[jorenham]

I have made things!

Several class-scoped generic type parameters were (implicitly) declared invariant, even though these were only used in either input positions (viz. contravariant) or output positions (viz. covariant). As can be seen in the typesafety tests, this could lead to unexpected type-checker complaints about e.g. Unwrappable[bool, ValueError] not being assignable to Unwrappable[int, Exception], even though that's perfectly type-safe.

This all matches the "variance" _{(in type theory this is called "polarity", but in Python we like naming things ourselves I guess)} inference algorithm from the typing spec (https://typing.python.org/en/latest/spec/generics.html#variance-inference) that's used for PEP 695 (py312+) type parameter syntax.

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

#256 (from 2020), kinda.

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[codspeed]

Merging this PR will not alter performance

1 note

ℹ️ 2 benchmarks have no flamegraph

The flamegraph wasn't generated: the symbols of the profiling trace could not be resolved, because no information about the language frames was collected.

See affected benchmarks

✅ 22 untouched benchmarks

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_{Comparing jorenham:typing/variance-fixes (76a57ed) with master (f3a6f54)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (82ef3ef) to head (76a57ed).
⚠️ Report is 656 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #2564    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           80        82     +2     
  Lines         2485      2700   +215     
  Branches       437        45   -392     
==========================================
+ Hits          2485      2700   +215     

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[sobolevn]

Thank you! Very nice!