[Bug c++/12242] g++ should warn about out-of-range int->enum conversions

["manu at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #9 from manu at gcc dot gnu dot org  2008-02-15 11:09 -------
(In reply to comment #0)
> enum E { A };
> ...
> E e = static_cast<E> (10);
> 
> Yes, i know that result of this is undefined.

Is this true? I haven't found any mention of this.

> However, previous versions of g++ (and every other c++ compiler i've used)
> has implemented this in what seems like the simplest way: the compiler
> simply assigns the value `10' to the enum value, regardless of the
> fact that this is not a valid value for the enum.

We assing 10 in GCC 4.3.

> Therefore, the compiler is silently changing the behavior of the
> code from what was likely intended.  So i think a warning would
> be appropriate in this situation.  The current verison of 3.4 does
> not issue any warning, even with `-Wall -pedantic'.

The following patch implements the warning for this testcase. Do you know other
cases that may be interesting to warn about?

Index: gcc/testsuite/g++.dg/warn/pr12242.C
===================================================================
--- gcc/testsuite/g++.dg/warn/pr12242.C (revision 0)
+++ gcc/testsuite/g++.dg/warn/pr12242.C (revision 0)
@@ -0,0 +1,4 @@
+// { dg-do compile }
+// { dg-options "-Wall -Wextra -Wconversion" }
+enum E { A };
+E e = static_cast<E> (10); // { dg-warning "warning.*undefined" }
Index: gcc/cp/typeck.c
===================================================================
--- gcc/cp/typeck.c     (revision 132310)
+++ gcc/cp/typeck.c     (working copy)
@@ -5069,10 +5069,15 @@ build_static_cast_1 (tree type, tree exp
      types which are integral, floating, or enumeration types can be
      performed.  */
   if ((INTEGRAL_TYPE_P (type) || SCALAR_FLOAT_TYPE_P (type))
       && (INTEGRAL_TYPE_P (intype) || SCALAR_FLOAT_TYPE_P (intype)))
     {
+      if (TREE_CODE (expr) == INTEGER_CST
+         && TREE_CODE (type) == ENUMERAL_TYPE
+         && !int_fits_type_p (expr, type))
+       warning (0, "this conversion is undefined");
+
       expr = ocp_convert (type, expr, CONV_C_CAST, LOOKUP_NORMAL);

       /* Ignore any integer overflow caused by the cast.  */
       expr = ignore_overflows (expr, orig);
       return expr;


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12242