Conditional types do not work in generic functions

[ns-vpanfilov]

Bug Report

I've created a type that converts all undefined properties to optional. This types works well, however fails when used in generic functions:

type UndefinedKeys<T> = { [P in keyof T]: undefined extends T[P] ? P : never }[keyof T]
type OptionalUndefined<T> = Partial<Pick<T, UndefinedKeys<T>>> & Omit<T, UndefinedKeys<T>>

🔎 Search Terms

generic function

🕗 Version & Regression Information

This is the behavior in every version I tried ( 5.0.2 and 4.9.5 )

⏯ Playground Link

Playground link with relevant code

💻 Code

type UndefinedKeys<T> = { [P in keyof T]: undefined extends T[P] ? P : never }[keyof T]
type OptionalUndefined<T> = Partial<Pick<T, UndefinedKeys<T>>> & Omit<T, UndefinedKeys<T>>

type TSample<
  U extends Record<string, any> | undefined,
  D extends Record<string, any>
> = { undef: U, def: D }


function fn<
  U extends Record<string, any> | undefined,
  D extends Record<string, any>
>(params: OptionalUndefined<TSample<U, D>>) {
  {
    // ERROR: both properties do not exist on type 'OptionalUndefined<TSample<U, D>>'
    const { def, undef } = params
  }

  {
  // works as expected
  const { def, undef } = params as OptionalUndefined<
      TSample<
        Record<string, any> | undefined,
        Record<string, any>
      >
    > 
  }
}

🙁 Actual behavior

typescript cannot properly determine parameter type

🙂 Expected behavior

I expect typescript to use definition of U (extends Record<string, any> | undefined) and D (``) to be used to determine the type of parameters and params should have `{ undef?: Record<string, any> | undefined, def: Record<string, any> }` type?

Notice how, when I explicitly specify the type of params, it works properly.

[MartinJohns]

Resolving of conditional types including unbound type arguments is deferred. This is a design limitation.

TypeScript can't resolve your conditional type within the function because it doesn't know what types U and D are.

[fatcerberus]

Conveniently, this issue makes a good case against #53452. Partial by itself currently works in generic contexts.