mypy

7141d6bc - More principled approach for callable vs callable inference (#15910)

More principled approach for callable vs callable inference (#15910) Fixes https://github.com/python/mypy/issues/702 (one of the oldest open issues) The approach is quite simple, I essentially replicate the logic from subtyping check, while replacing each `is_subtype()` call with `infer_constraints()` call. Note that we don't have various options available in `constraints.py` so I use all checks, even those that may be skipped with some strictness flags (so we can infer as many constraints as possible). Depending on the output of `mypy_primer` we can try to tune this. Note that while I was looking at subtyping code, I noticed couple inconsistencies for ParamSpecs, I added TODOs for them (and updated some existing TODOs). I also deleted some code that should be dead code after my previous cleanup. Among inconsistencies most notably, subtyping between `Parameters` uses wrong (opposite) direction. Normally, `Parameters` entity behaves covariantly (w.r.t. types of individual arguments) as a single big argument, like a tuple plus a map. But then this entity appears in a contravariant position in `Callable`. This is how we handle it in `constraints.py`, `join.py`, `meet.py` etc. I tried to fix the left/right order in `visit_parameters()`, but then one test failed (and btw same test would also fail if I would try to fix variance in `visit_instance()`). I decided to leave this for separate PR(s).