re PR c++/34198 (-Wconversion gives apparent erroneous warning with g++ 4.3-20071109)

	PR c++/34198
	* c-common.c (conversion_warning): For INTEGER_TYPE to
	INTEGER_TYPE conversions call get_narrower on expr to avoid
	spurious warnings from binop shortening or when the implicit
	conversion can't change the value.

	* gcc.dg/Wconversion-5.c: New test.
	* g++.dg/Wconversion3.C: New test.

From-SVN: r130377

1 files changed, 30 insertions, 5 deletions

diff --git a/gcc/c-common.c b/gcc/c-common.c
index edc9b2c..6872e3a 100644
--- a/gcc/c-common.c
+++ b/gcc/c-common.c
@@ -1280,6 +1280,14 @@ conversion_warning (tree type, tree expr)
       else if (TREE_CODE (TREE_TYPE (expr)) == INTEGER_TYPE
                && TREE_CODE (type) == INTEGER_TYPE)
         {
+	  /* Don't warn about unsigned char y = 0xff, x = (int) y;  */
+	  int uns;
+	  tree orig_expr = expr;
+	  expr = get_narrower (expr, &uns);
+
+	  if (expr == orig_expr)
+	    uns = TYPE_UNSIGNED (TREE_TYPE (expr));
+
           /* Warn for integer types converted to smaller integer types.  */
           if (formal_prec < TYPE_PRECISION (TREE_TYPE (expr))) 
 	    give_warning = true;
@@ -1287,14 +1295,31 @@ conversion_warning (tree type, tree expr)
 	  /* When they are the same width but different signedness,
 	     then the value may change.  */
 	  else if ((formal_prec == TYPE_PRECISION (TREE_TYPE (expr))
-		    && TYPE_UNSIGNED (TREE_TYPE (expr)) != TYPE_UNSIGNED (type))
+		    && uns != TYPE_UNSIGNED (type))
 		   /* Even when converted to a bigger type, if the type is
 		      unsigned but expr is signed, then negative values
 		      will be changed.  */
-		   || (TYPE_UNSIGNED (type) && !TYPE_UNSIGNED (TREE_TYPE (expr))))
-	    warning (OPT_Wsign_conversion,
-		     "conversion to %qT from %qT may change the sign of the result",
-		     type, TREE_TYPE (expr));
+		   || (TYPE_UNSIGNED (type) && !uns))
+	    {
+	      if (uns != TYPE_UNSIGNED (TREE_TYPE (expr)))
+		{
+		  /* For signed char s1, s2 = (int) (unsigned char) s1;
+		     get_narrower returns s1, but uns = 1.  Find the
+		     narrowest type with uns == TYPE_UNSIGNED (type).  */
+		  tree unsexpr = orig_expr;
+
+		  while (TREE_CODE (unsexpr) == NOP_EXPR
+			 && unsexpr != expr
+			 && TYPE_UNSIGNED (TREE_TYPE (TREE_OPERAND (unsexpr,
+								    0)))
+			    == uns)
+		    unsexpr = TREE_OPERAND (unsexpr, 0);
+		  expr = unsexpr;
+		}
+	      warning (OPT_Wsign_conversion,
+		       "conversion to %qT from %qT may change the sign of the result",
+		       type, TREE_TYPE (expr));
+	    }
         }
 
       /* Warn for integer types converted to real types if and only if