TS 4.2: Type alias differs from original type

[YauheniKhadanionak]

Bug Report

🔎 Search Terms

4.2 Type alias

🕗 Version & Regression Information

• This changed between versions 4.1.5 and 4.2.3
• Also reproducible in 4.3.0-beta and nightly

⏯ Playground Link

Playground link

💻 Code

const func1 = (arg: Record<'field', string>) => { }

type Type1 = Record<'field', string>
const func2 = (arg: Type1) => { }

const arg: Record<string, string> = {}

func1(arg) // ok
func2(arg) // Property 'field' is missing in type 'Record<string, string>' but required in type 'Type1'.

🙁 Actual behavior

Type alias Type1 differs from original type Record<'field', string>

🙂 Expected behavior

Types of func1 and func2 should be equal and behave the same way, as it worked on 4.1.5 and earlier

[ahejlsberg]

The direct cause of this issue is #42284. We now preserve the alias for Type1 and thus when relating to Record<'field', string> we are no longer relating instantiations of the same type alias. Therefore we relate the types structurally and we discover they're not compatible. The reason we view Record<'field', string> and Record<string, string> as compatible has to do with the way we measure variance. We find that the first type parameter is contra-variant when in reality variance is strictly speaking unmeasurable for mapped types because a string key turns into an index signature which doesn't exactly behave as an infinite number of properties.

I'm pretty sure we already have a bug for this, but haven't been able to find it yet.

[ahejlsberg]

I should add, I think we already tried marking variance of mapped types as unmeasurable, but it lead to an explosion in structural type comparisons with significant effects on performance.

[ahejlsberg]

I found the issue we already have for this. Duplicate of #29698.