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Untagged variant switch with a default arm matches instances and literals in the wrong case

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評估

難度
4/5
預估耗時
1-2 天
新手友好度
55/100
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 compiler/ml/lam_compile.ml, especially compile_untagged_cases and compile_general_cases, then compare the behavior with rules 7 and 8 in compiler/ml/UntaggedVariants.md. Run or add compiler cases for the instance and literal examples in the issue; done means both produce other for the shown inputs while preserving the documented matching rules.

由索引模型根據 Issue 內容生成。

描述

A switch on an untagged variant with a _ arm can match a value in the wrong case. This happens when the default survives into the untagged dispatch, which takes a variant with several cases; with fewer cases the match compiler expands _ into the remaining cases and the result is correct.

An instance lands in the object case.

@unboxed type t = D(Date.t) | R(RegExp.t) | O({x: int}) | S(string) | I(int) | F(int => int)
@inline(never) let id = (x: t) => x
let f = v => switch v { | O(_) => "O" | S(_) => "S" | _ => "other" }
Console.log(f(id(D(Date.make()))))

function f(v) {
  switch (typeof v) {
    case "object" :
      return "O";
    case "string" :
      return "S";
    default:
      return "other";
  }
}

Output: O; expected other. The case "object" lacks the !(v instanceof Date) && !(v instanceof RegExp) exclusion that the object check gets elsewhere.

A literal lands in the string case.

@unboxed type t = | @as("lit") Lit | S(string) | I(int) | B(bool) | O({x: int}) | F(int => int)
@inline(never) let id = (x: t) => x
let f = v => switch v { | S(_) => "S" | O(_) => "O" | _ => "other" }
Console.log(f(id(Lit)))

function f(v) {
  switch (typeof v) {
    case "string" :
      return "S";
    case "object" :
      return "O";
    default:
      return "other";
  }
}

Output: S; expected other. No test for the literal "lit" is emitted before the typeof switch.

Both contradict compiler/ml/UntaggedVariants.md: rule 7 ("A value equal to a literal therefore belongs to the literal's case") and rule 8 ("In both forms a value of an instance case never lands in the object case"). The cause is presumably in lam_compile.ml:compile_untagged_cases / compile_general_cases when the switch keeps a Default and the instance and literal cases it would need to exclude have no arm of their own (not yet confirmed).

主要語言
OCaml
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