How to impose generic constraints to the parameters and return values of a function argument of a ge-CodePudding

I want to define a Kotlin generic function that takes a lambda as a parameter, but I want to restrict the allowed types of lambdas parameters and return types. How do I do this in Kotlin?

In the following example, I expected the constraint: where T: Base, V: (T) -> Unit to mean that V can only be functions whose first parameter implements Base.

However, I see that the compiler ignores the T: Base part of the constraint and will accept any (Any) -> Unit.

interface Base

fun <T, V> exampleGenericFunction(func: V) where T: Base, V: (T) -> Unit {
    println("func is $func")
}

class ImplementsBase : Base

class DoesNotImplementBase

fun main() {
    val f1: (ImplementsBase) -> Unit = { }
    exampleGenericFunction(f1)
    
    val f2: (DoesNotImplementBase) -> Unit = { }
    exampleGenericFunction(f2) // expected this to be a compilation error
}

CodePudding user response：

This unexpected behavior was caused by a bug in the Kotlin >=1.4 compiler. This bug can be tracked here: https://youtrack.jetbrains.com/issue/KT-48935.