fix Constrained TypeVar lost after binop #3220

[asukaminato0721]

Summary

Fixes #3220

constrained TypeVar binary ops preserve the original quantified T when evaluating over constraints produces the same constraint set.

Test Plan

add test

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[Copilot]

Pull request overview

Fixes a type inference bug in Pyrefly where constrained TypeVars could be widened to their constraint union after binary operations, leading to false-positive operator errors (issue #3220).

Changes:

• Add a regression test covering constrained TypeVar preservation through chained binops.
• Preserve constrained TypeVar quantification in binop_types when the computed result matches the TypeVar’s constraint union.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/test/operators.rs	Adds a regression testcase reproducing the constrained-TypeVar binop scenario from #3220.
pyrefly/lib/alt/operators.rs	Adjusts binop type computation to keep constrained TypeVars quantified when binop results equal their constraint union.

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