Uniqueness error after fusion

[athas]

let iota_2d (n: i64) : [n][n](i64, i64) =
  tabulate_2d n n (\i j -> (i, j))

let first_anti_diag [n] 't (i: i64) (xs: [n][n]t) =
  if i >= 0 then
   xs
    |> flatten
    |> (.[i:n * i + 1:n - 1])
  else []

let process_block [b]
                  (prev_anti_diag: [][b][b]i32)
                  (block: [b][b]i32)
                  (i: i64) : *[b][b]i32 =
  let inds = tabulate b (\tx -> (tx, tx))
  let vals = tabulate b (\tx -> prev_anti_diag[i, tx - 1, b - 1])
  in scatter_2d (copy block) inds vals

let main [b][num_blocks] (i: i64) (matb: *[num_blocks][num_blocks][b][b]i32) =
  #[unsafe]
  let ip1 = i + 1
  let res = map2 (process_block (first_anti_diag (i - 1) matb))
                 (first_anti_diag i matb :> [ip1][b][b]i32)
                 (iota ip1)
  in scatter_2d matb (iota_2d num_blocks |> first_anti_diag i :> [ip1](i64, i64)) res

My guess is that we're not being careful about the aliases of that if expression.

[athas]

Much simpler:

let main [n] (xs: *[n]i32) =
  let vals = map (+2) (reverse xs)
  in scatter xs (iota n) vals

How did we not notice this before?