Example from
box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint
// FILE: TestJ.java
public class TestJ {
public static <T> In<T> materialize() { return null; }
}
// FILE: test.kt
class In<in T>
fun <T> inferred(e: In<T>?, l: () -> T): T = l()
fun box() {
inferred(TestJ.materialize<Unit>(), { null })
}
`materialize` has flexible type, both for `In<T>` and `T`.
When analyzing `{ null }`, collected type constraints include:
ft<Unit?, Unit> <: T (from ft<In<ft<Unit, Unit?>>, In<ft<Unit, Unit?>>?>)
By allowing the lower bound of flexible type, FIR resolution can visit
`{ null }` with the expected type Unit, which will lead to proper
coercion to Unit at the end.
Jinseong Jeon
0ea6b32c01