// Original version, uses any 
export function isArrayOld(array: any): array is any[] {
      return Array.isArray(array);
}

// New, funky-looking, but retains readonly safely
function isArray<T>(array: T | {}): array is T extends readonly any[] ? (unknown extends T ? never : readonly any[]) : any[] {
    return Array.isArray(array);
}

const mutableArray: string[] = []
const immutableArray: readonly string[] = []

if (isArray(mutableArray)) {
  mutableArray.push("")
}

// Fails correctly
if(isArray(immutableArray)) {
  immutableArray.push("")
  // would error here
}


/// Previous behavior

if (isArrayOld(mutableArray)) {
  mutableArray.push("")
}

if (isArrayOld(immutableArray)) {
  immutableArray.push("")
  // No error on pushing to an readonly array
}