optional params don't work with block argument(s) in last position

[timotheecour]

passing a block as last argument doesn't work with optional params.

This is a common use case; fixing this would simplify a lot of code.

Example

template fun*(a = 1, b = 2, body: untyped) = discard
fun(a = 1, 2): discard
fun(3, 4): discard
fun(b = 5, a = 6): discard
fun(a = 1, body = 1)
fun(a = 1, body = block:
    discard)
fun(a = 1): discard  # CT error
fun(): discard  # CT error

template fun2*(a = 1, b = 2, body: untyped): untyped = (a, b)
let c = fun2(1,2): discard
let c2 = fun2(1): discard # CT error

Current Output

Error: type mismatch: got <a: int literal(1), void>
but expected one of:
template fun(a = 1; b = 2; body: untyped)
  first type mismatch at position: 2
  required type for b: int
  but expression '
discard' is of type: void

expression: fun(a = 1):
  discard
  fun(a = 1): discard  # CT error
     ^

Expected Output

should work

Possible Solution

• improve sigmatch

Additional Information

• 5fb40af
• D20200514T010829
• related (but different) from varargs + optional params doesn't work #14436
• this should also handle do notation (ie for passing multiple block arguments)

links

• https://nim-lang.github.io/Nim/manual.html#templates-passing-a-code-block-to-a-template
• as noted here Let arguments be left as optional when using "do" notation. #884 (comment) there is no ambiguity so spec (and implementation) should allow this

As the "do" can only fill the arguments starting from the last one, there is no ambiguity in letting the optional arguments before it be left unfiled.