Inconsistent extends check when comparing a directly written generic (Template<'a'>) to a type-aliased version of the same generic (TA = Template<'a'>)

[Gguigre]

🔎 Search Terms

"Alias equality", "extends broken by alias template", "generic type extension", "extends transitivity typescript"

🕗 Version & Regression Information

• This changed between versions 4.1.5 and 4.2.3

4.1.5	4.2.3

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAgg8gJwEJQLxQOQEMNQD6YBGGAUCaJLIkgAoID2YAzmlAN4lRQUQBcmOTlCaQAxvwwiIoqIIC++dkJ78ARIVVCp4zNqjEScsjygAVCAFswAGyzAIAHlNQIAD3sA7ACYt4yVti4BBjEAHys7FAA9FHcABYAliyiWACuTBAswHHQhKkA5srgfGZaYvwAou4IWKLADn60DMwANOwqZnKhANqq2qoAuobGxWYwrOZWtvYOgaEjlABKmQCME5Y2do5zLu4Q3iyTm-ZQAPzcCKnQ-ABmWNYZ0bFMcfSp1l760HcP0FjeUCSUB+j34gFByEgxLhQAB6pwW0GWTAATBNxm5PD4zBtptBzsBLtdgfdHlCXm8Pl9ib9ZACgQSrlB+IAZckhsVh8LIZKwFlyBUBNygAEkoOloDx6IKALQy2Vy+VykiiegeJjAKBgVgcLgdQItMrSCR6eSyZIqtUIqBIgDMrAlgs1GP2WKOuLOF0ZtxJ0DJr3en0I329tM+9MJTKgrKhXDhQA

💻 Code

type AOrB = 'a' | 'b'

type AOrBProps = {
  type: 'a'
  spec: 'spec a'
} | {
  type: "b"
  spec: 'spec b'
}

type Template<T extends AOrB = 'a' | 'b'> =  { // this causes the bug
  type: T
  spec: Extract<AOrBProps, {type: T}>["spec"]
}

type TA = Template<'a'>

type Res1 = Template<'a'> extends Template ? true : false // should be false and is false : ✅
//   ^?

type Res2 = TA extends Template ? true : false // should be false and is true : ❌
//  ^?


// same bug if I use typeof -------------------
const p = {
  type: 'a',
  spec: 'spec a'
} as const

type Res3 = typeof p extends Template ? true : false // should be false and is true : ❌
//   ^?

🙁 Actual behavior

Types declared as equals (TA and Template<'a'>) does not behave the same when compared to a third type (Template)

🙂 Expected behavior

As TA = Template<'a'> I would expect Res1 and Res2 to be the same (false)

Additional information about the issue

Working on this issue with @GuillaumeEgret

[MartinJohns]

Sounds like a duplicate of #60503.

[GuillaumeEgret]

Yes, seems likea duplicate but the other issue was automatically closed because there was no update. Should we investigate what broke the type aliasing ?

[MartinJohns]

the other issue was automatically closed because there was no update

It was closed because it was marked as a "Design Limitation", which means there's nothing they can do about it in the foreseeable future.

Should we investigate what broke the type aliasing ?

🤷‍♂ I'm not a team member. But if you have a fix that doesn't have horrendous performance costs and doesn't break anything else I'm sure the team would be happy to accept it.