Misleading error message when interface tries to extend constrained type parameter

🔎 Search Terms

"statically known members."
"could be instantiated with a different subtype of constraint 'object'."

🕗 Version & Regression Information

• This changed between versions 3.6.2 and 3.7.5

⏯ Playground Link

https://www.typescriptlang.org/play?ts=3.7.5#code/JYOwLgpgTgZghgYwgAgKIA84FsAOAbCAHgBVkJ1IQATAZ2QHsAjAKwgTAD4yKJq7SA3gF8AUEA

💻 Code

interface Example<T extends object> extends T {}

🙁 Actual behavior

I get the following message:

Interface 'Example' incorrectly extends interface 'T'.
'Example' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'object'.

This error message is misleading. The interface does not vary with T and since it declares no new members, is completely compatible with T.

🙂 Expected behavior

I expect a descriptive message. Prior to 3.6.2, the error message was more helpful:

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

This is still the error you get if the type variable T does not have the extends constraint.

Additional information about the issue

No response

Start with the TypeScript 3.7.5 Playground example: interface Example extends T {}. Compare its diagnostic with the unconstrained type-parameter case and the pre-3.6.2 wording. Done means the constrained case reports a descriptive diagnostic without regressing the existing behavior.