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SC2003 'string | undefined' when a method returning an element of its array field is used inline

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評価

難易度
4/5
見積もり時間
3〜5日
初心者へのやさしさ
30/100
issue の種類
バグ
明瞭さ
おおむね明確
活発さ
活発
技術スタック
typescript
領域
compilers

調査の方向性

Reproduce first: save the snippet as repro.ts and run scriptc build repro.ts; the rejection is emitted by the SC2003 union-conversion layout check. Locate where that diagnostic fires and how an inline method-call result reaches it differently than a const-bound value; the const and plain-function variants that already compile are the contrast. Read the #2319 fix for "string concatenation with possibly missing reads" as prior art. Done means the repro compiles and prints 2 with a regression test guarding it.

索引モデルが issue の本文から書いたものです。

説明

A class method declared to return string returns an element of its own string[] field. Using the call result inline, as in obj.last().length, is rejected with SC2003 got 'string | undefined'. Binding the result to a const first compiles.

Reproduction

class Args {
  list: string[] = ["a", "bc"];
  last(): string {
    return this.list[this.list.length - 1];
  }
}
console.log(new Args().last().length);

$ node repro.ts
2
$ scriptc build repro.ts -o repro
repro.ts:7:13 - error SC2003: union conversion has no unambiguous layout mapping: expected 'string', got 'string | undefined' (each source arm needs an identical destination arm, a unique compatible record/array layout, or a shared literal discriminant selecting compatible layouts)

It fails 3 out of 3 builds.

What matters

variant result
repro as above SC2003
const value = new Args().last(); console.log(value.length); compiles, prints 2
same body as a plain function last(list: string[]): string, used inline compiles, prints 2
plain function reading the array from a Map, with undefined/empty guards, used inline compiles

So it needs both a method that returns an element read and an inline use of the call result. At the call site the declared return type is string, and there's no union in the source. The limitations page (array-bounds-and-runtime-errors) says a missing element that reaches storage that can't hold undefined throws a TypeError at run time. A build-time rejection that disappears once the value goes through a const looks inconsistent with that.

Possibly related

#2141 (fixed by #2319, after 0.4.5) has the same shape: an inline call result, a body that reads an array element, and binding to a variable avoids it. Its trigger was string concatenation ("x-" + PAD[i]), and the fix describes "string concatenation with possibly missing reads". This repro has no concatenation, and no release after #2319 exists yet, so I couldn't check whether it's covered.

Versions

  • scriptc 0.0.36: compiles and prints 2.
  • 0.1.1, the next version published to npm: SC2003.
  • 0.4.5: still SC2003.
  • SCRIPTC_CHECKER=tsgo gives the same result.

Where it came from

A CLI argument parser, args.value("as", "").length > 0. value() returns the last element of a string[] or a fallback. This compiled with 0.0.36 and blocks upgrading to 0.4.5.

Environment

  • scriptc 0.4.5 (@scriptc/cli-darwin-arm64)
  • macOS 26 arm64
  • Node v24.16.0 as the reference
主要言語
TypeScript
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