isArray type guard regression

[mjbvz]

TypeScript Version: 4.2.0-dev.20201208

Search Terms:

Code

interface IAction {
    x: number;
}

function foo(arg: IAction | ReadonlyArray<IAction>) {
    const actions: ReadonlyArray<IAction> = Array.isArray(arg) ? arg : [arg];
}

Expected behavior:
This compiles fine in TS 4.1.2

Actual behavior:
In the latest nightly, this produces this error:

Type '(IAction & any[]) | (readonly IAction[] & any[]) | (IAction | readonly IAction[])[]' is not assignable to type 'readonly IAction[]'.
  Type '(IAction | readonly IAction[])[]' is not assignable to type 'readonly IAction[]'.
    Type 'IAction | readonly IAction[]' is not assignable to type 'IAction'.
      Property 'x' is missing in type 'readonly IAction[]' but required in type 'IAction'.ts(2322)

Playground Link: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201208#code/JYOwLgpgTgZghgYwgAgJIEEFmAexMgbwChlTkAPALmRAFcBbAI2gG4iBfIomWkLXfDBw4AFHCgBzahn55kAH2QAlCHAAmeADYBPdFChxtAHhnY8APgCUhEmQR4AzmGSIzIB9RXqtu-YZOYbubIALzIegbaAHTADhGGYpLWAPwuksjUANriEgC6bOxAA

Related Issues:

[mjbvz]

The type of arg after the isArray guard ends up being:

(IAction & any[]) | (readonly IAction[] & any[])

[MartinJohns]

Related: #41808. They mentioned they plan to reverse the previous breaking changes with 4.1.3.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.