[Bug c/51294] spurious warning from -Wconversion in C and C++ in conditional expressions

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51294

--- Comment #10 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-05-12 
18:38:33 UTC ---
Latest patch by Paolo, who run out of time:

http://gcc.gnu.org/ml/gcc-patches/2012-05/msg00861.html

BTW, I think the patch is not wrong, and if fixes some cases, then it is a step
forward.

I think a more complete fix would be something like:

Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c     (revision 187427)
+++ gcc/c-family/c-common.c     (working copy)
@@ -2262,10 +2262,15 @@ conversion_warning (tree type, tree expr
   location_t loc = EXPR_LOC_OR_HERE (expr);

   if (!warn_conversion && !warn_sign_conversion)
     return;

+  STRIP_USELESS_TYPE_CONVERSION (expr);
+  /* Don't warn about unsigned char y = 0xff, x = (int) y;  */
+  expr = get_unwidened (expr, 0);
+  expr_type = TREE_TYPE (expr);
+
   switch (TREE_CODE (expr))
     {
     case EQ_EXPR:
     case NE_EXPR:
     case LE_EXPR:
@@ -2294,26 +2299,18 @@ conversion_warning (tree type, tree expr
                    type, expr_type);
       return;

     case COND_EXPR:
       {
-       /* In case of COND_EXPR, if both operands are constants or
-          COND_EXPR, then we do not care about the type of COND_EXPR,
-          only about the conversion of each operand.  */
-       tree op1 = TREE_OPERAND (expr, 1);
-       tree op2 = TREE_OPERAND (expr, 2);
-
-       if ((TREE_CODE (op1) == REAL_CST || TREE_CODE (op1) == INTEGER_CST
-            || TREE_CODE (op1) == COND_EXPR)
-           && (TREE_CODE (op2) == REAL_CST || TREE_CODE (op2) == INTEGER_CST
-               || TREE_CODE (op2) == COND_EXPR))
-         {
-           conversion_warning (type, op1);
-           conversion_warning (type, op2);
-           return;
-         }
-       /* Fall through.  */
+        /* In case of COND_EXPR, we do not care about the type of
+           COND_EXPR, only about the conversion of each operand.  */
+        tree op1 = TREE_OPERAND (expr, 1);
+        tree op2 = TREE_OPERAND (expr, 2);
+ 
+        conversion_warning (type, op1);
+        conversion_warning (type, op2);
+        return;
       }

     default: /* 'expr' is not a constant.  */
       if (unsafe_conversion_p (type, expr, true))
        warning_at (loc, OPT_Wconversion,


Totally untested, and it lacks testcases, ChangeLog, etc. Neither Paolo nor I
have time to pursue this, so someone new will have to step forward.