Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5387,6 +5387,9 @@
   "dereference of type %1 that was reinterpret_cast from type %0 has undefined "
   "behavior">,
   InGroup<UndefinedReinterpretCast>, DefaultIgnore;
+def warn_taking_address_of_packed_member : Warning<
+  "taking address of packed member %0 of class or structure %q1 may result in an unaligned pointer value">,
+  InGroup<DiagGroup<"address-of-packed-member">>;
 
 def err_objc_object_assignment : Error<
   "cannot assign to class object (%0 invalid)">;
Index: cfe/trunk/lib/Sema/SemaExpr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaExpr.cpp
+++ cfe/trunk/lib/Sema/SemaExpr.cpp
@@ -10514,6 +10514,30 @@
     return QualType();
   }
 
+  // Taking the address of a data member/field of a packed
+  // struct may be a problem if the pointer value is dereferenced.
+  Expr *rhs = OrigOp.get();
+  const auto *ME = dyn_cast<MemberExpr>(rhs);
+  while (ME && isa<FieldDecl>(ME->getMemberDecl())) {
+    QualType BaseType = ME->getBase()->getType();
+    if (ME->isArrow())
+      BaseType = BaseType->getPointeeType();
+    RecordDecl *RD = BaseType->getAs<RecordType>()->getDecl();
+
+    ValueDecl *MD = ME->getMemberDecl();
+    bool ByteAligned = Context.getTypeAlignInChars(MD->getType()).isOne();
+    if (ByteAligned) // Attribute packed does not have any effect.
+      break;
+
+    if (!ByteAligned &&
+        (RD->hasAttr<PackedAttr>() || (MD->hasAttr<PackedAttr>()))) {
+      Diag(OpLoc, diag::warn_taking_address_of_packed_member)
+          << MD << RD << rhs->getSourceRange();
+      break;
+    }
+    ME = dyn_cast<MemberExpr>(ME->getBase());
+  }
+
   return Context.getPointerType(op->getType());
 }
 
