013 fold missing null arm

✓ Passing Passing: the compiler rejects this program as expected.

Code

// A fold with no `| null` arm refuses: every `?` edge and every `?*Node`
// root reaches the empty case — the base is a law, not a default.
import std/tree

std/tree:node(Node) { item: i64, left: ?Node }

pub tor mk { } -> *Node<owned!>
mk = if(true)
    | then |> std/tree:mint(Node) { item: 1, left: null }: h -> h
    | else -> null

mk(): t
|> std/tree:fold(t, as: i64): c
    | node n -> n.item + n.left
|> std/tree:dispose(t): _

input.k

Actual compiler output

error[KORU184]: std/tree:fold(t): missing `| null -> <expr>` — every `?` edge and every `?*Node` root reaches the empty case; spell it
  --> tests/regression/600_STDLIB/682_TREE/682_013_fold_missing_null_arm/input.k:13:1

Must contain:

KORU184

Flows

flow ~node click a branch to expand · @labels scroll to their anchor

node (Node, source: item: i64, left: ?Node)

subflow ~mk click a branch to expand · @labels scroll to their anchor

if (true)

flow ~mk click a branch to expand · @labels scroll to their anchor

Test Configuration

MUST_ERROR

← 012 each body fold hoists 014 fold heterogeneous edge →