c++: fix missing lifetime extension [PR119383]

Since r15-8011 cp_build_indirect_ref_1 won't do the *&TARGET_EXPR ->
TARGET_EXPR folding not to change its value category.  That fix seems
correct but it made us stop extending the lifetime in this testcase,
causing a wrong-code issue -- extend_ref_init_temps_1 did not see
through the extra *& because it doesn't use a tree walk.

This patch reverts r15-8011 and instead handles the problem in
build_over_call by calling force_lvalue in the is_really_empty_class
case as well as in the general case.

	PR c++/119383

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Use force_lvalue to ensure op= returns
	an lvalue.
	* cp-tree.h (force_lvalue): Declare.
	* cvt.cc (force_lvalue): New.
	* typeck.cc (cp_build_indirect_ref_1): Revert r15-8011.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/temp-extend3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/call.cc

@@ -10828,10 +10828,8 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
       if (is_really_empty_class (type, /*ignore_vptr*/true))
 	{
 	  /* Avoid copying empty classes, but ensure op= returns an lvalue even
-	     if the object argument isn't one. This isn't needed in other cases
-	     since MODIFY_EXPR is always considered an lvalue.  */
-	  to = cp_build_addr_expr (to, tf_none);
-	  to = cp_build_indirect_ref (input_location, to, RO_ARROW, complain);
+	     if the object argument isn't one.  */
+	  to = force_lvalue (to, complain);
 	  val = build2 (COMPOUND_EXPR, type, arg, to);
 	  suppress_warning (val, OPT_Wunused);
 	}
@@ -10852,6 +10850,9 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	  tree array_type, alias_set;
 
 	  arg2 = TYPE_SIZE_UNIT (as_base);
+	  /* Ensure op= returns an lvalue even if the object argument isn't
+	     one.  */
+	  to = force_lvalue (to, complain);
 	  to = cp_stabilize_reference (to);
 	  arg0 = cp_build_addr_expr (to, complain);
 

gcc/cp/cp-tree.h

@@ -7079,6 +7079,7 @@ extern tree convert_to_reference		(tree, tree, int, int, tree,
 						 tsubst_flags_t);
 extern tree convert_from_reference		(tree);
 extern tree force_rvalue			(tree, tsubst_flags_t);
+extern tree force_lvalue			(tree, tsubst_flags_t);
 extern tree ocp_convert				(tree, tree, int, int,
 						 tsubst_flags_t);
 extern tree cp_convert				(tree, tree, tsubst_flags_t);

gcc/cp/cvt.cc

@@ -575,6 +575,19 @@ force_rvalue (tree expr, tsubst_flags_t complain)
   return expr;
 }
 
+/* Force EXPR to be an lvalue, if it isn't already.  */
+
+tree
+force_lvalue (tree expr, tsubst_flags_t complain)
+{
+  if (!lvalue_p (expr))
+    {
+      expr = cp_build_addr_expr (expr, complain);
+      expr = cp_build_indirect_ref (input_location, expr, RO_ARROW, complain);
+    }
+  return expr;
+}
+
 
 /* If EXPR and ORIG are INTEGER_CSTs, return a version of EXPR that has
    TREE_OVERFLOW set only if it is set in ORIG.  Otherwise, return EXPR

gcc/cp/typeck.cc

@@ -3870,13 +3870,11 @@ cp_build_indirect_ref_1 (location_t loc, tree ptr, ref_operator errorstring,
 	  return error_mark_node;
 	}
       else if (do_fold && TREE_CODE (pointer) == ADDR_EXPR
-	       && same_type_p (t, TREE_TYPE (TREE_OPERAND (pointer, 0)))
-	       /* Don't let this change the value category.  '*&TARGET_EXPR'
-		  is an lvalue, but folding it into 'TARGET_EXPR' would turn
-		  it into a prvalue of class type.  */
-	       && lvalue_p (TREE_OPERAND (pointer, 0)))
+	       && same_type_p (t, TREE_TYPE (TREE_OPERAND (pointer, 0))))
 	/* The POINTER was something like `&x'.  We simplify `*&x' to
-	   `x'.  */
+	   `x'.  This can change the value category: '*&TARGET_EXPR'
+	   is an lvalue and folding it into 'TARGET_EXPR' turns it into
+	   a prvalue of class type.  */
 	return TREE_OPERAND (pointer, 0);
       else
 	{

gcc/testsuite/g++.dg/cpp0x/temp-extend3.C

@@ -0,0 +1,32 @@
+// PR c++/119383
+// { dg-do run { target c++11 } }
+
+int g;
+
+struct base {
+  virtual base *clone() const = 0;
+  ~base() { }
+};
+
+struct impl : virtual base {
+  base *clone() const { return new impl; }  // #1
+  impl() { ++g; }
+  ~impl() { --g; }
+};
+
+const base *
+make_a_clone ()
+{
+  const base &base = impl{}; // #2
+  return base.clone();
+}
+
+int
+main ()
+{
+  make_a_clone ();
+  // impl::impl() is called twice (#1 and #2), impl::~impl() once,
+  // at the end of make_a_clone.
+  if (g != 1)
+    __builtin_abort ();
+}