Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -38,7 +38,9 @@
 def StringConversion : DiagGroup<"string-conversion">;
 def SignConversion : DiagGroup<"sign-conversion">;
 def PointerBoolConversion : DiagGroup<"pointer-bool-conversion">;
-def BoolConversion : DiagGroup<"bool-conversion", [ PointerBoolConversion ] >;
+def UndefinedBoolConversion : DiagGroup<"undefined-bool-conversion">;
+def BoolConversion : DiagGroup<"bool-conversion", [PointerBoolConversion,
+                                                   UndefinedBoolConversion]>;
 def IntConversion : DiagGroup<"int-conversion">;
 def EnumConversion : DiagGroup<"enum-conversion">;
 def FloatConversion : DiagGroup<"float-conversion">;
@@ -298,10 +300,12 @@
 def TautologicalOutOfRangeCompare : DiagGroup<"tautological-constant-out-of-range-compare">;
 def TautologicalPointerCompare : DiagGroup<"tautological-pointer-compare">;
 def TautologicalOverlapCompare : DiagGroup<"tautological-overlap-compare">;
+def TautologicalUndefinedCompare : DiagGroup<"tautological-undefined-compare">;
 def TautologicalCompare : DiagGroup<"tautological-compare",
                                     [TautologicalOutOfRangeCompare,
                                      TautologicalPointerCompare,
-                                     TautologicalOverlapCompare]>;
+                                     TautologicalOverlapCompare,
+                                     TautologicalUndefinedCompare]>;
 def HeaderHygiene : DiagGroup<"header-hygiene">;
 def DuplicateDeclSpecifier : DiagGroup<"duplicate-decl-specifier">;
 def CompareDistinctPointerType : DiagGroup<"compare-distinct-pointer-types">;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2388,10 +2388,29 @@
     "address of%select{| function| array}0 '%1' will always evaluate to "
     "'true'">,
     InGroup<PointerBoolConversion>;
+def warn_this_bool_conversion : Warning<
+  "this pointer can only be false in undefined contexts, which may be "
+  "removed during optimization">,
+  InGroup<UndefinedBoolConversion>, DefaultIgnore;
+def warn_address_of_reference_bool_conversion : Warning<
+  "pointer to reference can only be false in undefined contexts, which may be "
+  "removed during optimization">,
+  InGroup<UndefinedBoolConversion>, DefaultIgnore;
+
 def warn_null_pointer_compare : Warning<
     "comparison of %select{address of|function|array}0 '%1' %select{not |}2"
     "equal to a null pointer is always %select{true|false}2">,
     InGroup<TautologicalPointerCompare>;
+def warn_this_null_compare : Warning<
+  "this pointer %select{not |}0equal to null pointer can only be "
+  "%select{false|true}0 in undefined contexts, which may be removed during "
+  "optimization">,
+  InGroup<TautologicalUndefinedCompare>, DefaultIgnore;
+def warn_address_of_reference_null_compare : Warning<
+  "pointer to reference %select{not |}0equal to null pointer can only be "
+  "%select{false|true}0 in undefined contexts, which may be removed during "
+  "optimization">,
+  InGroup<TautologicalUndefinedCompare>, DefaultIgnore;
 
 def note_function_warning_silence : Note<
     "prefix with the address-of operator to silence this warning">;
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -6168,6 +6168,13 @@
 
   const bool IsCompare = NullKind != Expr::NPCK_NotNull;
 
+  if (isa<CXXThisExpr>(E)) {
+    unsigned DiagID = IsCompare ? diag::warn_this_null_compare
+                                : diag::warn_this_bool_conversion;
+    Diag(E->getExprLoc(), DiagID) << E->getSourceRange() << Range << IsEqual;
+    return;
+  }
+
   bool IsAddressOf = false;
 
   if (UnaryOperator *UO = dyn_cast<UnaryOperator>(E)) {
@@ -6197,9 +6204,14 @@
     // Address of function is used to silence the function warning.
     if (IsFunction)
       return;
-    // Address of reference can be null.
-    if (T->isReferenceType())
+
+    if (T->isReferenceType()) {
+      unsigned DiagID = IsCompare
+                            ? diag::warn_address_of_reference_null_compare
+                            : diag::warn_address_of_reference_bool_conversion;
+      Diag(E->getExprLoc(), DiagID) << E->getSourceRange() << Range << IsEqual;
       return;
+    }
   }
 
   // Found nothing.
Index: test/SemaCXX/warn-tautological-undefined-compare.cpp
===================================================================
--- test/SemaCXX/warn-tautological-undefined-compare.cpp
+++ test/SemaCXX/warn-tautological-undefined-compare.cpp
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wtautological-undefined-compare %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wtautological-compare %s
+
+void test1(int &x) {
+  if (x == 1) { }
+  if (&x == 0) { }
+  // expected-warning@-1{{pointer to reference equal to null pointer can only be true in undefined contexts, which may be removed during optimization}}
+  if (&x != 0) { }
+  // expected-warning@-1{{pointer to reference not equal to null pointer can only be false in undefined contexts, which may be removed during optimization}}
+}
+
+class test2 {
+  test2() : x(y) {}
+
+  void foo() {
+    if (this == 0) { }
+    // expected-warning@-1{{this pointer equal to null pointer can only be true in undefined contexts, which may be removed during optimization}}
+    if (this != 0) { }
+    // expected-warning@-1{{this pointer not equal to null pointer can only be false in undefined contexts, which may be removed during optimization}}
+  }
+
+  void bar() {
+    if (x == 1) { }
+    if (&x == 0) { }
+    // expected-warning@-1{{pointer to reference equal to null pointer can only be true in undefined contexts, which may be removed during optimization}}
+    if (&x != 0) { }
+    // expected-warning@-1{{pointer to reference not equal to null pointer can only be false in undefined contexts, which may be removed during optimization}}
+  }
+
+  int &x;
+  int y;
+};
Index: test/SemaCXX/warn-undefined-bool-conversion.cpp
===================================================================
--- test/SemaCXX/warn-undefined-bool-conversion.cpp
+++ test/SemaCXX/warn-undefined-bool-conversion.cpp
@@ -0,0 +1,35 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wundefined-bool-conversion %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wbool-conversion %s
+
+void test1(int &x) {
+  if (x == 1) { }
+  if (&x) { }
+  // expected-warning@-1{{pointer to reference can only be false in undefined contexts, which may be removed during optimization}}
+
+  if (!&x) { }
+  // expected-warning@-1{{pointer to reference can only be false in undefined contexts, which may be removed during optimization}}
+}
+
+class test2 {
+  test2() : x(y) {}
+
+  void foo() {
+    if (this) { }
+    // expected-warning@-1{{this pointer can only be false in undefined contexts, which may be removed during optimization}}
+
+    if (!this) { }
+    // expected-warning@-1{{this pointer can only be false in undefined contexts, which may be removed during optimization}}
+  }
+
+  void bar() {
+    if (x == 1) { }
+    if (&x) { }
+    // expected-warning@-1{{pointer to reference can only be false in undefined contexts, which may be removed during optimization}}
+
+    if (!&x) { }
+    // expected-warning@-1{{pointer to reference can only be false in undefined contexts, which may be removed during optimization}}
+  }
+
+  int &x;
+  int y;
+};