Unbound TypeVar apparently undetected in function overload

[AlexWaygood]

I just merged a PR over at typeshed to improve the stub for builtins.sum. The first draft of the PR proposed defining sum like this:

import sys
from typing import *

class _SupportsSum(Protocol):
     def __add__(self, __x: Any) -> Any: ...

 _SumT = TypeVar("_SumT", bound=_SupportsSum)
 _SumS = TypeVar("_SumS", bound=_SupportsSum)

 @overload
 def sum(__iterable: Iterable[_SumT]) -> Any | Literal[0]: ...

if sys.version_info >= (3, 8):
     @overload
     def sum(__iterable: Iterable[_SumT], start: _SumS) -> _SumT | _SumS: ...

 else:
     @overload
     def sum(__iterable: Iterable[_SumT], __start: _SumS) -> _SumT | _SumS: ...

I think this should have triggered pyright's reportInvalidTypeVarUse check, as the TypeVar in the first overload is only used once. However, pyright reported no errors in our CI check.

(The PR was revised to correct this mistake before it was merged.)

[erictraut]

The reportInvalidTypeVarUse exempts constrained and bound TypeVars because those have legitimate (or at least well-defined) uses when only one instance appears within a signature. Since this is a bound TypeVar, the check didn't emit a diagnostic in this case.

[AlexWaygood]

Thanks!