expected-error {{constant expression}}
+  constexpr bool v7 = qv <= (void*)&s.b; // ok
+  constexpr bool v8 = qv > (void*)&s.a; // expected-error {{constant expression}} expected-note {{unequal pointers to void}}
 
   // FIXME: Implement comparisons of pointers to members.
   // [expr.eq]p2: If either is a pointer to a virtual member function and