Allow non-literals when indexing uniform tuples

[JukkaL]

Currently mypy doesn't allow indexing fixed-length tuples using a non-literal index. We should probably support this at least for tuples with homogeneous types.

For example, code like this should probably be okay:

t = (1, 3, 7)
for i in range(len(t)):
    print(i, t[i])

The example below is actually already valid so rejecting the above code is inconsistent (this works due to how we set the fallback attribute for tuples to be Tuple[t, ...]):

t = (1, 2, 3)
for i, x in enumerate(t):
    print(i, x)

[JukkaL]

The relevant bit of code is visit_index_expr_helper in mypy/checkexpr.py.

[o11c]

I think there's a good argument for allowing it for all tuples, just with a return type of Union[T, U, V...].

Rather than forcing the error immediately, let the error be deferred if necessary.

[momandine]

I had a generator expression that constructed a tuple, that I knew from context would be of length 3

self.tpl = tuple(int(x) for x in some_string.split('.'))  # type: Tuple[int, int, int]

And then later __getitem__ that indexed into the tuple

def __getitem__(self, key):
    # type: (int) -> int
    return self.tpl[key]  

There were two mypy errors:

error: Incompatible types in assignment (expression has type Tuple[int, ...], variable has type "Tuple[int, int, int]")
error: Tuple index must be an integer literal

The first is easier to understand, the latter I found very confusing because it went away as soon as I typed the tuple-generator exception without a specific length, e.g. Tuple[int, ...]. Why would indexing into a less-specific tuple be ok but a fixed-length one bad?