Hey, I have a question regarding bounds on generic...

# announcements

[alllex]

Hey, I have a question regarding bounds on generic types. ❓

In the Kotlin stdlib there this function:

@SinceKotlin("1.3")
@kotlin.internal.InlineOnly
public inline fun <C, R> C.ifEmpty(defaultValue: () -> R): R where C : CharSequence, C : R =
    if (isEmpty()) defaultValue() else this

You can call it and it works great. However, if I simply copy this function into my code (with intention to later have a slightly modified version), it simply does not compile:

public inline fun <C, R> C.ifEmpty2(defaultValue: () -> R): R where C : CharSequence, C : R =
    if (isEmpty()) defaultValue() else this

Produces error highlighting `CharSequence`:

Type parameter cannot have any other bounds if it's bounded by another type parameter

You can find the example here: https://pl.kotl.in/oGzbGuXah

Why is this the case? Why can’t I use the same features used in stdlib?

[Albert Chang]

It's because kotlin compiler skips the check when @kotlin.internal.InlineOnly annotation is present. Source here.

[alllex]

Thank you for the explanation!

This does not seem fair for the language users though…

[Youssef Shoaib [MOD]]

well there's multiple ways that you can surpass that error for now (because yes eventually this'll be stopped but according to the folks in the Kotlin team useful annotations like inline only will be available to the public somehow):

@Suppress("BOUNDS_NOT_ALLOWED_IF_BOUNDED_BY_TYPE_PARAMETER")

Or for the "future-proof" solution:

@file:Suppress("INVISIBLE_MEMBER, INVISIBLE_REFERENCE")

@kotlin.internal.InlineOnly fun...

Or if you don't want any compiler error suppression, then create a file with the package kotlin.internal and copy over the code for @InlineOnly and then you'll be able to use it right away.