> people looking at the low pass rates for mypy and ty and concluding they must not be very useful

Yeah, that would be the wrong takeaway from this blog. The point of the blog was to add context to what the conformance results mean and clarify their limitations, since I saw quite a few people sharing links to the tracker online w/o context.

I dont know, I've seen some truly simple things check out as ok in mypy

Example

def foo(bar: bool) -> bool:

  if bar:
 
    m = True
 
  return m

No error that m is defined conditionally? What's going on?

ty and zuban also don't give an error. pyright and pyrefly do.

We provide this diagnostic in ty (https://docs.astral.sh/ty/reference/rules/#possibly-unresolv...), but it's disabled by default because it can have false positives in many scenarios where the code works at runtime (this is true also in the type checkers that enable it by default). A typical example is code like

  def _(flag: bool):
    if flag:
      x = True
    ...
    if flag:
      read(x)

But it's easy to turn the rule on if you want it:

  [tool.ty.rules]
  possibly-unresolved-reference = "error"