// @target: es2015
// https://github.com/microsoft/TypeScript/issues/41984

interface TestCase<T extends string | number> {
  readonly val1: T | ReadonlyArray<T>;
  readonly val2: ReadonlyArray<T>;
}

interface MultiCaseFixture<T> {
  cases: T[];
}

function subDataFunc(): TestCase<string | number>[] {
  return [
      { val1: "a", val2: ["a", "b", "c"] },
      { val1: 2, val2: [1, 2, 3] },
      { val1: ["a", "z"], val2: ["x", "y", "z"] },
      { val1: [5, 10], val2: [10, 100, 1000] },
  ];
}

function dataFunc<T>(subFunc: () => T[]): MultiCaseFixture<T> {
  return { cases: subFunc() };
}

function testFunc() {
  const fixture = dataFunc<TestCase<string | number>>(subDataFunc);
  fixture.cases.forEach(({ val1, val2 }) => {
      if (Array.isArray(val1)) {
          // This should retain val1 as being an array
          const reversedVal1 = val1.slice().reverse();
          console.log(reversedVal1);
      } else {
          console.log(val1);
      }
      console.log(val2);
  });
}

testFunc();
