I was working on a simple project with Kotlin in IntelliJ IDEA, when I encountered an odd compile-time error by the IDE, which read:

Type argument is not within its bounds.

This was being shown for a type argument that was within its bounds!

It took a while to produce an MRU for the error, and here it is:

fun interface I { fun f() }

annotation class A<T : I>

@A<C>
class C : I { override fun f() {} }

In this code, as you can see, C is a subtype of I, but the IDE still shows a compile-time error at the C in the @A<C> line, which says:

Type argument is not within its bounds.
Expected: I
Found: C

This is clearly a bug.

The weird part is, the bug goes away if I is not a fun interface and is just a simple interface. Very strange.

My question is, what do I do now? Is this a bug or is my code wrong?