re PR c++/39551 (C++ frontend not warn about unused dereference operator with -Wunused-value)

        PR c++/39551
	* gcc/cp/call.c (build_over_call): Set TREE_NO_WARNING on the
	compiler-generated INDIRECT_REF expression.
	* gcc/cp/cvt.c (convert_to_void): Emit warning when stripping off
	INDIRECT_REF.
	* gcc/testsuite/g++.dg/warn/Wunused-13.C: New testcase.

From-SVN: r146132

gcc/cp/ChangeLog

+2009-04-15  Le-Chun Wu  <lcwu@google.com>
+
+	PR c++/39551
+	* call.c (build_over_call): Set TREE_NO_WARNING on the
+	compiler-generated INDIRECT_REF expression.
+	* cvt.c (convert_to_void): Emit warning when stripping off
+	INDIRECT_REF.
+
 2009-04-14  Diego Novillo  <dnovillo@google.com>
 
 	* parser.c (cp_parser_type_specifier_seq): Move call to

gcc/cp/call.c

@@ -5419,6 +5419,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	  if (test)
 	    t = build3 (COND_EXPR, TREE_TYPE (t), test, arg0, t);
 	  val = cp_build_indirect_ref (t, 0, complain);
+          TREE_NO_WARNING (val) = 1;
 	}
 
       return val;

gcc/cp/cvt.c

@@ -870,7 +870,20 @@ convert_to_void (tree expr, const char *implicit, tsubst_flags_t complain)
                        implicit ? implicit : "void context");
           }
 	if (is_reference || !is_volatile || !is_complete || TREE_ADDRESSABLE (type))
-	  expr = TREE_OPERAND (expr, 0);
+          {
+            /* Emit a warning (if enabled) when the "effect-less" INDIRECT_REF
+               operation is stripped off. Note that we don't warn about
+               - an expression with TREE_NO_WARNING set. (For an example of
+                 such expressions, see build_over_call in call.c.)
+               - automatic dereferencing of references, since the user cannot
+                 control it. (See also warn_if_unused_value() in stmt.c.)  */
+            if (warn_unused_value
+                && (complain & tf_warning)
+                && !TREE_NO_WARNING (expr)
+                && !is_reference)
+              warning (OPT_Wunused_value, "value computed is not used");
+            expr = TREE_OPERAND (expr, 0);
+          }
 
 	break;
       }

gcc/testsuite/ChangeLog

+2009-04-15  Le-Chun Wu  <lcwu@google.com>
+
+	PR c++/39551
+	* g++.dg/warn/Wunused-13.C: New testcase.
+
 2009-04-15  Ian Lance Taylor  <iant@google.com>
 
 	* gcc.dg/Wenum-compare-1.c: New testcase.

gcc/testsuite/g++.dg/warn/Wunused-13.C

+// Test whether -Wunused handles effectless indirect_ref operation ('*').
+// { dg-do compile }
+// { dg-options "-Wunused" }
+
+void Foo(int* x) {
+  *x++; // { dg-warning "value computed is not used" }
+}