Erlang: erlangArityAt doesn't skip <<binary>> literals (contrary to its docstring), so binary-arg dispatch sites get an inflated arity and the behaviour-callback edge is dropped or mis-linked — verified

[inth3shadows]

Summary

erlangArityAt (the arity counter for the Erlang behaviour-dispatch synthesizer) does not skip <<binary>> literals, despite its docstring claiming it does. Commas inside a binary literal are counted as top-level argument separators, so a dispatch site like Mod:decode(<<1,2,3>>, Opts) is computed as arity 4 instead of 2. The behaviour-dispatch lookup then keys on the wrong name/arity, so the edge to the real -callback is silently dropped (missed edge) — or collides with an unrelated behaviour that happens to declare that inflated arity (wrong edge). Verified against the real compiled function.

Root cause

// src/resolution/callback-synthesizer.ts (erlangArityAt)
/** ... Skips nested (), [], {}, <<>> content, "strings", 'atoms', and $c char literals ... */
function erlangArityAt(src: string, openIdx: number): number {
  let depth = 1; let commas = 0; ...
  for (let i = openIdx + 1; i < limit; i++) {
    const ch = src[i]!;
    if (ch === '"' || ch === "'") { /* skip string/atom */ }
    if (ch === '$') { /* skip char literal */ }
    if (ch === '(' || ch === '[' || ch === '{') { depth++; ... }
    if (ch === ')' || ch === ']' || ch === '}') { depth--; ... }
    if (ch === ',' && depth === 1) { commas++; continue; }   // <- binary's inner commas land here
    ...
  }
}

The scan special-cases ()/[]/{} but never <</>>, so the , inside <<1,2,3>> is seen at depth === 1 and counted. The docstring's own worked example (init(fun((a, b) -> ok), #{k => v}) → 2) passes only because () and {} balance; the <<>> claim in the same sentence is unimplemented.

Repro (executed against the real compiled function)

dispatch site	real arity	erlangArityAt
Mod:decode(<<1,2,3>>, Opts)	2	4
Mod:encode(<<"pre", Rest/binary>>, Ctx)	2	3
Mod:handle(Msg, State) (control, no binary)	2	2
init(fun((a, b) -> ok), #{k => v}) (docstring example)	2	2

Binary literals with commas are ordinary Erlang (<<1,2,3>>, <<"prefix", Rest/binary>>), so any Var:fn(<<...>>, ...) dispatch site through a behaviour hits this.

Suggested direction

Track <</>> depth alongside the existing bracket depth: on a two-char << increment depth and advance past both chars, on >> decrement (guarding the two-char match so a lone </> comparison isn't misread). Then the docstring's <<>> claim holds and the arity is correct.

Environment

main @ 5955d04, package 1.4.1, Node 22.23.1 (Linux). Function extracted from the built dist/ and exercised directly.

[inth3shadows]

Suggested labels for triage (I lack permission to apply them myself): bug, correctness (graph returns wrong/missing data).