can't pass @[] to template (even typed) => internal error or sigmatch error

[timotheecour]

Example

when defined case1:
  template fn(b: seq[string]) =
    let d = $(b, )
  fn(@[])

when defined case2:
  template fn(b: seq[string]) =
    let d = $b
  fn(@[])

Current Output

-d:case1:
Error: internal error: getTypeDescAux(tyEmpty)

-d:case2:
Error: type mismatch: got <seq[empty], string, string, string>

Expected Output

works

Additional Information

1.5.1 6b3c77e
works with proc instead of template

mildly related: https://github.com/nim-lang/Nim/issues?q=is%3Aissue+is%3Aopen+%22tyEmpty%22

[juancarlospaco]

The question is if theres a literal empty seq thats not seq[empty] ?,
because @[] can match for seq[int] or any other type. The error message needs to be improved.

[solo989]

It's literally turning

let d = $(b, )

into

let d = $(@[],)

You would have to special case for all sorts of stuff inside templates to get this and other similar situations to work.

It's also why

template hi(a : int8) =
  let
    x = a
  echo type(x) #int
hi(5)

x is an int instead of an int8.

The error messages when echo doesn't work should definitely be improved though.

The best solution would probably to have literals carry around their type information after they passed through a template with a
specific typed argument. I don't think @[] is currently a literal though.

Unfortunately you also can't do this

template hi(a : seq[int]) : untyped =
  var
    x : seq[seq[int]] = @[a]
hi(@[])

which would ideally work even if a is untyped.