Index: compilers/imcc/imcc.y
===================================================================
--- compilers/imcc/imcc.y	(revision 47282)
+++ compilers/imcc/imcc.y	(working copy)
@@ -568,13 +568,23 @@
 {
     ASSERT_ARGS(func_ins)
     int i;
+    Instruction *ins;
+
     /* shift regs up by 1 */
     for (i = n - 1; i >= 0; --i)
         r[i+1] = r[i];
     r[0] = lhs;
+
     /* shift keyvec */
     keyv <<= 1;
-    return INS(interp, unit, op, "", r, n+1, keyv, emit);
+
+    ins = INS(interp, unit, op, "", r, n+1, keyv, emit);
+
+    /* check that 'assigned' register isn't treated as an input by the op */
+    if (ins->flags & 1)
+        IMCC_warning(interp, "assignment form used for input argument to `%s'\n", op);
+
+    return ins;
 }
 
 /*