Covariant generic type context results in confusing message with __add__ #5971
Description
This generates a false positive on the last line:
from typing import TypeVar, Generic, Union, Any
T = TypeVar('T')
Tcov = TypeVar('Tcov', covariant=True)
S = TypeVar('S')
class A(Generic[Tcov]): pass
class B(A[T]):
def __add__(self, x: B[S]) -> B[Union[T, S]]: ...
a: A[object]
b1: B[Any]
b2: B[int]
a = b1 + b2 # Unsupported operand types for + ("B[Any]" and "B[int]")The error message is confusing, since B[Any] and B[int] are compatible.
We perhaps shouldn't use the type context from a for type inference in this case.
This issue affects built-in list and Sequence type context with the latest typeshed master.