Why in this code type `GT` cannot be inferred ``` class A lt T gt val t T class B lt T out U A lt T gt gt val t T val at U fun lt T gt B lt T gt withCapturedU B lt T A lt T gt gt = this private fun lt FT gt f x B lt FT gt g x ERROR why g x withCapturedU OK private fun lt GT gt g y B lt GT A lt GT gt gt println y ```

class A<T>(val t: T)
class B<T, out U: A<T>>(val t: T, val at: U)

fun <T> B<T, *>.withCapturedU(): B<T, A<T>> = this

private fun <FT> f(x: B<FT, *>) {
  g(x) // ERROR, why?
  g(x.withCapturedU()) // OK
}

private fun <GT>  g(y: B<GT, A<GT>>) {
  println(y)
}

Andreas Sinz

01/04/2018, 3:00 PM

seems to put the compiler off. if I change it to

x: B<FT, A<FT>

it works fine

@snrostov ok, its not the

, but rather the variance of your generics, if you define

inside

and

out

, it works fine

snrostov

01/04/2018, 5:17 PM

@Andreas Sinz, yes, but why first substitution in compiller error message is not suitable? Moroover it is not highlighted as red. Looks like compiller (or at least diagnostic formatting) error. And why variant using `withCpaturedU`is working?

Andreas Sinz

01/04/2018, 6:14 PM

@snrostov because

can be

A<FT>

or any subclass of

A<FT>

together with

FT

being invariant means the compiler is not able to ensure that you'll always have

A<GT>

inside your

B<GT, *>

. not sure about the red highlighting

snrostov

01/05/2018, 10:33 AM

Ah, I see now. Thanks for explanation!

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