re PR c++/38958 ('unused variable' warning emitted when extending the lifetime…

re PR c++/38958 ('unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor])

/cp
2013-06-12  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/38958
	* decl.c (poplevel): For the benefit of -Wunused-variable see
	through references.

/testsuite
2013-06-12  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/38958
	* g++.dg/warn/Wunused-var-20.C: New.

From-SVN: r200042

gcc/cp/ChangeLog

 2013-06-12  Paolo Carlini  <paolo.carlini@oracle.com>
 
+	PR c++/38958
+	* decl.c (poplevel): For the benefit of -Wunused-variable see
+	through references.
+
+2013-06-12  Paolo Carlini  <paolo.carlini@oracle.com>
+
 	* parser.c (cp_parser_nested_name_specifier_opt): Fix typo in comment.
 
 2013-06-12  Paolo Carlini  <paolo.carlini@oracle.com>

gcc/cp/decl.c

@@ -622,17 +622,20 @@ poplevel (int keep, int reverse, int functionbody)
 	   push_local_binding where the list of decls returned by
 	   getdecls is built.  */
 	decl = TREE_CODE (d) == TREE_LIST ? TREE_VALUE (d) : d;
+	// See through references for improved -Wunused-variable (PR 38958).
+	tree type = non_reference (TREE_TYPE (decl));
 	if (VAR_P (decl)
 	    && (! TREE_USED (decl) || !DECL_READ_P (decl))
 	    && ! DECL_IN_SYSTEM_HEADER (decl)
 	    && DECL_NAME (decl) && ! DECL_ARTIFICIAL (decl)
-	    && TREE_TYPE (decl) != error_mark_node
-	    && (!CLASS_TYPE_P (TREE_TYPE (decl))
-		|| !TYPE_HAS_NONTRIVIAL_DESTRUCTOR (TREE_TYPE (decl))))
+	    && type != error_mark_node
+	    && (!CLASS_TYPE_P (type)
+		|| !TYPE_HAS_NONTRIVIAL_DESTRUCTOR (type)))
 	  {
 	    if (! TREE_USED (decl))
 	      warning (OPT_Wunused_variable, "unused variable %q+D", decl);
 	    else if (DECL_CONTEXT (decl) == current_function_decl
+		     // For -Wunused-but-set-variable leave references alone.
 		     && TREE_CODE (TREE_TYPE (decl)) != REFERENCE_TYPE
 		     && errorcount == unused_but_set_errorcount)
 	      {

gcc/testsuite/ChangeLog

+2013-06-12  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/38958
+	* g++.dg/warn/Wunused-var-20.C: New.
+
 2013-06-12  Richard Sandiford  <rdsandiford@googlemail.com>
 
 	* gcc.target/mips/mips.exp: Handle -f{no-,}common.

gcc/testsuite/g++.dg/warn/Wunused-var-20.C

+// PR c++/38958
+// { dg-options "-Wunused" }
+
+volatile int g;
+
+struct Lock
+{
+  ~Lock() { g = 0; }
+};
+
+Lock AcquireLock() { return Lock(); }
+
+int main()
+{
+  const Lock& lock = AcquireLock();
+  g = 1;
+  g = 2;
+  g = 3;
+}