[ty] Handle covariant "deep" mutual typevar constraints

[dcreager]

This is the first step in supporting https://github.com/astral-sh/ty/issues/2045. This handles just the covariant (i.e. easy) case. With a quick list / Sequence switcheroo, we get:

def invoke[A, B](fn: Callable[[A], B], value: A) -> B:
    return fn(value)

def head[T](xs: Sequence[T]) -> T: ...
def lift[T](x: T) -> Sequence[T]: ...

reveal_type(invoke(head, [1, 2, 3]))  # revealed: int | Unknown
reveal_type(invoke(lift, 1))  # revealed: Sequence[Literal[1]]

[astral-sh-bot[bot]]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #22043 will not alter performance

_{Comparing dcreager/generic-horn-clauses (e1f9ba7) with main (cdb7a9f)}

Summary

✅ 22 untouched
⏩ 30 skipped[^skipped]

[^skipped]: 30 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports.

[dcreager]

Lovely. The latest mypy_primer timeout happens on homeassistant/core. I can reproduce locally. In the debug profile, it times out on every run (exit code 124 comes from the timeout command):

$ ./time.sh
15:58:13 124
15:58:58 124
15:59:43 124
16:00:28 124
16:01:13 124
16:01:58 124
16:02:43 124
16:03:28 124
16:04:13 124
16:04:58 124

Under profiling, the times out 30-50ish% of the time:

$ ./time.sh
15:50:38 1
15:51:08 124
15:51:14 1
15:51:20 1
15:51:51 124
15:51:56 1
15:52:27 124
15:52:57 124
15:53:02 1
15:53:32 124

When limited to TY_MAX_PARALLELISM=1, it doesn't time out at all.

(Also no timeouts on main, so this definitely seems to be a regression introduced by this PR)

[MichaReiser]

When limited to TY_MAX_PARALLELISM=1, it doesn't time out at all.

That's scary. It could mean that There's a fixpoint issue (where salsa fails to establish an owner thread). Or does it hang in the sense that cpu usage deops to zero and all threads are blocked?