re PR c++/19199 (Wrong warning about returning a reference to a temporary)

2005-04-03  Roger Sayle  <roger@eyesopen.com>
	    Alexandre Oliva  <aoliva@redhat.com>

	PR c++/19199
	* fold-const.c (non_lvalue): Split tests into...
	(maybe_lvalue_p): New function.
	(fold_cond_expr_with_comparison): Preserve lvalue-ness for the
	C++ front-end prior to lowering into gimple form.

	* g++.dg/expr/lval2.C: New.

	* expr2.C: Fixed.

From-SVN: r97522

1 files changed, 28 insertions, 9 deletions

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index c224257..0b9a71f 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -2003,16 +2003,13 @@ fold_convert (tree type, tree arg)
     }
 }
 
+/* Return false if expr can be assumed not to be an value, true
+   otherwise.  */
 /* Return an expr equal to X but certainly not valid as an lvalue.  */
 
-tree
-non_lvalue (tree x)
+static bool
+maybe_lvalue_p (tree x)
 {
-  /* While we are in GIMPLE, NON_LVALUE_EXPR doesn't mean anything to
-     us.  */
-  if (in_gimple_form)
-    return x;
-
   /* We only need to wrap lvalue tree codes.  */
   switch (TREE_CODE (x))
   {
@@ -2052,8 +2049,24 @@ non_lvalue (tree x)
     /* Assume the worst for front-end tree codes.  */
     if ((int)TREE_CODE (x) >= NUM_TREE_CODES)
       break;
-    return x;
+    return false;
   }
+
+  return true;
+}
+
+/* Return an expr equal to X but certainly not valid as an lvalue.  */
+
+tree
+non_lvalue (tree x)
+{
+  /* While we are in GIMPLE, NON_LVALUE_EXPR doesn't mean anything to
+     us.  */
+  if (in_gimple_form)
+    return x;
+
+  if (! maybe_lvalue_p (x))
+    return x;
   return build1 (NON_LVALUE_EXPR, TREE_TYPE (x), x);
 }
 
@@ -4273,7 +4286,13 @@ fold_cond_expr_with_comparison (tree type, tree arg0, tree arg1, tree arg2)
      a number and A is not.  The conditions in the original
      expressions will be false, so all four give B.  The min()
      and max() versions would give a NaN instead.  */
-  if (operand_equal_for_comparison_p (arg01, arg2, arg00))
+  if (operand_equal_for_comparison_p (arg01, arg2, arg00)
+      /* Avoid these transformations if the COND_EXPR may be used
+	 as an lvalue in the C++ front-end.  PR c++/19199.  */
+      && (in_gimple_form
+	  || strcmp (lang_hooks.name, "GNU C++") != 0
+	  || ! maybe_lvalue_p (arg1)
+	  || ! maybe_lvalue_p (arg2)))
     {
       tree comp_op0 = arg00;
       tree comp_op1 = arg01;