Check covariant variable used in an invariant position in a return type

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
52/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
python
Domain
compilers, devtools

Research direction

Start by locating the existing checks for covariant and contravariant type variables in return annotations, then find the related diagnostic tests. Use the issue's Generic[CV] example with List[CV] as the initial case, and add coverage for the analogous contravariant rule if the existing checks support it. Done means invariant positions are rejected with tests covering both variance directions.

Written by the indexing model from the issue text.

Description

bug priority-1-normal topic-type-variables

Covariant type variable should not be used in the return type in an invariant position, for example like this:

CV = TypeVar('CV', covariant=True)

class A(Generic[CV]):
    def foo(self) -> List[CV]: ...   # Error, should be 
        # Sequence[CV] or similar, since List is invariant

There is probably a similar rule for contravariance.

Dominant language
Python
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