Union of different-length Tuple types should agree for corresponding items

[gvanrossum]

See python/typeshed#721 (comment). The issue is that after

T = Union[Tuple[int, int], Tuple[int, int, str]]
def f() -> T: pass
a = f()
reveal_type(a[0])
reveal_type(a[2])

the item types are considered to be Any. A better approach would be to understand that a[0] is always an int while a[2] is invalid without some check (however we can't do isinstance() checks on tuple lengths so that's problematic).

[JukkaL]

We could support len(x) checks for tuple unions: #1178

[rwbarton]

I'm a little surprised this doesn't go through the path for a method call on a union. For example, this works fine:

class A:
    def f(self) -> int: pass

class B:
    def f(self) -> int: pass

U = Union[A, B]
def g() -> U: pass

b = g()
reveal_type(b.f())   # Revealed type is 'builtins.int'

It might just take some code reorganization (translate the IndexExpr into a __getitem__ method call, then handle __getitem__ on tuples in check_op).