recursive default call in type sections: invalid type in this context for proc

[mratsim]

The following doesn't compile

type
  Enqueueable* {.explain.} = concept x, type T
    x is ptr
    x.next is T

  Queue*[T: Enqueueable] = object
    count: int
    back: T
    front: typeof(default(T)[])

  Foo = ptr object
    next: Foo

  FooWrapper = ptr object
    next: FooWrapper
    foo: Foo
    bar: Bar

  Bar = ptr object
    queue: Queue[FooWrapper]

While I use concept here I think it's related to the nested default calls.

This builds up on the following issues:

• the lack of proper dereference at the type level
• https://github.com/nim-lang/Nim/issues/12714
• https://github.com/nim-lang/Nim/issues/13046 Changing typeof to type will lead to #12714 and changing type declaration order will lead to #13046

[mratsim]

The following custom deref prevents the issue from happening:

macro deref*(T: typedesc): typedesc =
  echo T.getTypeInst.treerepr
  let instantiated = T.getTypeInst
  instantiated.expectkind(nnkBracketExpr)
  doAssert instantiated[0].eqIdent"typeDesc"

  let ptrT = instantiated[1]
  let ptrTImpl = instantiated[1].getImpl

  ptrTimpl.expectKind(nnkTypeDef)
  ptrTImpl[2].expectKind(nnkPtrTy)
  ptrTImpl[2][0].expectKind(nnkObjectTy)

  result = getType(ptrTImpl[2][0])

type
  Enqueueable* {.explain.} = concept x, type T
    x is ptr
    x.next is T

  Queue*[T: Enqueueable] = object
    count: int
    back: T
    front: deref(T)

  Foo = ptr object
    next: Foo

  FooWrapper = ptr object
    next: FooWrapper
    foo: Foo
    bar: Bar

  Bar = ptr object
    queue: Queue[FooWrapper]

var q: Queue[FooWrapper]
echo q

[mratsim]

But unfortunately, creating a pointer from the "deref"-ed type doesn't match the original

  type
    Task = ptr object
      prev, next: Task
      parent: Task
      fn: proc (param: pointer) {.nimcall.}
      # User data
      data: array[10, byte]

  var t = create(deref(Task))
  doAssert t is Task