Array concat has wrong types

Bug Report

A not-uncommon JS idiom is [].concat(x || [], y || [], z || []) as a faster, simpler, more backwards-compatible [x, y, z].flatMap(x => x || []). However, when I try, I get type errors.

🔎 Search Terms

array concat concat overload

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

const a = [].concat(
    true ? 'a' : [],
    false ? 'b' : [],
);

const x = [].concat(
    false ? 'b' : [],
    true ? 'a' : [],
);

🙁 Actual behavior

No overload matches this call.
  Overload 1 of 2, '(...items: ConcatArray<never>[]): never[]', gave the following error.
    Argument of type 'never[] | "a"' is not assignable to parameter of type 'ConcatArray<never>'.
      Type 'string' is not assignable to type 'ConcatArray<never>'.
  Overload 2 of 2, '(...items: ConcatArray<never>[]): never[]', gave the following error.
    Argument of type 'never[] | "a"' is not assignable to parameter of type 'ConcatArray<never>'.
      Type 'string' is not assignable to type 'ConcatArray<never>'.(2769)

🙂 Expected behavior

No type errors (except perhaps that the variable may be any until i provide an explicit type for the resulting array), because this is valid JavaScript since at least ES3.

[MartinJohns]

Duplicate of https://github.com/microsoft/TypeScript/issues/36554#issuecomment-580924501. Used search terms: never array concat

[MartinJohns]

@SancheZz He’s intentionally not using the spread operator. See his first sentence, where he mentions:

more backwards-compatible