Concepts: a type matching a concept must be forward declared even in a single type section

[mratsim]

Even within a single type section which is supposed to accept any order of declaration a type matching a concept must be forward declared.

Example

type
  Enqueueable = concept x, type T
    x is ptr
    x.next is T

  Queue[T: Enqueueable] = object
    front: T
    back: T

type

  # Declaring before works

  # Val = ptr object
  #   next: Val
  #   val: int

  Foo = object
    queue: Queue[Val]

  # Declaring after fails to match the concept

  Val = ptr object
    next: Val
    val: int

Note that in that caise the {;explain.} pragma does not explain anything.