Literal broadened to superclass erroneously

Bug Report

In some situations, the type checker will broaden a Literal to its superclass, causing type errors.

To Reproduce

Create the following test.py file:

from typing import *

def foo() -> Iterable[Union[Tuple[Literal["foo"], int], Tuple[Literal["bar"], str]]]:
    for i in range(10):
        if i % 2 == 0:
            yield "foo", i
        else:
            yield "bar", str(i)

Expected Behavior

I would expect these type hints to be correct.

Actual Behavior

$ mypy test.py 
test.py:7: error: Incompatible types in "yield" (actual type "Tuple[str, int]", expected type "Union[Tuple[Literal['foo'], int], Tuple[Literal['bar'], str]]")
test.py:9: error: Incompatible types in "yield" (actual type "Tuple[str, str]", expected type "Union[Tuple[Literal['foo'], int], Tuple[Literal['bar'], str]]")
Found 2 errors in 1 file (checked 1 source file)

Here, "actual type" should not be Tuple[str, int] or Tuple[str, str], it should be Tuple[Literal['foo'], int] or Tuple[Literal['bar'], str].

Your Environment

Mypy version used: 0.920
Mypy command-line flags: N/A
Mypy configuration options from mypy.ini (and other config files): N/A
Python version used: 3.9.9
Operating system and version: macOS 10.15.7

Another example of similar behaviour:

from typing import Any, Literal, TypeVar


T = TypeVar('T', bound=Any)


def original() -> Literal["a", "b", "c"] | None:
    return "a"


def get_some(val: T | None) -> T:
    assert val is not None
    return val


val = get_some(original())
reveal_type(val)

Gives:

demo2.py:17: note: Revealed type is "builtins.str*"

...the "literal-ness" of T got lost in the process of narrowing the type from Optional[T] to T.

This appears to have been fixed in mypy 0.93. I think it can be closed.

Fixed in #11236

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Literal broadened to superclass erroneously #12215

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Literal broadened to superclass erroneously #12215

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anentropic commented on Apr 25, 2022

erictraut commented on Aug 13, 2023

hauntsaninja commented on Aug 13, 2023

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