Add support for conditionally defined overloads

[cdce8p]

Description

This PR allows users to define overloads conditionally, e.g., based on the Python version. At the moment this is only possible if all overloads are contained in the same block which requires duplications.

from typing import overload, Any
import sys

class A: ...
class B: ...

if sys.version_info >= (3, 9):
    class C: ...


@overload
def func(g: int) -> A: ...

@overload
def func(g: bytes) -> B: ...

if sys.version_info >= (3, 9):
    @overload
    def func(g: str) -> C: ...

def func(g: Any) -> Any: ...

Closes #9744

Test Plan

Unit tests have been added.

Limitations

Only if is supported. Support for elif and else might be added in the future. However, I believe that the single if as shown in the example is also the most common use case.

The change itself is fully backwards compatible, i.e. the current workaround (see below) will continue to function as expected.

Update: Seems like support for elif and else is required for the tests to pass.

Update: Added support for elif and else.

Current workaround

from typing import overload, Any
import sys

class A: ...
class B: ...

if sys.version_info >= (3, 9):
    class C: ...


if sys.version_info >= (3, 9):
    @overload
    def func(g: int) -> A: ...

    @overload
    def func(g: bytes) -> B: ...

    @overload
    def func(g: str) -> C: ...

    def func(g: Any) -> Any: ...

else:
    @overload
    def func(g: int) -> A: ...

    @overload
    def func(g: bytes) -> B: ...

    def func(g: Any) -> Any: ...

[cdce8p]

@JukkaL Maybe you would like to take a look at this, too? I believe it could be another good usability improvement, especially considering that pyright already supports it.

[JukkaL]

Sorry for the slow response! Left a question about elif/else blocks. Overall it seems like this could be a useful feature, especially in stubs.

[JukkaL]

What happens in cases like this:

@overload
def f(x: A) -> A: ...
if x:
    @overload
    def f(x: B) -> B: ...
elif y:
    @overload
    def f(x: C) -> C: ...
else:
    @overload
    def f(x: D) -> D: ...

Consider all combinations of bool values for x and y. For example, if x is always false but y is always true, should we pick up C -> C from the elif block? We should have a test case that covers this. If this isn't supported, there should be a useful error message.

[97littleleaf11]

Any progress on this?

[cdce8p]

Any progress on this?

@97littleleaf11 I hadn't had much time to look at it so far. Sorry about that. If it's urgent I can look at it sooner but from what I've seen the cutoff for the next release has already happened. I hope to address the comment next week.

[97littleleaf11]

@cdce8p Oh, I was checking the status of existing PRs. Just take your time.

[JelleZijlstra]

Why is this needed?

[cdce8p]

Fixed in 3d0397a. Starting If stmts are no longer merged if they don't contain at least one overload.

[JelleZijlstra]

What if maybe_true is defined but of type bool? It seems like you just silently ignore all conditionally defined overloads. It would make more sense to me to produce an error.

[cdce8p]

914d517, mypy would now emit an error for it.

Condition cannot be inferred, unable to merge overloads [misc]

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[cdce8p]

Thanks for taking the time to review my PR @JelleZijlstra!
I've been looking forward to this one for a long time.