Inconsistent behavior with Literals depending on the amount of elements it contains

[ethan-leba]

This is an issue I ran into working on #9097.
Code adapted from this test.

Literals with one value:

from typing import List
from typing_extensions import Literal

lit1: Literal[1]
lit2: Literal[2]

arr2 = [lit1, lit2]

reveal_type(arr2)

➜  mypy git:(master) python3 -m mypy leba_misc/test2.py
leba_misc/test2.py:9: note: Revealed type is 'builtins.list[builtins.int*]'

Literals with multiple values:

from typing import List
from typing_extensions import Literal

lit1: Literal[1]
lit2: Literal[2, 3]

arr2 = [lit1, lit2]

reveal_type(arr2)

➜  mypy git:(master) python3 -m mypy leba_misc/test2.py
leba_misc/test2.py:9: note: Revealed type is 'builtins.list[Union[Literal[1], Literal[2], Literal[3]]]'

On Python 3.7 and mypy 0.790.

[ethan-leba]

As far as I understand this, the inconsistency is stemming from the fact that literals are joined by their fallback types, while Unions are simply merged together, and that mypy treats Literals with multiple values as a Union of literals for each value.

So my thoughts on fixing this would to join Literals in the same way that Unions are joined, but I'm not sure if this would cause problems elsewhere.

[ethan-leba]

Thoughts on this @Michael0x2a? It looks like you wrote the join visitor for literals poking around on git blame