Refuse returns a generic callback's inferred result would drop members of

[cramforce]

The callback here is passed to a generic call and has no return annotation, and its returns extend the elements it was given:

const items = run(() => {
  if (plain.length === 0) return plain;            // Plain[]
  return plain.map((x) => ({ ...x, extra: true })); // { name; n; extra }[]
});
const asItems: Item[] = items;                     // Item = Plain & { extra?: boolean }

The checker unifies both returns into the narrower Plain[], so run's T is Plain[]. Node keeps extra on the returned objects, and the later wider view reads it back. Natively, converting the extended records into the inferred type was a width copy that dropped extra silently. On main the last line printed [{"name":"a","n":1},…] where Node prints [{"name":"a","n":1,"extra":true},…].

Such a return is now refused by name. The message names the dropped members and points at the fix: an annotation on the callback's return type or on the call's type argument. That annotation names a type that keeps the members, and those programs compile and match Node.

Only conversions the refusal targets change, and gate covers it. A return is refused only when all of these hold:

• it sits in an unannotated arrow or function expression;
• that function is a call argument;
• the conversion would drop declared members, through array elements and promise payloads.

Callbacks whose results the checker types by their own returns keep every member already, so they are unaffected: map with conditional or spread-extended returns, filter + map chains, and reduce returning a spread-extended accumulator. All of them were probed and match Node.

Evidence

• tests/diagnostics/inferred-callback-drops.ts (snapshot) pins the refusal.
• tests/corpus/6480-generic-callback-extended-results.ts covers the annotated callback, the type-argument form, the async form, and the map/reduce shapes. It is byte-identical to Node in both lanes.
• Zero-cost gate against origin/main 4bbf31b: PASS, 2502 of 2502 identical (bench, self-hosted compiler, corpus).
• pnpm lint and pnpm format:check are clean, pnpm test:ts7 --baselines-only passes, and tests/harness/diagnostics.test.ts passes.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated
scriptc	Ready	Preview, v0	Oct 20, 2026 8:24am UTC

[vercel]

Conditional and satisfies-wrapped callback returns bypass the new refusal and still silently lose extended members.