Fixes issue #336

[pcjordan]

Requesting bug bounty :-)

[nielstron]

Hey, thanks for participating in the bug bounty (for #336). Please share your payment address / ada handle while I will review your contribution and provide feedback. Payments will be done if the PR results in a merge. More details as usual can be found in the bug bounty documentation.

[nielstron]

Please incorporate the noted changes. Please also run the code on two example contracts and provide the output with that (I think creating a proper unit test for logged warnings would be beyond the scope of this PR)

[nielstron]

If I understand correctly you are trying to cover two cases here: either the line is assert None or the line is assert foo() where foo returns None.

I think it is good to explicitly cover the foo case and provide a more specific error message for it. However the handling would be more generic if after that check we check for the type of the whole expression (I.e. ta.test.args[0].typ or ta.test.typ)

[pcjordan]

Picking up on this again, I made the Exception more specific, and added two examples which give the following warnings:

1. python3 -m opshin compile spending examples/smart_contracts/assert_none.py gives WARNING for examples/smart_contracts/assert_none.py:6 Asserting 'None' (see assert with message 'Always fails!') is equivalent to asserting False, which always fails.
2. python3 -m opshin compile spending examples/smart_contracts/assert_foo_none.py gives WARNING for examples/smart_contracts/assert_foo_none.py:10 Asserting 'None' (see assert with message 'Always fails!') is likely to stem from a procedure already doing internal assertions and returning 'None'. Asserting this is equivalent to asserting False, which always fails (likely unintended).

Regarding your comment on more generic handling: I'm not sure if I understand what you mean by the ta.test.args[0].value.typ thing. If the line is assert None, then ta.test.args[0].value is None and there is no attribute typ. If the line is assert foo() where foo returns None, then ta.test.args[0] is a Call object which has no attribute value.