extends within dt function is not safe?

[loynoir]

Bug Report

🔎 Search Terms

extends dt function

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

    type dtExtends<X, Y> = X extends Y ? true : false

    // Expected `true`, but actually it's `never`
    type y = dtExtends<never, never>

🙁 Actual behavior

never extends never is true.

But _dtExtends<never, never> is not true, but never.

  {
    'describe: extends within dt function is unsafe'

    type _dtExtends<X, Y> = X extends Y ? true : false

    'describe: never extends never'
    // $ExpectType true
    type x = never extends never ? true : false

    'describe: unexpectedly, never is not extends never within dt function
    // Expected `true`, but actually it's `never`
    // $ExpectType never
    type y = _dtExtends<never, never>

  }

🙂 Expected behavior

never extends never is true.

_dtExtends<never, never> is true.

  {
    'describe: extends within dt function is unsafe'

    type _dtExtends<X, Y> = X extends Y ? true : false

    'describe: never extends never'
    // $ExpectType true
    type x = never extends never ? true : false

    'describe: never extends never within dt function'
    // $ExpectType true
    type y = _dtExtends<never, never>

  }

[JeroenoBoy]

try dtR<X, Y>() you currently use an Array for the first type, and nothing as the second.

[loynoir]

try dtR<X, Y>() you currently use an Array for the first type, and nothing as the second.

@JeroenoBoy
Ah, that is not the problem, just add reference to avoid ... is declared but never used.

never extends never is true

The problem is _dtExtends<never, never> is not true, but never

🤔

[whzx5byb]

Duplicate of #23182 (comment)

[MartinJohns]

Your conditional type distributes over the type arguments. But since never is an empty union, there's nothing to distribute over and you end up with never. If you disable distribution in the conditional type (as documented) you end up with true.

https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types

[loynoir]

You guys are awesome!!!!!!!! 😘