TS 4.1+ Key remapping in mapped types allows bypassing statically known members check

[Gerrit0]

Bug Report

🔎 Search Terms

key remapping static member, arbitrary interface keys, bypassing interface dynamic member check

🕗 Version & Regression Information

With the key remapping introduced in TS 4.1 for mapped types, the following error can be bypassed:

An interface can only extend an object type or intersection of object types with statically known members. (2312)

I was unable to test this on prior versions because key remapping didn't exist.

⏯ Playground Link

• Playground

💻 Code

// @ts-expect-error -- this isn't allowed
interface RecordInterface<K extends keyof any, V> extends Record<K, V> { }

type RemapRecord<K extends keyof any, V> = { [_ in never as K]: V }

// @ts-expect-error -- but this is!
interface RecordInterface<K extends keyof any, V> extends RemapRecord<K, V> { }

🙁 Actual behavior

TypeScript inconsistently prevents using interfaces with dynamic members.

🙂 Expected behavior

Either RecordInterface to have an error, or to be able to replace Record1 with Record without any error.

CC: @tjjfvi, who discovered this, but hasn't submitted a bug.

There is a feature request (#41383) about changing the definition of Pick which includes a comment about this.

[tjjfvi]

Seems like a feature to me 🤷

[MartinJohns]

I'm confused what in never is even supposed to be.

When hovering over RemapRecord<> the tooltip shows that it maps to {}, so it's an empty interface and a result assignable to any other type.

When mapping it to another type and providing type arguments, e.g. type Foo = RemapRecord<...>, then the type Foo has properties.

[tjjfvi]

My version uses [_ in "" as K]. Apparently the in never also works, but that seems like a separate bug?

[RyanCavanaugh]

Please don't take a production dependency on this; I have no idea what might be hiddenly broken when that type starts getting used.

[crowlKats]

it would be great if this was an actual feature. "dynamically" adding typings would be great in some scenarios

[tjjfvi]

Please don't take a production dependency on this; I have no idea what might be hiddenly broken when that type starts getting used.

👍 ; thinking back now, I remember having some really cryptic bugs that were fixed when I stopped using this behavior