const VALUES = ["a1", "a2", "a3", "b1", "b2", "b3", "c1", "c2", "c3"] as const;

// Please ignore the non-sensical nature of these types. This is a simplified version of my real use case.
type Value = (typeof VALUES)[number];
type TransformFunction = (values: Value[]) => Value[];
type TransformDefinition = [RegExp] | [TransformFunction];

// This doesn't work, but seems like it should.
function errorExample(definition: TransformDefinition): TransformFunction {
  let [condition] = definition;

  if (condition instanceof RegExp) {
    return () => VALUES.filter((value) => condition.test(value));
  }

  return condition;
}

// This works, and is almost the same thing
function successExample([condition]: TransformDefinition): TransformFunction {
  if (condition instanceof RegExp) {
    return () => VALUES.filter((value) => condition.test(value));
  }

  return condition;
}