Switch on an option of an untagged variant with an `@as(undefined)` case takes the wrong arm for `Some` of that case

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評估

難度
4/5
預估耗時
3-5 天
新手友好度
28/100
Issue 類型
缺陷
描述清晰度
基本清楚
活躍度
活躍
技術堆疊
javascript, ocaml
領域
compilers

研究方向

Start with rejects_nested_some in compiler/ml/matching.ml, which decides whether a pattern reads the payload with Pval_from_option_not_nest and skips unwrapping the nested Some box. Reproduce with the g(Some(U)) snippet from the issue and inspect the compiled JavaScript for the typeof and === undefined tests. Done when Some(U) reaches the Some(_) arm and prints SU, with the constant and runtime-built cases agreeing.

由索引模型根據 Issue 內容生成。

描述

Found by differential semantic drift analysis from b123b7c3 to c10223e8.

What is broken

@unboxed type t = @as(undefined) U | S(string) | I(int)

let g = (x: option<t>) =>
  switch x {
  | None => "None"
  | Some(S(_)) => "SS"
  | Some(I(_)) => "SI"
  | Some(_) => "SU"
  }

Console.log(g(Some(U)))

Expected SU. At c10223e8 it prints SI; at b123b7c3 it printed None (the construction bug fixed by #9008). A Some(U) built at run time (let h = v => g(Some(v)), h(U)) prints SI at both commits, so the match bug predates the range; #9008 makes constant values reach it too. Matches that name Some(U) explicitly, and tagged variants, are correct.

The compiled g tests x !== undefined, then x === undefined, then typeof x === "string", and otherwise returns "SI", without unwrapping the nested-Some box first: the boxed Some(U) fails the first two tests and lands in the last case.

rejects_nested_some returns true for _ and for untagged payload patterns, so the match reads the payload with Pval_from_option_not_nest, which skips unwrapping.

Possible cause (speculation only)

rejects_nested_some assumes that each pattern rejects the box on its own test, but the untagged dispatch decides the last remaining case by elimination, without a test of its own.

🤖 Generated with Claude Code

主要語言
OCaml
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