Why does unpacking a record that has a key containing bounded data fail with `to_value sp.TBounded` in smartpy?

Question

Tests are throwing a to_value error in the scenario when trying to unpack a record that contains a key with a bounded type TNat inside the entrypoint being called.

There's a minimal reproduction below, and a link to the SmartPy IDE at the bottom.

Example Contract:

import smartpy as sp

test_type = sp.TRecord(
    num=sp.TNat,
    select=sp.TBounded([1, 2, 3, 4, 5], sp.TNat),
)

class PackUnpackBoundedTypes(sp.Contract):
    def __init__(self):
        self.init(
            rec = sp.pack(
                sp.record(
                    num=sp.nat(2),
                    select=sp.bounded(5)
                )
            ),
            done = sp.record(
                num=sp.nat(1),
                select=sp.bounded(1)
            )
        )
        
    @sp.entry_point()
    def get_unpacked_info(self):
        res = sp.unpack(self.data.rec, test_type).open_some()
        self.data.done = res

Contract Tests:

@sp.add_test(name = "PackUnpackBoundedTypes")
def test():
    scenario = sp.test_scenario()
    scenario.h1("PackUnpackBoundedTypes")
    alice = sp.test_account('alice')
    
    c = PackUnpackBoundedTypes()
    scenario += c

    c.get_unpacked_info().run(sender=alice)

Failure:

Error: Error in Scenario
Unexpected error in transaction, please use .run(valid=False, ..)
Failure: to_value sp.TBounded([1, 2, 3, 4, 5], t=sp.TNat) 5;
get_unpacked_info

SmartyPy IDE Link

Is it not possible to pack/unpack bounded types?

Answer 1

It is unsafe without a dynamic check. We need to determine what the most natural and/or appropriate decision is (very possibly: continue forbidding it).