Get rid of make_simplified_union() except in error messages

ilevkivskyi commented 4 years ago

It used to cause various issues and is still causing known bugs (like false negatives in per-file strict optional when map_instance_to_supertype() is involved). Ideally we should replace all calls to it (except in error message formatting code) with UnionType.make_union(). Union constructor already flattens nested unions, we can also make it remove exact duplicates minimize perf impact.

Akuli commented 4 years ago

Ideally there would be a test case to ensure #9169 won't happen again once the refactoring is done

97littleleaf11 commented 3 years ago

I've noticed that make_simplified_union has been modified several times since this issue opened:

https://github.com/python/mypy/pull/9192 adds a fast path for union of literal strings and later https://github.com/python/mypy/pull/9394 extends the fast path to enums
https://github.com/python/mypy/pull/9097/ adds a helper function for combining union of enum literals
https://github.com/python/mypy/pull/10373 fixes instances with last_known_value by adding a helper func is_redundant_literal_instance

We probably need to reconsider the necessity and ways to refactor make_union and make_simplified_union

python / mypy

Get rid of make_simplified_union() except in error messages #8624