Feature requests: recognise that some types always evaluate to False

[reinhrst]

• Are you reporting a bug, or opening a feature request?
  Feature Request
• Please insert below the code you are checking with mypy,

import typing as t


class NotPresent:
    def __bool__(self) -> t.Literal[False]:
        return False


x: t.Union[NotPresent, t.Sequence[int]] = [1]
reveal_type(x)
if x:
    print(x[0])

• What is the actual behavior/output?
  test.py:12:11: error: Value of type "Union[NotPresent, Sequence[int]]" is not indexable

• What is the behavior/output you expect?
  The if x branch should recognise that x cannot be of type NotPresent (as it would do if it was None rather than NotPresent)

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

• What are the mypy flags you are using? (For example --strict-optional)
  None

[povilasb]

This is awesome.
A funny thing is that I posted exactly the same question on Stackoverflow yesterday :)

If I were to contribute some of my precious time, how would I approach this problem given I haven't worked with mypy source code?

[hauntsaninja]

Might need some discussion, since you could subclass NotPresent to do something else.

[reinhrst]

I think that (rightly) mypy does not allow you to subclass NotPresent with a __bool__ method with a more permissive returntype than t.Literal[False], right?

[hauntsaninja]

Yup, you're right. I believe this is also fixed on master by #9297