class A has one type parameter, but type B has one

Question

Recently I stumbled across a strange (to me) compiler error message. Consider the following code:

trait Foo {
  type Res <: Foo
  type Bar[X <: Res]
}

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Answer 1

I only have a vague understanding of the exact workings of the type inferrer in Scala so consider this ideas not definitive answers.

1. Type inferring has problems with inferring more then one type at once.

2. You use an existential type in the definition of FOO, which translates to: there exists a type such, not sure if this is compatible with the specific type given in MyFoo