TypedDict accepts an Enum member as a key and reveals the value type, but KeyErrors at runtime

[silviogutierrez]

mypy lets you index a TypedDict with an Enum member, and reveal_type gives the correct value type. But it's a KeyError at runtime. Under the hood mypy substitutes the member's .value for the key, a substitution it refuses to make anywhere else.

To Reproduce

from enum import Enum
from typing import Literal, TypedDict

class Foo(Enum):
    One = "One"

class TD(TypedDict):
    One: int

td: TD = {"One": 1}
d: dict[str, int] = {"One": 1}

reveal_type(td[Foo.One])       # Revealed type is "int"  -- accepted
x: Literal["One"] = Foo.One    # error: incompatible types  -- member is not the literal
d[Foo.One]                     # error: invalid index type  -- not a valid str key

Only the TypedDict line gets through. The other two are rejected, and correctly so: Foo.One is Literal[Foo.One], not Literal["One"], and it isn't a str. So mypy already knows the member isn't its value, everywhere except TypedDict subscription.

At runtime a plain Enum member doesn't compare equal to its value, so the "type-safe" access is the thing that breaks:

>>> td[Foo.One]
KeyError: <Foo.One: 'One'>

Expected Behavior

td[Foo.One] should be an error, the same way d[Foo.One] on a dict[str, int] is. pyright rejects it: "Could not access item in TypedDict."

Actual Behavior

Accepted. reveal_type is int. KeyError when you run it.

One wrinkle worth noting: the only case where the runtime agrees with mypy is a str-mixin enum (StrEnum, or class Foo(str, Enum)), because there the member genuinely is the string. So the acceptance happens to be sound for str enums and is a false negative for every other Enum.

Your Environment

• mypy 2.2.0 (compiled). Reproduces with and without --strict.
• Python 3.12.8
• No flags beyond the above; no plugins.
• For comparison: pyright 1.1.411 flags it.