Error message about invalid TypeVar definition is cryptic

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
32/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

No source file or test is named. Reproduce the TypeVar example and its current diagnostics first, then trace the validation and error-reporting path for TypeVar definitions. Done should mean the example either produces a clear, actionable diagnostic or accepts the syntax consistently with the requested Python typing behavior.

Written by the indexing model from the issue text.

Description

priority-1-normal topic-usability

With the following

from itertools import groupby
from typing import Iterator, Iterable, TypeVar, Callable, Tuple, Any

X, Key = TypeVar('X'), TypeVar('Key')
def sorted_groupby(sequence: Iterable[X], key: Callable[[X], Key]) -> Iterator[Tuple[Key, Iterator[X]]]:
    return groupby(sorted(sequence, key=key), key=key)

You'll get

a.py:4: error: "object" not callable
a.py: note: In function "sorted_groupby":
a.py:5: error: Invalid type "a.X"
a.py:5: error: Invalid type "a.Key"

It would be nice if TypeVars didn't have restrictions compared to the full python syntax that can otherwise be used in non-typechecked code

Dominant language
Python
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