async proc inside a generic proc gives CT error

[timotheecour]

Example

when true:
  import asyncdispatch
  # ok
  block:
    proc main(a: int|string)=
      proc bar(b: int|string) = echo b
      bar(a)
    main(1)

  block:
    proc main(a: int) : Future[void] {.async.} =
      proc bar(b: int): Future[void] {.async.} = echo b
      await bar(a)
    waitFor main(1)

  block:
    proc main(a: int) : Future[void] {.async.} =
      proc bar(b: int | string): Future[void] {.async.} = echo b
      await bar(a)
    waitFor main(1)

  block:
    # bug
    proc main(a: int|string) =
      proc bar(b: int): Future[void] {.async.} = echo b
      waitFor bar(a)
    main(1) # uncomment and it'll work, otherwise Error: invalid type: 'void' for var

Current Output

Error: invalid type: 'void' for var

Expected Output

works

Additional Information

1.5.1 2b5841c
root cause for #16741