join is not associative in the presence of unions

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#3,339 3 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers

Research direction

Start with the two list examples and their reveal_type results, then read the context in issue #3322. The fix is complete when equivalent joins are associative for these union-containing lists and the spurious --strict-optional errors no longer occur.

Written by the indexing model from the issue text.

Description

bug topic-join-v-union topic-strict-optional

While working on #3322 I have found this problem:

lst1 = [1, 'a', None]
lst2 = [1, None, 'a']

reveal_type(lst1) # builtins.list[builtins.object*]
reveal_type(lst2) # builtins.list[Union[builtins.str, builtins.int, builtins.None]]

This turns out to be the cause of spurious --strict-optional errors I mentioned there.

Dominant language
Python
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