Conditional types about types HOT 2 CLOSED

expectType<string | undefined>(R.defaultTo('This file', undefined)); // << incorrect

export function defaultTo<Default, Value>(a: Default, b: Value): (Value extends (null | undefined) ? Default | NonNullable<Value> : Value);
export function defaultTo<Default>(a: Default): <Value>(b: Value) => (Value extends (null | undefined) ? Default | NonNullable<Value> : Value);

expectType<string>(defaultTo('default value', undefined));

expectType<string>(defaultTo(0, 'two'));
expectType<number>(defaultTo('zero', 2));

const numberOrUndefined = Math.random() < 0.5 ? 0 : undefined;
expectType<string | number>(defaultTo('default value', numberOrUndefined));

// `string` is not correct here
expectType<string>(R.defaultTo('This file', undefined)); // error

// `string | undefined` is
expectType<string | undefined>(R.defaultTo('This file', undefined)); // ok!

// `undefined` is taken literally above, no differently than how the return type is `string | number` for this
expectType<string | number>(R.defaultTo('This file', 123)); // ok!

R.compose(
  R.defaultTo("This file"), // :(
  R.nth(1),
  R.match(/.*\.(.*)/),
  R.propOr("", "name")
)

R.compose(
  R.defaultTo("This file"), // 
  R.match(/.*\.(.*)/),
  R.propOr("", "name")
)

export function defaultTo<T, U>(a: T, b: U | null | undefined): T | U;
// new overload that lets you set `U` ahead of time
export function defaultTo<T, U>(a: T): (b: U | null | undefined) => T | U;
export function defaultTo<T>(a: T): <U>(b: U | null | undefined) => T | U;