Allow users to customize the variance of built-in Array

[devanshj]

Bug Report

🔎 Search Terms

Array covariance hardcoded?, Array invariance, --noLib

🕗 Version & Regression Information

Tested with 4.9.5

⏯ Playground Link

https://tsplay.dev/WPzdYN

💻 Code

// @noLib: true

interface Array<T>
  { [i: number]: T | undefined
  , push: (t: T) => number
  }

interface _Array<T>
  { [i: number]: T | undefined
  , push: (t: T) => number
  }

declare let as: Array<"a">
declare let abs: Array<"a" | "b">
abs = as

declare let _as: _Array<"a">
declare let _abs: _Array<"a" | "b">
_abs = _as

🙁 Actual behavior

The assignment abs = as compiles.

🙂 Expected behavior

The assignment abs = as should not compile, just like the assignment _abs = _as does not compile.

[RyanCavanaugh]

This behavior is not userland-configurable. You can remove the check in getVariances if you want to try this out in your own fork to see how it would go, though

[devanshj]

This behavior is not userland-configurable.

I don't think it's unreasonable to want to use a sound (or sounder) version of TypeScript by rewritings the types.

If not a bug then consider this a feature request.

[rotu]

Strangely (or maybe not so strangely), this does work as you expect when using type aliases instead of interfaces playground:

type Array<T> = 
  { [i: number]: T | undefined
  , push: (t: T) => number
  }

Even if you explicitly use a variance annotation on the interface (interface Array<in out T>), it gets silently ignored, which is strange.

[rotu]

I also notice that when defining Array as a generic type alias, the compiler's tooltips get mucked up:

And defining Array in terms of [] syntax causes a hard compiler crash #57009.

So maybe this isn't a good workaround for now.