Disjoint unions with exact types - poor inference of typeof undefined check

[Andarist]

I honestly dont know if Im right, but this one seems easily inferrable and I just cannot make it work. It might be a bug (cannot know that for sure) or maybe there is some reason why this is happening and Im just not aware of the problem in my code.

Would be greatly appreciated if somebody can clarify things up for me. From my point of view this should behave quite exactly the same as strict shapes example in the docs here

EDIT:// I got it to work (see below), but I have no idea whats the difference between those 2:

• m.iwcs !== undefined (working)
• typeof m.iwcs !== 'undefined' (not working)

Shouldnt they both just work? Also started testing this code branch with such

• typeof m.iwcs === 'string'
• 'iwcs' in m

and they are not working too, which is rly odd

Original problematic code
Working example

[rkurbatov]

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgAeAUgMoDyAcmALxgDeAPmAFYDOcAdgFxjkBGLPAGMMYRgF9MuAgEkA6gGFStBswCWCYW15sMAJ3WcA5uKnZ8YALJ42bAIbGCdEhWrMFygNzphXPWA49vpseDZ2js5gABQAtrYOTjpgAIL6+vZYADzhiXgAfACUvGkZ2a5U+bRV8RFJAHSx9jjRqGAxsby5kYXVMfRt7UND7Fy8sfWa2mAA-GBkVPVBIXhxk1ps9Ww4MOoY0QDkAHoHhQDaAIwAur3j9aOc4sz0UsPDEr1vqIVAA

this works for me. And agree, union types is the most problematic part of flow. It's interesting to check how it behaves in older version. We're trying to switch to the new flow since 0.44 (we're using 0.43.1 currently) but still have a lot of warnings on redux actions with union typing.

UPD: Just checked your version with flow 0.43.1, the same errors. And my version still works there. So even if it's a bug, it's not a new bug.

[Andarist]

@rkurbatov thanks for the answer! I finally managed to make it work, but some things still aint working like expected (see EDIT note in the original post)

[rkurbatov]

Yeah, seems like if (m.iwcs) and if (m.iwcs !== undefined) are equivalent. Looks like a bug.

[Andarist]

not rly - m.iwcs is not working, while m.iwcs !== undefined is the only one making it to work - while other mentioned checks should be OK too

m.iwcs is not working, as it shouldnt - because empty string wouldnt go into this block of the code, which is responsible for handling that case

[toxic-johann]

bump into same problem here.
my emample

flow can't do well in Disjoint unions with exact types