Limitation of Higher Kinded Inference

higher-kinded-inference-bug.scala

trait PartialApply1Of2[T[_, _], A] {
  type Apply[B] = T[A, B]
}

trait Foo[M[_]]

def foo[M[_]]: Foo[M] = null

// Why can't the type parameter be inferred here?
//
// (fragment of pure-bind-monad-state.scala):9: error: polymorphic expression cannot be instantiated to expected type;
//  found   : [M[_]]this.Foo[M]
//  required: this.Foo[[B]scala.collection.immutable.Map[Int,B]]
// foo: Foo[PartialApply1Of2[Map, Int]#Apply]  
//  ^
// one error found
// !!!
// discarding <script preamble>
foo: Foo[PartialApply1Of2[Map, Int]#Apply]  

//
// Explicitly passing the Type Constructor PartialApply1Of2[Map, Int]#Apply works.
//
foo[PartialApply1Of2[Map, Int]#Apply]: Foo[PartialApply1Of2[Map, Int]#Apply]  

//
// The Type Constructor List[_] is inferred here.
//
foo: Foo[List]