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typescript: type a callback parameter from context through an optional callback and without a staged lib - #1740
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swapnilpaliwal-sd
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suyashpaliwal26
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September 29, 2026 02:00
…l callback and without a staged lib `p.then(u => u.m())` left `u` untyped: `then`'s callback parameter is written `((value: T) => R) | undefined | null`, and the rule only looked for a bare function type. Unwrap unions and parentheses to reach it. With no standard library staged (a default index), `xs.map(u => u.m())`, `forEach`, `filter`, `reduce` and `promise.then` had no declaration to read a signature from, so the parameter stayed untyped while `for (const u of xs)` resolved. Type the element parameter of the built-in element callbacks from the receiver's container reference, and `then`'s value from a Promise/PromiseLike reference. Co-authored-by: axiomcode-bot[bot] <334110751+axiomcode-bot[bot]@users.noreply.github.com>
…hrough aliases and nested callbacks The no-library route typed a `forEach` / `then` callback parameter from the receiver's type argument by NAME, so a program's own `class Set<T>` or `class Promise<T>`, whose callbacks receive something else, got the type argument as well as its own parameter type. A container name that resolves to a client declaration is no longer read as the built-in. A union of two different function types gives the compiler no contextual type; the union is now looked through only when exactly one member is a function type. The element now travels as a type reference, so the route also reaches `type Users = User[]`, `Alias<User>` over `Array<T>`, a callback over a parameter that was itself typed this way (`xss.forEach(xs => xs.forEach(...))`, `p.then(us => us.map(...))`), and `Map.forEach((value, key) => ...)`. Co-authored-by: axiomcode-bot[bot] <334110751+axiomcode-bot[bot]@users.noreply.github.com>
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TypeScript: type an arrow's parameter from its callback context when the callback is optional (
p.then(u => u.m())) and when no standard library is staged (xs.map/forEach/filter/reduce/…,promise.then,Map.forEach). The type is read from the receiver's container or Promise reference, following type aliases and nested callbacks.class Set<T>/class Promise<T>isn't read as the built-in: its callbacks keep their own parameter type.80-contextual-lambda-paramand81-contextual-lambda-param-no-lib; both agree with tsc 35/35. Controls: userSet/Promise/map/then, union of callbacks, annotated params.