Add repr_change_type op, for asking the repr to do a type change on t…

…he object.

src/ops/nqp.ops

@@ -374,6 +374,33 @@ inline op repr_is_attr_initialized(out INT, in PMC, in PMC, in STR) :base_core {
 }
 
 
+/*
+
+=item repr_change_type()
+
+Tries to change the type of the object in $1 to the type $2. This
+delegates to the REPR to do the work, so it is ultimately up to the
+REPR to decide what changes it will or won't allow.
+
+=cut
+
+*/
+inline op repr_change_type(in PMC, in PMC) :base_core {
+    PMC *obj      = decontainerize(interp, $1);
+    PMC *new_type = decontainerize(interp, $2);
+    if (obj->vtable->base_type == smo_id && new_type->vtable->base_type == smo_id) {
+        if (REPR(obj)->change_type)
+            REPR(obj)->change_type(interp, obj, new_type);
+        else
+            Parrot_ex_throw_from_c_args(interp, NULL, EXCEPTION_INVALID_OPERATION,
+                "Representation does not implement change_type");
+    }
+    else {
+        Parrot_ex_throw_from_c_args(interp, NULL, EXCEPTION_INVALID_OPERATION,
+            "Can only use repr_change_type on SixModelObjects");
+    }
+}
+
 /*
 
 =item type_check(obj, wanted_type)