Index: cfe/trunk/test/Sema/address-packed.c
===================================================================
--- cfe/trunk/test/Sema/address-packed.c
+++ cfe/trunk/test/Sema/address-packed.c
@@ -0,0 +1,126 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+extern void f1(int *);
+extern void f2(char *);
+
+struct Ok {
+  char c;
+  int x;
+};
+
+struct __attribute__((packed)) Arguable {
+  char c0;
+  int x;
+  char c1;
+};
+
+union __attribute__((packed)) UnionArguable {
+  char c;
+  int x;
+};
+
+typedef struct Arguable ArguableT;
+
+struct Arguable *get_arguable();
+
+void g0(void) {
+  {
+    struct Ok ok;
+    f1(&ok.x); // no-warning
+    f2(&ok.c); // no-warning
+  }
+  {
+    struct Arguable arguable;
+    f2(&arguable.c0); // no-warning
+    f1(&arguable.x);  // expected-warning {{packed member 'x' of class or structure 'Arguable'}}
+    f2(&arguable.c1); // no-warning
+  }
+  {
+    union UnionArguable arguable;
+    f2(&arguable.c); // no-warning
+    f1(&arguable.x); // expected-warning {{packed member 'x' of class or structure 'UnionArguable'}}
+  }
+  {
+    ArguableT arguable;
+    f2(&arguable.c0); // no-warning
+    f1(&arguable.x);  // expected-warning {{packed member 'x' of class or structure 'Arguable'}}
+    f2(&arguable.c1); // no-warning
+  }
+  {
+    struct Arguable *arguable = get_arguable();
+    // These do not produce any warning because of the parentheses.
+    f2(&(arguable->c0)); // no-warning
+    f1(&(arguable->x));  // no-warning
+    f2(&(arguable->c1)); // no-warning
+  }
+  {
+    ArguableT *arguable = get_arguable();
+    // These do not produce any warning because of the parentheses.
+    f2(&(arguable->c0)); // no-warning
+    f1(&(arguable->x));  // no-warning
+    f2(&(arguable->c1)); // no-warning
+  }
+}
+
+struct S1 {
+  char c;
+  int i __attribute__((packed));
+};
+
+int *g1(struct S1 *s1) {
+  return &s1->i; // expected-warning {{packed member 'i' of class or structure 'S1'}}
+}
+
+struct S2_i {
+  int i;
+};
+struct __attribute__((packed)) S2 {
+  char c;
+  struct S2_i inner;
+};
+
+int *g2(struct S2 *s2) {
+  return &s2->inner.i; // expected-warning {{packed member 'inner' of class or structure 'S2'}}
+}
+
+struct S2_a {
+  char c;
+  struct S2_i inner __attribute__((packed));
+};
+
+int *g2_a(struct S2_a *s2_a) {
+  return &s2_a->inner.i; // expected-warning {{packed member 'inner' of class or structure 'S2_a'}}
+}
+
+struct __attribute__((packed)) S3 {
+  char c;
+  struct {
+    int i;
+  } inner;
+};
+
+int *g3(struct S3 *s3) {
+  return &s3->inner.i; // expected-warning {{packed member 'inner' of class or structure 'S3'}}
+}
+
+struct S4 {
+  char c;
+  struct __attribute__((packed)) {
+    int i;
+  } inner;
+};
+
+int *g4(struct S4 *s4) {
+  return &s4->inner.i; // expected-warning {{packed member 'i' of class or structure 'S4::(anonymous)'}}
+}
+
+struct S5 {
+  char c;
+  struct {
+    char c1;
+    int i __attribute__((packed));
+  } inner;
+};
+
+int *g5(struct S5 *s5) {
+  return &s5->inner.i; // expected-warning {{packed member 'i' of class or structure 'S5::(anonymous)'}}
+}
Index: cfe/trunk/test/SemaCXX/address-packed.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/address-packed.cpp
+++ cfe/trunk/test/SemaCXX/address-packed.cpp
@@ -0,0 +1,100 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+extern void f1(int *);
+extern void f2(char *);
+
+struct __attribute__((packed)) Arguable {
+  int x;
+  char c;
+  static void foo();
+};
+
+extern void f3(void());
+
+namespace Foo {
+struct __attribute__((packed)) Arguable {
+  char c;
+  int x;
+  static void foo();
+};
+}
+
+struct Arguable *get_arguable();
+
+void g0() {
+  {
+    Foo::Arguable arguable;
+    f1(&arguable.x);   // expected-warning {{packed member 'x' of class or structure 'Foo::Arguable'}}
+    f2(&arguable.c);   // no-warning
+    f3(&arguable.foo); // no-warning
+  }
+  {
+    Arguable arguable1;
+    Arguable &arguable(arguable1);
+    f1(&arguable.x);   // expected-warning {{packed member 'x' of class or structure 'Arguable'}}
+    f2(&arguable.c);   // no-warning
+    f3(&arguable.foo); // no-warning
+  }
+  {
+    Arguable *arguable1;
+    Arguable *&arguable(arguable1);
+    f1(&arguable->x);   // expected-warning {{packed member 'x' of class or structure 'Arguable'}}
+    f2(&arguable->c);   // no-warning
+    f3(&arguable->foo); // no-warning
+  }
+}
+
+struct __attribute__((packed)) A {
+  int x;
+  char c;
+
+  int *f0() {
+    return &this->x; // expected-warning {{packed member 'x' of class or structure 'A'}}
+  }
+
+  int *g0() {
+    return &x; // expected-warning {{packed member 'x' of class or structure 'A'}}
+  }
+
+  char *h0() {
+    return &c; // no-warning
+  }
+};
+
+struct B : A {
+  int *f1() {
+    return &this->x; // expected-warning {{packed member 'x' of class or structure 'A'}}
+  }
+
+  int *g1() {
+    return &x; // expected-warning {{packed member 'x' of class or structure 'A'}}
+  }
+
+  char *h1() {
+    return &c; // no-warning
+  }
+};
+
+template <typename Ty>
+class __attribute__((packed)) S {
+  Ty X;
+
+public:
+  const Ty *get() const {
+    return &X; // expected-warning {{packed member 'X' of class or structure 'S<int>'}}
+               // expected-warning@-1 {{packed member 'X' of class or structure 'S<float>'}}
+  }
+};
+
+template <typename Ty>
+void h(Ty *);
+
+void g1() {
+  S<int> s1;
+  s1.get(); // expected-note {{in instantiation of member function 'S<int>::get'}}
+
+  S<char> s2;
+  s2.get();
+
+  S<float> s3;
+  s3.get(); // expected-note {{in instantiation of member function 'S<float>::get'}}
+}