I'm a little bit confused what causes this error `...

# compiler

[smallshen]

I'm a little bit confused what causes this error There's a cycle in the inheritance hierarchy for this type

class Test : Test2.Children {

    sealed interface Children
}

class Test2 : Test.Children {

    sealed interface Children
}

[dmitriy.novozhilov]

Inside a class you can observe all nested classes/interfaces from superclasses
To achieve this compiler need to transitively resolve all supertypes of a class before starting resolution of supertypes of nested classes/interfaces

So here you have a following cycle:
1. start resolution of Test supertypes (Test2.Children)
2. before resolving supertypes of Test2.Children, supertypes of Test2 should be resolved
3. start resolution of Test.Children supertype, its own supertypes should be resolved
4. before resolving supertypes of Test1.Children, supertypes of Test1 should be resolved
5. go to 1

[smallshen]

Thank you, I understand it now. Are there any solutions for this?

[dmitriy.novozhilov]

You need to change the hierarchy
For example, move one of Children interfaces to top-level