Why does this Kotlin code with generics produce a Type Mismatch compiler error?

Question

Misc.kts contains:

class E
fun <B> foo(): B {
    return E() // <--- Error here
}
fun bar() {
    val r = foo<E>()
}

Error is Type mismatch: inferred type is Misc.E, but B was expected. This compiler behavior surprises me. I'd be grateful for an explanation.

Alexander · Accepted Answer · 2019-05-18 21:09:54Z

6

You can think of generic functions as cookie cutters for functions. For any value of B, your generic function definition can stamp out a different function cookie.

There is one value of B that can make return E() legal. That's when B is E, which is the case in the call too foo<E>(). However, this is the only such case. What happens if I call foo<Int>()? You can't return an E() when an Int is expected.

For your function definition to be valid, it's insufficient for there to merely exist one return value which is compatible with B. Your returned value must be compatible with all possible values of B.

answered May 18, 2019 at 21:09

Alexander

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