Generic concept has itself as generic parameters cause type mismatch

[7sDream]

type
  Iterable[T] = concept x
    for v in x:
      type(v) is T

proc map2[T, U](itit: Iterable[Iterable[T]], f: proc(_: T): U): seq[U] =
  result = @[]
  for it in itit:
    for v in it:
      result.add(f(v))

echo map2(@[@[1, 2, 3], @[4, 5, 6]], proc (x: int): int = x + 1)

this code compile failed(try it on glot):

main.nim(12, 10) Error: type mismatch: got <seq[seq[int]], proc (x: int): int{.noSideEffect, gcsafe, locks: 0.}>
but expected one of: 
proc map2[T, U](itit: Iterable[Iterable[T]]; f: proc (_: T): U): seq[U]
  first type mismatch at position: 2
  required type: proc (_: T): U{.closure.}
  but expression 'proc (x: int): int = result = x + 1' is of type: proc (x: int): int{.noSideEffect, gcsafe, locks: 0.}

expression: map2(@[@[1, 2, 3], @[4, 5, 6]], proc (x: int): int = result = x + 1)

but if I change Iterable[Iterable[T]] to Iterable[seq[T]], it will run as expected.

[narimiran]

this code compile failed ... but if I change Iterable[Iterable[T]] to Iterable[seq[T]], it will run as expected.

Still the same behaviour in July 2019 with the latest Nim (v0.20.99).