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Malte UblandClaude Opus 5.5
378901df95
`["one"] | string[]` flowing into a `string[]` slot compiled to the checked extraction against the array arm alone, so a tuple value threw at runtime. Node accepts it: the value is an array, and nothing about a `string[]` position distinguishes which arm produced it. Let the union-width plan cover a mixed union that holds the destination arm beside a convertible one, and place that plan before the checked-extraction rule, so each arm converts on its tag (a tuple rebuilds, an array copies). A union whose only non-unit arm is the destination (`T[] | undefined`) still has nothing to convert and keeps its tested extraction. tests/corpus/8513-tuple-union-array.ts pins the binding, call-argument, readonly, and single-tuple shapes against Node. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
26 lines
984 B
TypeScript
26 lines
984 B
TypeScript
// A union that holds an array arm beside tuple arms, flowing into that array
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// type: Node has one array either way, so both arms answer as arrays instead
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// of the tuple arm trapping in the checked extraction.
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type Mixed = ["one"] | string[];
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function pick(flag: boolean): Mixed {
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return flag ? ["one"] : ["two", "three"];
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}
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function take(value: string[]): string {
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return `${value.length}:${value.join(",")}`;
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}
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for (const flag of [true, false]) {
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const m = pick(flag);
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const arr: string[] = m;
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console.log(flag, arr.length, arr.join(","), take(m), arr[0]);
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}
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type Readonly = readonly ["a"] | readonly string[];
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const r: Readonly = ["b", "c"];
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const plain: readonly string[] = r;
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console.log("readonly", plain.length, plain.join(","));
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const narrow: ["one"] = ["one"];
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const one: string[] = narrow;
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console.log("single", one.length, one.join(","));
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const optional: string[] | undefined = ["x"];
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const lifted: string[] = optional!;
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console.log("optional", lifted.length);
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