Remove (meant to be) dead code

The front-ends handle type conversion when doing arithmetic over enum types. The
case of an enum tag is not expected here. That, however, was only the case for
some operators. Made the C front-end properly handle all assignment operators.

Author: tautschnig

regression/cbmc/enum8/main.c

@@ -0,0 +1,22 @@
+#include <assert.h>
+
+enum E
+{
+  A = 1,
+  B = 16
+};
+
+int main()
+{
+  enum E e = A;
+  e <<= 4;
+  assert(e == B);
+  e >>= 4;
+  assert(e == A);
+  e |= B;
+  e ^= A;
+  assert(e == B);
+  e -= 15;
+  assert(e == A);
+  return 0;
+}

regression/cbmc/enum8/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring

src/ansi-c/c_typecheck_expr.cpp

@@ -3350,6 +3350,7 @@ void c_typecheck_baset::typecheck_side_effect_assignment(
   else if(statement==ID_assign_shl ||
           statement==ID_assign_shr)
   {
+    implicit_typecast_arithmetic(op0);
     implicit_typecast_arithmetic(op1);
 
     if(is_number(op1.type()))
@@ -3364,12 +3365,6 @@ void c_typecheck_baset::typecheck_side_effect_assignment(
 
         typet underlying_type=op0.type();
 
-        if(underlying_type.id()==ID_c_enum_tag)
-        {
-          const auto &c_enum_type = to_c_enum_tag_type(underlying_type);
-          underlying_type=c_enum_type.subtype();
-        }
-
         if(underlying_type.id()==ID_unsignedbv ||
            underlying_type.id()==ID_c_bool)
         {
@@ -3405,7 +3400,7 @@ void c_typecheck_baset::typecheck_side_effect_assignment(
             o_type0.id()==ID_signedbv ||
             o_type0.id()==ID_c_bit_field)
     {
-      implicit_typecast(op1, o_type0);
+      implicit_typecast_arithmetic(op0, op1);
       return;
     }
     else if(o_type0.id()==ID_vector &&

src/goto-programs/goto_convert_side_effect.cpp

@@ -105,19 +105,10 @@ void goto_convertt::remove_assignment(
       tmp.op0().make_typecast(expr.op1().type());
       rhs=typecast_exprt(is_not_zero(tmp, ns), expr.op0().type());
     }
-    else if(op0_type.id() == ID_c_enum_tag)
-    {
-      // We convert c_enums to their underlying type, do the
-      // operation, and then convert back
-      const auto &enum_type = ns.follow_tag(to_c_enum_tag_type(op0_type));
-      auto underlying_type = enum_type.subtype();
-      auto op0 = typecast_exprt(expr.op0(), underlying_type);
-      auto op1 = typecast_exprt(expr.op1(), underlying_type);
-      binary_exprt tmp(op0, new_id, op1, underlying_type);
-      rhs = typecast_exprt(tmp, expr.op0().type());
-    }
     else
     {
+      PRECONDITION(
+        op0_type.id() != ID_c_enum_tag && op0_type.id() != ID_c_enum);
       rhs.id(new_id);
       rhs.copy_to_operands(expr.op0(), expr.op1());
       rhs.type()=expr.op0().type();