Cannot infer the value of static param

[mratsim]

Sorry for the long repro code, it's quite tricky to extract it from my codebase and keep the issue reproducible as 3 differents procs/statements need to be present

This is a follow-up on my Go playing bot project which triggered many bugs last April every time I tried to avoid one:

• Set + static (+ range ?): Ordinal type expected #7546 (Set + static (+ range ?): Ordinal type expected)
• Macro + Static in object field: undeclared identifier N if proc result #7547 (Macro + Static in object field: undeclared identifier N if proc result)
• Static + distinct: Cannot borrow when static is involved #7552 (Static + distinct: Cannot borrow when static is involved)
• [Codegen] distinct + Borrow + var seq: expression is not assignable #7553 ([Codegen] distinct + Borrow + var seq: expression is not assignable)
• Static + type conversion in type section for ranges #7609 (Static + type conversion in type section for ranges)
• Static[non-int] + $ string conversion not matching #7611 (Static[non-int] + $ string conversion not matching)
• Array constructor for range types #7616 (Array constructor for range types)
• strformat doesn't work properly inside generics #7632 (strformat doesn't work properly inside generics and templates)

With this new example I get:

cannot_infer.nim(90, 15) template/generic instantiation of `search_best_move` from here
cannot_infer.nim(81, 7) template/generic instantiation of `run_rollout` from here
cannot_infer.nim(19, 42) Error: cannot infer the value of the static param 'N'

import hashes, tables

type
  ################################ Coordinates ###################################
  GoInt* = int32

  Point*[N: static[GoInt]] = distinct range[-1.GoInt .. (N+2) * (N+2) - 1]
    # Can't use plain static[GoInt]: https://github.com/nim-lang/Nim/issues/7609

  ################################ Coordinates ###################################

  ################################ Color & Moves ###################################

  Intersection* = enum
    Empty, Black, White, Border

  ################################ Board  ###################################

  Board*[N: static[GoInt]] = array[(N+2) * (N+2), Intersection]

  BoardState*[N: static[GoInt]] = ref object
    board*: Board[N]

  ################################ Monte-Carlo ###################################

  Zobrist* = Hash
  Node*[N: static[GoInt]] = object
  NodeTable*[N: static[GoInt]] = Table[Zobrist, Node[N]]

  MCTS_Context*[N: static[GoInt]] = object
    nodes*: NodeTable[N]

################################ Procedures ###################################

func newBoardState*(size: static[GoInt]): BoardState[size] =
  new result

# Remove for no crash
func `$`*[N: static[GoInt]](board: Board[N]): string =
  # Display a go board - Triggers crash <------------------------------
  result = "foo"

proc run_rollout[N: static[GoInt]](
        self: var MCTS_Context[N],
        board_state: BoardState[N]) =

  var
    amaf_color_map: array[(N+2)*(N+2), Intersection]

  # Comment the following out and the project compiles:
  echo amaf_color_map # Triggers crash <------------------------------

proc search_best_move*[N: static[GoInt]](
  self: var MCTS_Context[N], board_state: BoardState[N],
  nb_simulations: Natural): Point[N] =

  self.run_rollout(board_state)

################################ Bug ###################################

var a = newBoardState(GoInt 19) # Triggers crash <------------------------------
# var a: BoardState[GoInt 19] # This is another possible fix

var ctx: MCTS_Context[GoInt 19]

let move = ctx.search_best_move(a, 10)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. If you think it is still a valid issue, write a comment below; otherwise it will be closed. Thank you for your contributions.

[narimiran]

var a = newBoardState(GoInt 19) # Triggers crash <------------------------------

This doesn't trigger crash anymore.

echo amaf_color_map # Triggers crash <------------------------------

This doesn't crash, but it produces:

Error: type mismatch: got <array[0..440, Intersection]>
but expected one of:
proc echo(x: varargs[typed, `$`])
  first type mismatch at position: 1
  required type for x: varargs[typed]
  but expression 'amaf_color_map' is of type: array[0..440, Intersection]

[metagn]

The compiler, I think rightfully, can't infer the N in Board[N] when an arbitrary array type is given. With #24098, this stops the $ overload with Board[N] from matching, and uses the $ overload for arrays instead. Would this be considered correct?

Giving amaf_color_map the type Board[N] does make it use the Board[N] overload, which is an interesting test IMO because an alternative implementation of the language arguably wouldn't support it.