Now this is what I wanted to see: no behavior changes andcolour brought down to 27 minutes from 50 - almost halved!

This is not ready for code review, removing draft status to get some feedback on the idea itself. I do not particularly like usingid(expr) as a cache key, but it should be OK?TempNode is excluded, and everything else should not be subject to ID rot because objects actually remain in the parsed tree for the whole lifetime of checker, their IDs cannot get reused, and we should not construct any nodes other thanTempNode while checking function calls.

That's why I only added caching toinfer_args_in_empty_context. It's a nice place to do that: diagnostics from this call arenever silenced and do not affect current overload resolution. We reject incompatible signatures later, accepting all arguments again anyway. Furthermore, it is only called when checking overload calls andAny calls, so I can safely remove overload stack check from it and replace it with a boolean flag. So I don't think that the generated errors can become relevant later. This might change overload resolution in presence of a globally invalid argument (foo(1 + 'a') wherefoo is overloaded), but any behavior is reasonable in such case IMO: we still emit an error that the argument is invalid itself. To prevent this, it is enough to record whether any errors were encountered, and skip the cache in such case.

Binder is indeed an implicit dependency, but it's still OK within a parent overload: re-accepting anAssignmentExpr should not change anything? Anyway, we can poison the cache invisit_assignment_expr to be extra sure, and that's probably a reasonable thing to do.

I'll add the changes mentioned above in a few hrs, but frankly my main "wtf" here is using IDs as cache keys.

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

Still quite nice in a single sample:

/tmp/mypy_primer/mypy_new/venv/bin/mypy on colour took 1988.16s/tmp/mypy_primer/mypy_old/venv/bin/mypy on colour took 2953.89s