Data flow-based exhaustiveness checking

[serras]

Exhaustiveness checking as currently implemented in Kotlin works in a local fashion, taking into account only the branches of the when being inspected. We propose to take a more global approach, by augmenting the current smart casting framework with negative information.

Link to the proposal text

ℹ️ This proposal is already implemented as part of 2.2.20. We initially saw this as just a "compiler improvement", but given that the scope has ended up being broader, we're publishing this to get additional community feedback.

[kyay10]

Any chance this will extend to contracts? Having a returns(true) implies (this@foo !is Bar) work in the same fashion might be useful

[serras]

Yes, contracts will support !is in the same way as is in the current form.