Mark reducer callbacks used when the type is instantiated

clang/lib/Sema/SemaType.cpp

@@ -2406,8 +2406,12 @@ QualType Sema::BuildReferenceType(QualType T, bool SpelledAsLValue,
 // Return value is always non-null.
 Expr *Sema::ValidateReducerCallback(Expr *E, unsigned NumArgs,
                                     SourceLocation Loc) {
-  if (!E)
+  if (E) {
+    if (!E->getType()->isDependentType() && isa<DeclRefExpr>(E))
+      cast<DeclRefExpr>(E)->getDecl()->markUsed(Context);
+  } else {
     E = new (Context) CXXNullPtrLiteralExpr(Context.NullPtrTy, Loc);
+  }
 
   QualType T = E->getType();
 

clang/test/Cilk/154.cpp

@@ -0,0 +1,17 @@
+// Bug 154: Spurious "not needed" warnings for reducer callbacks with -Wall.
+// RUN: %clang_cc1 %s -x c++ -fopencilk -fsyntax-only -Wall -verify
+// expected-no-diagnostics
+
+static void identity_64(void *p) { *(long *)p = 0; }
+static void sum_64(void *l, void *r) { *(long *)l += *(long *)r; }
+
+template <class T>
+T sum_cilk(){
+  long _Hyperobject(identity_64, sum_64) sum = 0;
+  return sum;
+}
+
+long f() {
+  long total = sum_cilk<long>();
+  return total;
+}