re PR c++/39875 (Wrong "value computed is not used" warning)

	PR c++/39875
	* cvt.c (convert_to_void) <case INDIRECT_REF>: Only warn about
	-Wunused-value if implicit.

	* g++.dg/warn/Wunused-15.C: New test.

From-SVN: r146825

gcc/cp/ChangeLog

+2009-04-27  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/39875
+	* cvt.c (convert_to_void) <case INDIRECT_REF>: Only warn about
+	-Wunused-value if implicit.
+
 2009-04-24  Ian Lance Taylor  <iant@google.com>
 
 	* call.c (build_temp): Change 0 to enum constant.

gcc/cp/cvt.c

@@ -787,7 +787,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags)
    make it impossible to ignore the reference return value from functions. We
    issue warnings in the confusing cases.
 
-   IMPLICIT is tells us the context of an implicit void conversion.  */
+   IMPLICIT is non-NULL iff an expression is being implicitly converted; it
+   is NULL when the user is explicitly converting an expression to void via
+   a cast.  When non-NULL, IMPLICIT is a string indicating the context of
+   the implicit conversion.  */
 
 tree
 convert_to_void (tree expr, const char *implicit, tsubst_flags_t complain)
@@ -885,6 +888,7 @@ convert_to_void (tree expr, const char *implicit, tsubst_flags_t complain)
                - automatic dereferencing of references, since the user cannot
                  control it. (See also warn_if_unused_value() in stmt.c.)  */
             if (warn_unused_value
+		&& implicit
                 && (complain & tf_warning)
                 && !TREE_NO_WARNING (expr)
                 && !is_reference)

gcc/testsuite/ChangeLog

+2009-04-27  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/39875
+	* g++.dg/warn/Wunused-15.C: New test.
+
 2009-04-26  Michael Matz  <matz@suse.de>
 
 	Expand from SSA.

gcc/testsuite/g++.dg/warn/Wunused-15.C

+// PR c++/39875
+// { dg-do compile }
+// { dg-options "-Wunused-value" }
+
+int *i;
+void
+foo ()
+{
+  *i++;		// { dg-warning "value computed is not used" }
+  (void) *i++;	// { dg-bogus "value computed is not used" }
+}