Avoid triggering reportUnreachable on exception raising?

[hauntsaninja]

(Acknowledging that this is a new and disabled-by-default rule)

Currently pyright will trigger reportUnreachable on the following code:

def foo(x: int | str):
    if isinstance(x, int):
        print("int")
    elif isinstance(x, str):
        print("str")
    else:
        raise ValueError  # error

mypy's version of the same rule ignores unreachable statements that are of a small set of forms, like raise (expr) or assert False, or statements are equivalent to a correctly used assert_never: https://github.com/python/mypy/blob/master/mypy/checker.py#L3139-L3165

This makes these diagnostics substantially higher signal

[erictraut]

If we're going to add exemptions to this rule, I'd prefer that it's based on a principle rather than an ad-hoc list of statement forms. Otherwise, it's hard to defend because it's somewhat subjective. Can you think of a way to define a general principle from which we could derive the list of exemptions?

[hauntsaninja]

I think "ignore unreachable statements that always raise" is the general principle. This covers raise (expr), assert False and correctly used assert_never (and correctly used assert_never-like user-defined functions)

(Mypy also ignores pass and ... statements. These do not fit with that principle and excluding them doesn't make a difference in codebases I'm familiar with)

[erictraut]

Yeah, I like that.

[randolf-scholz]

I think "ignore unreachable statements that always raise" is the general principle

Would that cover pytest.xfail() ? Or more generally, any function call that returns Never?

[hauntsaninja]

I'm proposing removing the exception for pass and ... statements in mypy in python/mypy#20488 , which will mean that mypy's behaviour matches the principle outlined here

Or more generally, any function call that returns Never?

Yes, not issuing unreachable diagnostics for these calls is my intention, and is mypy's current behaviour