Incorrect type inference on union type, unless using a third property as a discriminant about typescript

[Andarist]

kudos to @ssalbdivad: TS playground

it's not particularly intuitive how to put all the pieces together but this one works very well

[MartinJohns]

Essentially a duplicate of #30581. Not sure why that is closed.

[Andarist]

It's probably closed because it has been determined that it would be too computationally expensive to do it and the current best solution is to use the "indexed mapped type as a union" pattern from TS 4.6 (see here).

However, this discriminant value isn't easily used as a property key so while we can make the internal implementation of a function happy about it... it still doesn't quite infer a concrete K that is used and it doesn't discriminate the input argument for contextual parameters etc: TS playground

Using some intersections we can fix the latter: TS playground. It still doesn't infer K though and I'm not sure how to make true/false a viable inference source for "true"/"false"

[fatcerberus]

It looks like #30581 was auto-closed when #47109 got merged.

[typescript-bot]

This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.