[red-knot] Check the return type of dunder methods

[MichaReiser]

Description

Dunder methods like __bool__, __len__ (and binary/compare?) should return a value that is assignable to what's expected by their "convention". We currently don't check this.

[AlexWaygood]

I'm still not sure that this is actually something a type checker should check, rather than a type-aware linter. The type checker should check (where necessary/appropriate) that the return type is correct when these methods are (explicitly or implicitly) called. But if they're never called, there's no type safety issue. And we already have linter rules to guard against these antipatterns:

• https://docs.astral.sh/ruff/rules/invalid-length-return-type/
• https://docs.astral.sh/ruff/rules/invalid-bool-return-type/
• https://docs.astral.sh/ruff/rules/invalid-index-return-type/
• https://docs.astral.sh/ruff/rules/invalid-str-return-type/
• https://docs.astral.sh/ruff/rules/invalid-bytes-return-type/
• https://docs.astral.sh/ruff/rules/invalid-hash-return-type/

[MichaReiser]

Yeah sorry. That's what I meant. We should check the return type when implicitly calling those operations and error if the dunder is incorrectly implemented.

[AlexWaygood]

Okay, great! For __len__, we have a TODO here for this that's relevant for this issue:

ruff/crates/red_knot_python_semantic/src/types.rs

Line 1519 in e84985e

// TODO: Emit diagnostic for non-integers and negative integers

(and binary/compare?)

In general, these can return arbitrary objects and the call still succeeds, so there's nothing more for us to do on these, I don't think:

>>> class Foo:
...     def __lt__(self, other):
...         return Foo()
...         
>>> Foo() < Foo()
<__main__.Foo object at 0x1031cca50>

You could argue it's a bit of an antipattern to return a non-bool from these methods, but again, it's sort of a linter issue than something for a type checker to flag

[carljm]

Yeah I think this would apply to at least __len__, __bool__, __str__, __bytes__ and __repr__. Not sure if there are others I'm forgetting, haven't cross-checked with a comprehensive list of dunders.