Use Union for tuple fallback?

[mthuurne]

• Please insert below the code you are checking with mypy,

from typing import Iterable, Iterator, Union
from typing_extensions import Protocol

class MixedIter(Protocol):
    def __iter__(self) -> Iterator[Union[int, str]]: ...

def f(xs: Iterable[Union[int, str]]) -> str:
    return ''.join(str(x) for x in xs)

def g(xs: MixedIter) -> str:
    return ''.join(str(x) for x in xs)

mixedTuple = (56, 'A')
reveal_type(mixedTuple)
reveal_type(mixedTuple.__iter__)
print(f(mixedTuple))
print(g(mixedTuple))

• What is the actual behavior/output?

mixedtuple.py:14: note: Revealed type is 'Tuple[builtins.int, builtins.str]'
mixedtuple.py:15: note: Revealed type is 'def () -> typing.Iterator[builtins.object*]'
mixedtuple.py:17: error: Argument 1 to "g" has incompatible type "Tuple[int, str]"; expected "MixedIter"  [arg-type]

• What is the behavior/output you expect?

I'd expect both f(mixedTuple) and g(mixedTuple) to be accepted without errors.

• What are the versions of mypy and Python you are using?

mypy 0.730 on Python 3.7.2.

[msullivan]

The situation here is that the use of it as an Iterable is enabled as part of a bevy of unfortunate special cases in our subtype checker: https://github.com/python/mypy/blob/master/mypy/subtypes.py#L277-L286.

The type used to figure out things like the type of mixedTuple.__iter__ is the tuple's "fallback type", which is computed using a join: https://github.com/python/mypy/blob/master/mypy/typeops.py#L34

It seems possible that we could switch to using a union for tuple fallbacks instead of a join and drop those special cases subtyping special cases. The normal worry about switching from using a join to using a union--that it will break calls functions with invariant type constructors--seems less pronounced here since tuples are immutable.

I think the main concern would be when lists are created from tuples with list(mixedTuple). But constraints also has its own batch of special casing (https://github.com/python/mypy/blob/master/mypy/constraints.py#L366-L370) that seems to mitigate that...