Better error message for incompatible type assignment when using Generics

• Are you reporting a bug, or opening a feature request?

Feature Request

• Please insert below the code you are checking with mypy.

from typing import Generic, TypeVar


T = TypeVar("T", int, float)


class Foo(Generic[T]):
    def __init__(self, val: T):
        self.data: T = val 


def bar(x: T) -> Foo[T]:
    return Foo(x)


x = bar(1)
x = bar(2.0) # error thrown by mypy

• What is the actual behavior/output?
  mypy throws this error:

error: Argument 1 to "bar" has incompatible type "float"; expected "int"

• What is the behavior/output you expect?

This error message is thrown because of the combination of assigning a different type to x and using Generic. The error message is unclear because it indicates a type error on the argument passed to bar and can be interpreted as bar not accepting arguments of type float.

Having a different error message that mentions "Incompatible types in assignment" would be clearer.
A message to make it clearer that expression has type Foo[float], variable has type Foo[int]

• What are the versions of mypy and Python you are using?

mypy 0.770 and Python 3.7.6

• What are the mypy flags you are using? (For example --strict-optional)

None

Start by reproducing the Generic example with mypy 0.770 and Python 3.7.6, focusing on the assignment from Foo[int] to Foo[float]. Trace the diagnostic produced for the call to bar and assignment to x; done means the message clearly identifies the incompatible variable and expression types rather than implying that bar rejects float arguments.