[Pitch] Property-Based Type Narrowing and Union Types

[bbrk24]

@ensan-hcl

var value: Int? = 42
// some process
if value != nil {
    // some process     
    // finally make value nil
    value = nil
}

Kotlin works around this problem by only allowing this when value is an immutable local or stored variable, and not when it’s mutable or has a getter. In Swift terms, only lets can do narrowing here, not vars (and obviously not return values from functions).