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Unwrap type[Union[...]] when solving typevar constraints #18266

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sterliakov
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Closes #18265, closes #12115.

type[A | B] is internally represented as type[A] | type[B], and this causes problems for a typevar solver. Prevent using meet in such cases by unwraping type[...] if both sides have such shape.

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Primer diff is fine, it was a false positive.

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Thanks for the PR! I had a few comments but looks good overall.

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Hm, and this new diff is weird. I'll try to figure out the reason for the colour diff - don't merge yet.

… be unwrapped, so type should be restored when passing it further
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The upstream code has changed since, but I checked the previous version locally and confirmed the bug is gone. This should be ready for another review now.

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Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/annotations.py:1064: error: Incompatible return value type (got "object", expected "User | Role | Snowflake | _T")  [return-value]

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