potential bug related to generics

Bug Report

🔎 Search Terms

generics
pipe
functional programming

🕗 Version & Regression Information

5.2.0-dev

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

export type Func = (..._: any[]) => unknown;

export type Unary<Input, Output> = (_: Input) => Output;

export type Tail<L extends unknown[]> = L extends
  readonly [unknown, ...infer LTail] ? LTail
  : L;

type UnaryFn<A, R> = (a: A) => R;
type Arg<F> = F extends UnaryFn<infer A, unknown> ? A : never;
type Res<F> = F extends UnaryFn<any, infer R> ? R : never;

type ValidCompose<F1, F2> = Res<F1> extends (Arg<F2> | Promise<Arg<F2>>) ? F1
  : (arg: Arg<F1>) => Arg<F2>;

type ValidPipe<FS> = FS extends [infer F1, infer F2, ...infer Rest]
  ? [ValidCompose<F1, F2>, ...ValidPipe<[F2, ...Rest]>]
  : FS;

export const pipe = <Fs extends Func[]>(  ...fs: ValidPipe<Fs>) => {}

(<T, Fn extends (x: T) => number>(f: Fn) => {
  const g = (x: T) => x;
  const h = (_: T) => 1;
  pipe(g, f);  // This gives a typing error.
  pipe(g, h);  // This works fine.
});

🙁 Actual behavior

The first pipe invocation results in a typing error.

🙂 Expected behavior

None should have an error, because the generic typing ensures everything is in order.

Start with the linked TypeScript Playground and reduce the generic pipe example around ValidCompose, ValidPipe, and the two pipe invocations. Investigate the type-checking behavior for the generic function f versus h; done means both calls type-check without errors while preserving the reported generic constraints.