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Malte Ubl
1846753a3c
A function value's declared type may name fewer parameters than the function it holds, and a later widening call then passes arguments the declared type never spelled. JavaScript's function object carries all of its parameters, so nothing may be dropped on that path — that would be a silent divergence from Node. The narrowed store is already a named refusal (a compiled function value is one concrete native ABI: "'(string, number) => number' values where '(string) => number' is expected"), reported at the narrowing site for a binding and for an object-literal member slot. Record both in the diagnostics snapshot so the refusal cannot regress into a silent drop. Pin the shapes that DO keep every argument: a class member whose overloads spell fewer parameters than its implementation, the member read as a value (the read keeps the implementation's JS arity), and a value that entered through an untyped slot and is widened back. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> (cherry picked from commit 94d44e947c48dc0184c941d526380ebd1b1b443d)
26 lines
1.0 KiB
TypeScript
26 lines
1.0 KiB
TypeScript
// Declared-arity narrowing: a function value stored under a type that names
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// FEWER parameters than the value's own signature. Node's function object
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// still carries every parameter, so a later widened call would pass all of
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// its arguments (the surplus must not be dropped); a compiled function value
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// is one concrete native ABI, so the narrowing store is refused at every
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// observable use instead.
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const impl = (text: string, times: number) => text.length * times;
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// (1) Narrowing at a binding, then calling through the narrow type.
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const narrowed = impl as unknown as (text: string) => number;
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console.log(narrowed("abc"));
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// (2) The same narrowing in an object literal's member slot, with the member
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// read back and widened to the implementation's arity.
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interface Sink {
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write(text: string): number;
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}
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function makeWriter(): Sink {
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return { write: impl as unknown as (text: string) => number };
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}
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const sink = makeWriter();
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const widened = sink.write as (text: string, times: number) => number;
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console.log(widened("abcd", 3));
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