<!-- BUGS: Please use this template. --> <!-- QUESTIONS: This is not a general support forum! Ask Qs at http://stackoverflow.com/questions/tagged/typescript --> <!-- SUGGESTIONS: See https://github.com/Microsoft/TypeScript-wiki/blob/master/Writing-Good-Design-Proposals.md --> **TypeScript Version:** 2.4.1 **Code** ```ts let a: ReadonlyArray<number> = []; let b: number[] = []; // compile error let c = b.concat(a); // you see Array.concat() doesn't change the params ``` **Expected behavior:** **Actual behavior:**