False positive for disjoint type ranges

_Sorry if that's a known issue._

This code
```
def visit_goals(lst, f):
    for x in lst:
        f(x)

goals = set()
visit_goals([], lambda g: goals.add(g))

def foo():
    return [1, 2, 3]

goals = foo()
for g in goals:
    print(g)
```
causes pytype 2020.4.22 to emit
```
<lambda>: No attribute 'add' on List[int] [attribute-error]
```