Parameters does not factor in generics or overloads

Bug Report

🔎 Search Terms

• “parameters with generic function”
• “generic function wrapping”

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type System Behavior, Generics,

⏯ Playground Link

Playground link with relevant code

💻 Code

declare function foo<TState, T>(
  s1: TState,
  project: (s1: TState) => T): T;
declare function foo<TState1, TState2, T>(
  s1: TState1,
  s2: TState2,
  project: (s1: TState1, s2: TState2) => T): T;

// This is not what I would expect
type Args = Parameters<typeof foo>; // = [s1: unknown, s2: unknown, project: (s1: unknown, s2: unknown) => unknown]

// Something I would have expected
type Expected<TState, T, TState1, TState2> =
  [s1: TState, project: (s1: TState) => T]
  | [s1: TState1, s2: TState2, project: (s1: TState1, s2: TState2) => T];


// Even if it were to return the expected type, it still would not work
function wrapper<TState, T, TState1, TState2>(...args: Expected<TState, T, TState1, TState2>) {
  return foo(...args);
  //         ^^^^^^^ Expected 2-3 arguments, but got 0 or more.(2556)
}

🙁 Actual behavior

The Parameters type seems to choose either the last or a random overload of the given method instead of all of them. Also doesn’t factor in generics and converts them to unknown instead. Even if it would return what I would have expected (see Expected type in example), it still isn’t usable in practice.

🙂 Expected behavior

That Args would result in something similar to Expected. All of the overloads should be factored in and also generics. The Expected args should be usable on foo.

Real-world usages

Any wrapper function where the original function is complex, has overloads or is generic. In our case, it’s everything 😃

More specifically I want to reduce boilerplating with @ngrx so I wanted to wrap the function createSelector which is declared here, which is similar to the foo of my example, but with 31 overloads instead of only 2 and where the return type is dependent on the parameters (the project argument).

Wrapping this method is possible, but requires just an insane amount of copy-pasting with some little changes which isn’t feasible and does not exactly help in reducing the amount of code.

[MartinJohns]

Parameters<T> is based on type inferrence in conditional types, and type inferrence in conditional types does not support overloading: https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#inferring-within-conditional-types

This is a known design limitation.

[MartinJohns]

Because WrapperType is not generic. You can’t pass along generic types like this. I think this would require #1213.