Index: clang/lib/AST/Interp/Interp.h
===================================================================
--- clang/lib/AST/Interp/Interp.h
+++ clang/lib/AST/Interp/Interp.h
@@ -671,7 +671,9 @@
 
   if (!Pointer::hasSameBase(LHS, RHS)) {
     const SourceInfo &Loc = S.Current->getSource(OpPC);
-    S.FFDiag(Loc, diag::note_invalid_subexpr_in_const_expr);
+    S.FFDiag(Loc, diag::note_constexpr_pointer_comparison_unspecified)
+        << LHS.toDiagnosticString(S.getCtx())
+        << RHS.toDiagnosticString(S.getCtx());
     return false;
   } else {
     unsigned VL = LHS.getByteOffset();
Index: clang/lib/AST/Interp/Pointer.h
===================================================================
--- clang/lib/AST/Interp/Pointer.h
+++ clang/lib/AST/Interp/Pointer.h
@@ -79,6 +79,9 @@
   /// Converts the pointer to an APValue.
   APValue toAPValue() const;
 
+  /// Converts the pointer to a string usable in diagnostics.
+  std::string toDiagnosticString(const ASTContext &Ctx) const;
+
   /// Converts the pointer to an APValue that is an rvalue.
   APValue toRValue(const Context &Ctx) const;
 
Index: clang/lib/AST/Interp/Pointer.cpp
===================================================================
--- clang/lib/AST/Interp/Pointer.cpp
+++ clang/lib/AST/Interp/Pointer.cpp
@@ -150,6 +150,13 @@
   return APValue(Base, Offset, Path, IsOnePastEnd, IsNullPtr);
 }
 
+std::string Pointer::toDiagnosticString(const ASTContext &Ctx) const {
+  if (!Pointee)
+    return "nullptr";
+
+  return toAPValue().getAsString(Ctx, getType());
+}
+
 bool Pointer::isInitialized() const {
   assert(Pointee && "Cannot check if null pointer was initialized");
   Descriptor *Desc = getFieldDesc();
Index: clang/test/AST/Interp/literals.cpp
===================================================================
--- clang/test/AST/Interp/literals.cpp
+++ clang/test/AST/Interp/literals.cpp
@@ -125,6 +125,35 @@
   static_assert(!!FP, "");
 }
 
+namespace PointerComparison {
+
+  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 < (int*)0;
+  constexpr bool v2 = null < pv; // expected-error {{must be initialized by a constant expression}} \
+                                 // expected-note {{comparison between 'nullptr' and '&s.a' has unspecified value}} \
+                                 // ref-error {{must be initialized by a constant expression}} \
+                                 // ref-note {{comparison between 'nullptr' and '&s.a' has unspecified value}} \
+
+  constexpr bool v3 = null == pv; // ok
+  constexpr bool v4 = qv == pv; // ok
+
+  /// FIXME: These two are rejected by the current interpreter, but
+  ///   accepted by GCC.
+  constexpr bool v5 = qv >= pv; // ref-error {{constant expression}} \
+                                // ref-note {{unequal pointers to void}}
+  constexpr bool v8 = qv > (void*)&s.a; // ref-error {{constant expression}} \
+                                        // ref-note {{unequal pointers to void}}
+  constexpr bool v6 = qv > null; // expected-error {{must be initialized by a constant expression}} \
+                                 // expected-note {{comparison between '&s.b' and 'nullptr' has unspecified value}} \
+                                 // ref-error {{must be initialized by a constant expression}} \
+                                 // ref-note {{comparison between '&s.b' and 'nullptr' has unspecified value}}
+
+  constexpr bool v7 = qv <= (void*)&s.b; // ok
+}
+
 namespace SizeOf {
   constexpr int soint = sizeof(int);
   constexpr int souint = sizeof(unsigned int);