diff --git a/lib/Sema/SemaAttr.cpp b/lib/Sema/SemaAttr.cpp
index 0921156..d7290c3 100644
--- a/lib/Sema/SemaAttr.cpp
+++ b/lib/Sema/SemaAttr.cpp
@@ -288,6 +288,10 @@
       continue;
     }
 
+    // Warn if this was used before being marked unused.
+    if (VD->isUsed())
+      Diag(PragmaLoc, diag::warn_used_but_marked_unused) << Name;
+
     VD->addAttr(::new (Context) UnusedAttr(Tok.getLocation(), Context));
   }
 }
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 587a76e..ced3833 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -97,6 +97,10 @@
     }
   }
 
+  // Warn if this is used but marked unused.
+  if (D->hasAttr<UnusedAttr>())
+    Diag(Loc, diag::warn_used_but_marked_unused) << D->getDeclName();
+
   return false;
 }
 
@@ -7804,7 +7808,7 @@
   // -Wunused-parameters)
   if (isa<ParmVarDecl>(D) ||
       (isa<VarDecl>(D) && D->getDeclContext()->isFunctionOrMethod())) {
-    D->setUsed(true);
+    D->setUsed();
     return;
   }