Can't concat ReadonlyArray

[ghost]

TypeScript Version: nightly (2.5.0-dev.20170707)

Code

const x: ReadonlyArray<number> = [0];
x.concat(x);

Expected behavior:

No error.

Actual behavior:

src/a.ts(2,10): error TS2345: Argument of type 'ReadonlyArray<number>' is not assignable to parameter of type 'number | number[]'.
  Type 'ReadonlyArray<number>' is not assignable to type 'number[]'.
    Property 'push' is missing in type 'ReadonlyArray<number>'.