Casting away Optional

[MatthiasThul]

(See python/typing#645 for a related discussion.)

Consider the following file test.py:

from typing import TypeVar, Optional

T = TypeVar('T')

def cast_away_optional(x: Optional[T]) -> T:
    assert x is not None
    return x

def foo(i: int) -> Optional[int]:
    if i == 42:
        return i
    return None

x = foo(1)
reveal_type(x)

y = cast_away_optional(x)
reveal_type(y)

assert x is not None
reveal_type(x)

PyType 2020.12.2 gives:

File "/Users/someone/code/pytype/test.py", line 15, in <module>: Optional[int] [reveal-type]
File "/Users/someone/code/pytype/test.py", line 18, in <module>: Optional[int] [reveal-type]
File "/Users/someone/code/pytype/test.py", line 21, in <module>: int [reveal-type]

It is surprising to me that y is not inferred as int while x at the end is.

Mypy 0.790 gives as expected:

test.py:15: note: Revealed type is 'Union[builtins.int, None]'
test.py:18: note: Revealed type is 'builtins.int*'
test.py:21: note: Revealed type is 'builtins.int'

[rchen152]

Seems to be a bug in our union matching (

pytype/pytype/matcher.py

Line 331 in 2254593

elif isinstance(other_type, abstract.Union):

). In matching against Optional[T], once a value matches None, we should return a successful match, rather than going on and matching the value to T as well.