Check type variable bounds of self types

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#2,351 7 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Start by reading the self-type work referenced in issue #2193 and trace how mypy validates annotations on methods. Use the examples in this issue as focused cases: unbounded T and an incompatible bound should be rejected, while TA bound to A should remain valid. Done means these cases are covered by tests and the requested diagnostics behave consistently.

Written by the indexing model from the issue text.

Description

priority-2-low topic-self-types

As follow-up to #2193, check that if self is declared with a type variable type, the type variable has a compatible bound. For example, some of these should be rejected:

T = TypeVar('T')
TA = TypeVar('TA', bound='A')
TC = TypeVar('TC', bound='C')

class A:
    def f(self: T) -> T: ...  # Should be error
    def g(self: TC) -> TC: ...  # Should be error
    def h(self: TA) -> TA: ...  # OK

class C: pass

(We may also want to check variance, but it's a separate issue and less clear.)

Dominant language
Python
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