[ty] Intersection simplifications with gradual generic specializations

[sharkdp]

Summary

In isinstance(..., C) and TypeIs[..] narrowing, we currently intersect with the top materialization Top[C[Unknown]] of a generic type C. If we want to relax that to an intersection with C[Unknown] directly, then we will not just see intersections like C[P] & C[Unknown] in the if branch, we'll also see intersections like C[P] & ~C[Unknown] in the else branch. If we don't simplify/transform these intersections further, we are currently unable to infer that the bottom intersection of something like list[str] & ~list[Unknown] is Never. So here, we implement the following intersection transformation:

# For C a co-, contra-, or invariant generic class:
C[P] & ~C[Unknown] = C[P] & ~Bottom[C[Unknown]] & Unknown

In this presentation, due to the intersection with a plain Unknown, it is now obvious to ty that the gradual type extends all the way down to Never.

In addition, we also implement these transformations that we have derived before:

Co[P] & Co[Unknown] = Co[P & Unknown]
Contra[P] & Contra[Unknown] = Contra[P | Unknown]

Test Plan

• New and updated Markdown tests.
• Much better ecosystem results on [ty] Experiment: Relaxed isinstance narrowing #26361

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 94.47%. The percentage of expected errors that received a diagnostic held steady at 89.19%. The number of fully passing files held steady at 95/134.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)