mirror of
https://github.com/vercel-labs/scriptc.git
synced 2026-10-26 06:34:20 +08:00
Chris Tate
8d2b3bb2e0
- lowerUnitComparison folds ===/!== against a union lacking the unit arm to the constant answer, both operand orders - a non-droppable operand still evaluates exactly once, riding a throwaway tag test - union-switch case null / case undefined take the unit-comparison lowering, so a lacking arm never matches - the unionEq fallback had coerced the literal through the stranded-arm trap, throwing where Node answers false - corpus 2490-2494: the find-miss repro, every strict and loose spelling, switch unit cases, effect preservation
51 lines
1.2 KiB
TypeScript
51 lines
1.2 KiB
TypeScript
// `case null:` / `case undefined:` over union discriminants: a unit case the union HOLDS is a tag test; a unit case the union LACKS never matches (JS skips it — no coercion of the literal into the union). Grouped unit cases and defaults keep source order.
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const items = [1, 2, 3];
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const miss = items.find((n) => n > 50);
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const hit = items.find((n) => n > 2);
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switch (miss) {
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case null:
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console.log("never: null");
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break;
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case undefined:
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console.log("miss is undefined");
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break;
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default:
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console.log("never: default");
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break;
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}
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switch (hit) {
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case null:
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console.log("never: null");
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break;
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case 3:
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console.log("hit is three");
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break;
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default:
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console.log("never: default");
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break;
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}
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function tri(flag: number): string | null {
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return flag > 0 ? "pos" : null;
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}
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const triple = tri(-1);
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switch (triple) {
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case undefined:
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console.log("never: undefined");
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break;
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case "pos":
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console.log("never: pos");
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break;
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case null:
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console.log("tri gave null");
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break;
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}
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switch (miss) {
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case null:
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case undefined:
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console.log("grouped unit cases match the undefined arm");
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break;
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default:
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console.log("never: grouped default");
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break;
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}
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