Feature request: Detect type mismatch between pattern-matched variable and cases

[smheidrich]

Feature

It would be great if Mypy could detect and report as errors case patterns that are not possible given the type of the variable being matched on.

Pitch

Consider e.g.:

class A:
  ...

class B:
  ...

a: A = A()

match a:
  case B():
    ...

We know with certainty that variable a is of type A, so having a case to check whether it's an instance of B (which is not a subtype of A) makes no sense and indicates a type error. Yet the latest Mypy version (1.5.1) doesn't report it as one. In my opinion, it would be better if it did (and that would have been my expectation before I tested it), given that all necessary type information is explicit & static and doesn't even have to be inferred.

Objections I could think of

One could object that this isn't that different from a feature request asking that similarly "impossible" isinstance checks be treated as errors, but I would say they are different in that match/case is part of the syntax, while isinstance is "just a function" and giving its arguments special treatment (= extra type checking that takes into account its semantics) would seem odd.

Related issues

• Reachability/exhaustiveness checking for match statements #12010

[ikonst]

We have a check for unreachable code which also covers impossible isinstance checks. The thing is, unless A is @final, you can have C(A, B) such that

a: A = C()

[smheidrich]

Damn, I hadn't thought of that, sorry :-/ Closing this then, thanks for the quick response!