A union may now hold one promise beside data members (`T | Promise<T>`,
`Promise<R | void> | R | void`). The union tag is the `instanceof Promise`
test, and every operation keeps Node's microtask timing: `await` awaits the
promise arm or takes the one non-thenable turn, an async `return` (block
bodies, concise arrows, then handlers) adopts the promise arm after the
function's own finally blocks, `Promise.resolve` returns the promise arm as
is, and `typeof (u as any).then` reads the tag. Data arms that declare
`then`, two promise arms, and function arms beside a promise stay refused
with diagnostics. Inferred JavaScript literals that mix promises and values
keep the checked-dynamic array.
Rest parameters typed `unknown[]` or `any[]` in TypeScript use the
checked-dynamic variadic ABI that JavaScript rest parameters already use,
including spread forwarding (`(...args) => handler(...args)`).
Functions are now Map values, Map keys and Set elements, compared by
closure identity.
Two pre-existing bugs in the same area are fixed: an async lambda in a
`Promise<void> | void` slot got a `Promise<union>` ABI and threw when called
through the slot, and `return cb()` of such a callback in an async function
dropped the promise instead of adopting it.
The retired-engine 2251 and 2790 diagnostics fixtures (tagged templates
with `any[]` rest tags) now compile and match Node, so they move to the
corpus as 6248 and 6249.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
