`==` Narrows the type of an Enum in an if-elif block when following an `is`

[parsons-kyle-89]

is and == work strangely together when narrowing the type of an Enum in an if-elif block. It is known that you need to use is rather than == to narrow the type of an Enum, but it seems if the if statement uses is then the elif can have == and still narrow the type of the variable.

To Reproduce

from enum import Enum

class Foo(Enum):
    A = 1
    B = 2
    C = 3

def f0(x: Foo) -> str:
    if x is Foo.A:
        return 'one'
    elif x == Foo.B:
        return 'two'
    elif x == Foo.C:
        return 'three'
    reveal_type(x)  # Not revealed by mypy as it apparently considers this line unreachable

This is compared to how have come to expect == to work with an enum where it will not narrow the type.

def f1(x: Foo) -> str:
    if x == Foo.A:
        return 'one'
    elif x == Foo.B:
        return 'two'
    elif x == Foo.C:
        return 'three'
    reveal_type(x)  # Revealed type is Foo

Interestingly this doesn't just occur when an is is used in the if. It can occur in an elif and still "propogate" its narrowing ability to the following =='s.

def f2(x: Foo) -> str:
    if x == Foo.A:
        return 'one'
    elif x is Foo.B:
        return 'two'
    elif x == Foo.C:
        return 'three'
    reveal_type(x)  # Revealed type is Literal[Foo.A]

Expected Behavior

== would either narrow the type of an enum or not and it would not depend on whether or not it was preceded by an is.

Actual Behavior

== narrows the type of an enum only when it is preceded by an is.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: none or --strict will both demonstrate this behavior
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
disallow_any_decorated = True
disallow_any_explicit = True
disallow_any_generics = True
disallow_subclassing_any = True
disallow_untyped_defs = True
disallow_untyped_calls = True
disallow_incomplete_defs = True
disallow_untyped_decorators = True

• Python version used: 3.9.0
• Operating system and version: Ubuntu 20.04.1 LTS under WSL

[AlexWaygood]

Closing in favour of #10915, which is a newer issue but has more discussion attached to it.