diff --git a/clang/docs/LanguageExtensions.html b/clang/docs/LanguageExtensions.html
index 48d5aa64a1d0..8faa555f5cee 100644
--- a/clang/docs/LanguageExtensions.html
+++ b/clang/docs/LanguageExtensions.html
@@ -23,6 +23,7 @@ td {
 <li><a href="#has_include">Include File Checking Macros</a></li>
 <li><a href="#builtinmacros">Builtin Macros</a></li>
 <li><a href="#vectors">Vectors and Extended Vectors</a></li>
+<li><a href="#deprecated">Deprecated and Unavailable attribute with Message</a></li>
 <li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
   <ul>
   <li><a href="#cxx_exceptions">C++ exceptions</a></li>
@@ -266,6 +267,14 @@ float4 foo(float2 a, float2 b) {
 
 <p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
 
+<!-- ======================================================================= -->
+<h2 id="deprecated">Deprecated and Unavailable attribute with Message</h2>
+<!-- ======================================================================= -->
+
+<p> Optional string message can be added to Deprecated and Available attributes. </p>
+
+<p> Message will be added to deprecated warning or unavailable error if present. </p>
+
 <!-- ======================================================================= -->
 <h2 id="checking_language_features">Checks for Standard Language Features</h2>
 <!-- ======================================================================= -->
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index fbbaec27661a..506f76f004d9 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -374,6 +374,7 @@ def TransparentUnion : Attr {
 
 def Unavailable : Attr {
   let Spellings = ["unavailable"];
+  let Args = [StringArgument<"Message">];
 }
 
 def Unused : Attr {
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e43c83768035..2d2ed6376278 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1748,6 +1748,7 @@ def warn_deprecated : Warning<"%0 is deprecated">,
 def warn_deprecated_message : Warning<"%0 is deprecated: %1">,
     InGroup<DeprecatedDeclarations>;
 def err_unavailable : Error<"%0 is unavailable">;
+def err_unavailable_message : Error<"%0 is unavailable: %1">;
 def note_unavailable_here : Note<
   "function has been explicitly marked %select{unavailable|deleted}0 here">;
 def warn_not_enough_argument : Warning<
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 3e1ced29a8d0..e9ee50fe75c0 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -929,12 +929,26 @@ static void HandleDeprecatedAttr(Decl *d, const AttributeList &Attr, Sema &S) {
 
 static void HandleUnavailableAttr(Decl *d, const AttributeList &Attr, Sema &S) {
   // check the attribute arguments.
-  if (Attr.getNumArgs() != 0) {
-    S.Diag(Attr.getLoc(), diag::err_attribute_wrong_number_arguments) << 0;
+  int noArgs = Attr.getNumArgs();
+  if (noArgs > 1) {
+    S.Diag(Attr.getLoc(), diag::err_attribute_wrong_number_arguments) << "0 or 1";
     return;
   }
-
-  d->addAttr(::new (S.Context) UnavailableAttr(Attr.getLoc(), S.Context));
+  // Handle the case where unavailable attribute has a text message.
+  StringLiteral *SE;
+  if (noArgs == 1) {
+    Expr *ArgExpr = static_cast<Expr *>(Attr.getArg(0));
+    SE = dyn_cast<StringLiteral>(ArgExpr);
+    if (!SE) {
+      S.Diag(ArgExpr->getLocStart(), 
+             diag::err_attribute_not_string) << "unavailable";
+      return;
+    }
+  }
+  else
+    SE = StringLiteral::CreateEmpty(S.Context, 1);
+  d->addAttr(::new (S.Context) UnavailableAttr(Attr.getLoc(), S.Context,
+                                               SE->getString()));
 }
 
 static void HandleVisibilityAttr(Decl *d, const AttributeList &Attr, Sema &S) {
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index fa1ce7a21c57..f6dc441bff84 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -64,8 +64,12 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, SourceLocation Loc) {
   }
 
   // See if the decl is unavailable
-  if (D->getAttr<UnavailableAttr>()) {
-    Diag(Loc, diag::err_unavailable) << D->getDeclName();
+  if (const UnavailableAttr *UA = D->getAttr<UnavailableAttr>()) {
+    if (UA->getMessage().empty())
+      Diag(Loc, diag::err_unavailable) << D->getDeclName();
+    else
+      Diag(Loc, diag::err_unavailable_message) 
+        << D->getDeclName() << UA->getMessage().data();
     Diag(D->getLocation(), diag::note_unavailable_here) << 0;
   }
 
diff --git a/clang/test/Sema/attr-unavailable-message.c b/clang/test/Sema/attr-unavailable-message.c
new file mode 100644
index 000000000000..4d07a238a9c4
--- /dev/null
+++ b/clang/test/Sema/attr-unavailable-message.c
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// rdar: //6734520
+
+int foo(int)  __attribute__((__unavailable__("USE IFOO INSTEAD"))); // expected-note {{function has been explicitly marked unavailable here}}
+double dfoo(double)  __attribute__((__unavailable__("NO LONGER"))); // expected-note 2 {{function has been explicitly marked unavailable here}}
+
+void bar() __attribute__((__unavailable__)); // expected-note {{explicitly marked unavailable}}
+
+void test_foo() {
+  int ir = foo(1); // expected-error {{'foo' is unavailable: USE IFOO INSTEAD}}
+  double dr = dfoo(1.0); // expected-error {{'dfoo' is unavailable: NO LONGER}}
+
+  void (*fp)() = &bar; // expected-error {{'bar' is unavailable}}
+
+  double (*fp4)(double) = dfoo;  // expected-error {{'dfoo' is unavailable: NO LONGER}}
+}