Incorrectly reported `TS2322` error on array items after spread

🔎 Search Terms

"array spread", "TS2322"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgLhgQQE6rgTwDxgK4C2ARgKaowA+M0qAlmAOYB8MAvDANoC6A3AFChIsIkjQYc+YmRbsOAOgVwANDACMvIA

💻 Code

const a: Array<number | string> = [];
const b: Array<number> = [...a, 1];

🙁 Actual behavior

The compiler gives the following two errors, of which only one is appropriate:

~/prog/typescript ❯ tsc ./array-spread-error.ts
array-spread-error.ts:2:27 - error TS2322: Type 'string | number' is not assignable to type 'number'.
  Type 'string' is not assignable to type 'number'.

2 const b: Array<number> = [...a, 1];
                            ~~~~

array-spread-error.ts:2:33 - error TS2322: Type 'string | number' is not assignable to type 'number'.
  Type 'string' is not assignable to type 'number'.

2 const b: Array<number> = [...a, 1];
                                  ~

🙂 Expected behavior

The spread is clearly incorrect, as the original array could potentially contain strings as well, but the number literal should not be marked with the same error.

リンクされた TypeScript Playground の再現コードから始め、spread とそれに続く数値リテラルが別々の TS2322 診断を生成することを確認します。spread 要素を含む配列リテラルの checker の処理経路を追跡し、無効な spread だけが報告され、数値リテラルは受け入れられることを示す回帰テストを追加します。