Tweak constraints handling for splitting typevartuples #13716
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The existing logic for splitting mapped & template into prefix, middle, and suffix does not handle the case where the template middle itself is not a singleton unpack but rather itself has a prefix & suffix. In this case we need to pull out the prefix & suffix by doing a second round of splitting on the middle. Originally we weren't sure if the PEP required implementing this double split, but one of the PEP646 test cases requires it.
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JukkaL
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JukkaL
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jhance
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This adds a new Pep646 test case which demonstrates that like with the constraints from PR python#13716 we need to split twice for subtyping when handling Unpacks. It also demonstrates a weakness of the previous PR which is that the middle-prefix and the prefix may need to be handled differently so we introduce another splitting function that returns a 10-tuple instead of a 6-tuple and reimplement the 6-tuple version on top of the 10-tuple version. Complicating things further, the test case reveals that there are error cases where split_with_mapped_and_template cannot actually unpack the middle a second time because the mapped middle is too short to do the unpack. We also now have to deal with the case where there was no unpack in the template in which case we only do a single split. In addition we fix a behavioral issue where according to PEP646 we should assume that Tuple[Unpack[Tuple[Any, ...]]] is equivalent to Tuple[Any, Any] even if we don't actually know the lengths match. As such test_type_var_tuple_unpacked_variable_length_tuple changes from asserting a strict subtype to asserting equivalence. One of the messages was bad as well so we add a branch for UnpackType in message pretty-printing.
jhance
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Oct 13, 2022
This adds a new Pep646 test case which demonstrates that like with the constraints from PR #13716 we need to split twice for subtyping when handling Unpacks. It also demonstrates a weakness of the previous PR which is that the middle-prefix and the prefix may need to be handled differently so we introduce another splitting function that returns a 10-tuple instead of a 6-tuple and reimplement the 6-tuple version on top of the 10-tuple version. Complicating things further, the test case reveals that there are error cases where split_with_mapped_and_template cannot actually unpack the middle a second time because the mapped middle is too short to do the unpack. We also now have to deal with the case where there was no unpack in the template in which case we only do a single split. In addition we fix a behavioral issue where according to PEP646 we should assume that Tuple[Unpack[Tuple[Any, ...]]] is equivalent to Tuple[Any, Any] even if we don't actually know the lengths match. As such test_type_var_tuple_unpacked_variable_length_tuple changes from asserting a strict subtype to asserting equivalence. One of the messages was bad as well so we add a branch for UnpackType in message pretty-printing.
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The existing logic for splitting mapped & template into prefix, middle, and suffix does not handle the case where the template middle itself is not a singleton unpack but rather itself has a prefix & suffix.
In this case we need to pull out the prefix & suffix by doing a second round of splitting on the middle. Originally we weren't sure if the PEP required implementing this double split, but one of the PEP646 test cases requires it.