Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2886,6 +2886,10 @@
 def warn_partial_fwdclass_message : Warning<
     "%0 may be partial because the receiver type is unknown">,
     InGroup<UnguardedAvailability>, DefaultIgnore;
+def warn_at_available_unchecked_use : Warning<
+  "%select{@available|__builtin_available}0 does not guard availability here; "
+  "use if (%select{@available|__builtin_available}0) instead">,
+  InGroup<DiagGroup<"unsupported-availability-guard">>;
 
 // Thread Safety Attributes
 def warn_invalid_capability_name : Warning<
Index: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclAttr.cpp
+++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp
@@ -7284,6 +7284,12 @@
     return true;
   }
 
+  bool VisitObjCAvailabilityCheckExpr(ObjCAvailabilityCheckExpr *E) {
+    SemaRef.Diag(E->getLocStart(), diag::warn_at_available_unchecked_use)
+        << (!SemaRef.getLangOpts().ObjC1);
+    return true;
+  }
+
   bool VisitTypeLoc(TypeLoc Ty);
 };
 
Index: cfe/trunk/lib/Sema/SemaExpr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaExpr.cpp
+++ cfe/trunk/lib/Sema/SemaExpr.cpp
@@ -15762,6 +15762,13 @@
   if (Spec != AvailSpecs.end())
     Version = Spec->getVersion();
 
+  // The use of `@available` in the enclosing function should be analyzed to
+  // warn when it's used inappropriately (i.e. not if(@available)).
+  if (getCurFunctionOrMethodDecl())
+    getEnclosingFunction()->HasPotentialAvailabilityViolations = true;
+  else if (getCurBlock() || getCurLambda())
+    getCurFunction()->HasPotentialAvailabilityViolations = true;
+
   return new (Context)
       ObjCAvailabilityCheckExpr(Version, AtLoc, RParen, Context.BoolTy);
 }
Index: cfe/trunk/test/Parser/objc-available.m
===================================================================
--- cfe/trunk/test/Parser/objc-available.m
+++ cfe/trunk/test/Parser/objc-available.m
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -Wunguarded-availability -triple x86_64-apple-macosx10.10.0 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wunguarded-availability -Wno-unsupported-availability-guard -triple x86_64-apple-macosx10.10.0 -verify %s
 
 void f() {
 
Index: cfe/trunk/test/SemaObjC/unguarded-availability.m
===================================================================
--- cfe/trunk/test/SemaObjC/unguarded-availability.m
+++ cfe/trunk/test/SemaObjC/unguarded-availability.m
@@ -10,7 +10,7 @@
 #ifdef OBJCPP
 // expected-note@+2 6 {{marked partial here}}
 #endif
-int func_10_12() AVAILABLE_10_12; // expected-note 6 {{'func_10_12' has been explicitly marked partial here}}
+int func_10_12() AVAILABLE_10_12; // expected-note 7 {{'func_10_12' has been explicitly marked partial here}}
 
 int func_10_0() AVAILABLE_10_0;
 
@@ -155,6 +155,16 @@
   id y = x[42]; // expected-warning{{'objectAtIndexedSubscript:' is only available on macOS 10.12 or newer}} expected-note{{@available}}
 }
 
+void uncheckAtAvailable() {
+  if (@available(macOS 10.12, *) || 0) // expected-warning {{@available does not guard availability here; use if (@available) instead}}
+    func_10_12(); // expected-warning {{'func_10_12' is only available on macOS 10.12 or newer}}
+  // expected-note@-1 {{enclose 'func_10_12' in an @available check to silence this warning}}
+}
+
+void justAtAvailable() {
+  int availability = @available(macOS 10.12, *); // expected-warning {{@available does not guard availability here; use if (@available) instead}}
+}
+
 #ifdef OBJCPP
 
 int f(char) AVAILABLE_10_12;