python
Argument type `Union[T, List[T]]` fails to infer constaints
While working on #11128 I got hit by this.
This code does not type check:
from typing import TypeVar, Union, List
T1 = TypeVar("T1")
def promote_list(item: Union[T1, List[T1]]) -> List[T1]:
...
exprs: List[int]
reveal_type(promote_list(exprs)) # error
# ex.py:9: note: Revealed type is "builtins.list[<nothing>]"
# ex.py:9: error: Argument 1 to "promote_list" has incompatible type "List[int]"; expected "List[<nothing>]"
reveal_type(promote_list(1)) # ok
# Revealed type is "builtins.list[builtins.int*]"
Which is not what I expect. For example, TypeScript solves this properly:
function protomoteList<T>(input: T | T[]): T[] {
return []
}
function test(input: number[]) {}
test(protomoteList(1)) # ok
Link: https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABABwE5ynAthgpgGRgGcoAeAFQD4AKGMZEKALkXMQB9WBtAXQEoW5XogDeAWABQiaYlS4oIVEl6SAvpMmhIsBIii4SteoxZgQWAEa5UvPqPUTJ+w2gzY8hQwEY+fSUA
Cause
It happens somewhere here: https://github.com/python/mypy/blob/b3ff2a683bd41a176c597d3c3ba65f6a31f805c5/mypy/constraints.py#L168-L171
I will try to solve this, but I know that this is going to be complicated.
Adding an upper bound on T1 fixes this, see https://github.com/python/mypy/issues/6417. Although I agree it would be great if this worked without a bound. Proper solution for this is complex however.
