Faulty Protocol-TypeVar interactions

🐛 Bug Report

A bit of background: I'm trying the to describe certain operations (e.g. addition) of the numpy.ndarray class.
As the annotations of most operations are highly dependent on the type of underlying scalar, it is easier to let
aforementioned scalars handle all the details and dispatch results the actual ndarray class using protocols.

However, when trying to implementing this I encountered some odd interaction between the, herein created,
__radd__-protocol and a typevar in the scalars' actual __radd__ method:

from typing import Protocol, TypeVar, Generic

# Define a protocol for `__radd__`
T_co = TypeVar("T_co", covariant=True)
T_contra = TypeVar("T_contra", contravariant=True)

class SupportsRAdd(Protocol[T_contra, T_co]):
    def __radd__(self, other: T_contra) -> T_co: ...

With the protocol in hand we can define our scalar (e.g. bool_) and the array used for embedding the scalar:

class generic: ...  # baseclass for numpy scalars

T1 = TypeVar("T1", bound=generic)
T2 = TypeVar("T2", bound=generic)

class bool_(generic):
    def __radd__(self, other: T1) -> T1: ...
    def __add__(self, other: T1) -> T1: ...

class ndarray(Generic[T1]):
    def __add__(self, other: SupportsRAdd[T1, T2]) -> ndarray[T2]: ...

So far so good.
Additions involving ndarray[bool_] should now now be described by the underlying bool_, which is more or less what happens:

array: ndarray[bool_]
scalar: bool_

# note: Revealed type is 'op.bool_*'
reveal_type(scalar + scalar)

# error: Value of type variable "T1" of "__radd__" of "bool_" cannot be "ndarray[bool_]"  [type-var]
# note: Revealed type is 'op.ndarray*[op.bool_]'
reveal_type(array + scalar)

In both cases the return-type is correctly inferred, but in the array + scalar example it is accompanied by an error (a false positive?). Note removing the typevars in bool_.__add__ & .__radd__ does get rid of the error, but that's not exactly a viable solution due the sheer amount of generic subclasses.

Expected Behavior

Reveal the return type without any errors.

Actual Behavior

The return type is revealed in addition to producing a [type-var]-related error.

Your Environment

• Mypy version used: mypy 0.782
• Mypy command-line flags: --show-error-codes (including or excluding --strict makes no difference here)
• Mypy configuration options from mypy.ini (and other config files): n.a.
• Python version used: Python 3.8.3
• Operating system and version: macOS 10.15.6

Inizia eseguendo con mypy e --show-error-codes il reproducer fornito per Protocol, TypeVar, Generic, ndarray e bool_. Analizza l’interazione tra add e radd che emette l’errore type-var; il lavoro è completato quando il tipo restituito rivelato rimane corretto senza produrre tale errore.