async: per-iteration `let`/`const` binding collapses for closures created in a loop body (every closure sees the last value)

[proggeramlug]

Summary

Inside an async function, a let/const declared in a loop body and captured by a closure loses its per-iteration binding: every closure sees the last iteration's value. The same code in a non-async function is correct.

Silent wrong answer — no throw, exit 0.

Repro

async function main() {
  const fns: (() => void)[] = [];
  for (let i = 0; i < 9; i++) {
    const j = i;
    fns.push(() => console.log("j =", j));
  }
  fns.forEach((f) => f());
  await 0;
}
main();

	output
node 26	j = 0, j = 1, … j = 8
perry main	j = 8 × 9

Capturing the loop variable directly (fns.push(() => console.log(i))) is equally affected — node prints 0..8 (per-iteration let binding), perry prints 9 nine times (the single shared cell, read after the loop has exited).

Notes

• Drop the await (making main a plain function, or an async function with no await) and the output is correct — so the trigger is the async-to-generator transform, which hoists every body local into a single boxed mutable capture on the step closure. That one cell is shared across iterations, collapsing the per-iteration binding that let/const require.
• Independent of runtime: await Promise.all([...]) never resumes when inputs settle via executor-captured resolvers — program silently exits 0 #6328 / PR fix(runtime): numeric-key computed calls must read the element, not dispatch by name (#6328) #6344 (numeric-key computed calls). It reproduces on unpatched main via a call shape that already worked (const f = fns[i]; f(i)), and it still reproduces with fix(runtime): numeric-key computed calls must read the element, not dispatch by name (#6328) #6344 applied. Filing separately so the two fixes don't get tangled: fix(runtime): numeric-key computed calls must read the element, not dispatch by name (#6328) #6344 is a runtime fix, this one belongs in perry-transform's async lowering.
• Likely affects any for/while body that creates closures inside an async function — event-handler registration loops, per-item callbacks, setTimeout in a loop.