TS2589 regression in 6.0: implements without type parameters causes excessive type instantiation when property uses concrete generic type

🔎 Search Terms

Type instantiation is excessively deep and possibly infinite.

🕗 Version & Regression Information

• This changed between versions 5.9.3 and 6.0.2

⏯ Playground Link

https://www.typescriptlang.org/play/?target=7#code/JYWwDg9gTgLgBDAnmApnA3nAShArjFKAGQgGMBDGYCAOwBo4B1FAIwGEJxaUaYBJXoQBm5UmgC+cIVE5wARAAEAbuXIATYDQD0M-ITkBuAFBGkqbHgIBRAB7wAvBjgBnFKSq0A-AC4XMKJoA5nDixkZaWnAAKgDKAEwArAAcAJxwtAgAFmikADbkzs5wam75UJTUNHAsKLkQAO4AdOGRAGLANr5QKGD5YnAABszsnJA0PPyCUCJiA3D1wDCZg8McXOO8AgTToigAPDh6tjAAfAPNES1wAHIQBL5LaIQyUOk0uYgIAYGBhEU1FFwrkGdQoHhoc2ARTMKDU80Wy3IcFItFI3QICGQKCuYHI5RAKG2cAAFANDtsSGDKgdLChjmcAJSNOAAVWcQUG5MIlIqtDmCyWljgSJhOLx5AJRLUEBQRRody+wB+hDgi2aKBskFgyPyhTgAFlEFZcigCbw4BqCDQ1EUABJRfVEY2miaq8Ams0wIqrUbcTZTGZodBGOBwUG8mg+Cx6YhkCM0o52E7GcRGIA

💻 Code

import type { RouterLocation, WebComponentInterface } from "@vaadin/router";

type RouteExt = { section?: string };

// TS2589 on the class declaration below.
// Fix: replace `WebComponentInterface` with `WebComponentInterface<RouteExt>`.
//
// Note: the error only triggers because `location` is typed with a concrete type
// parameter (`RouterLocation<RouteExt>`). Using `RouterLocation` without a type
// parameter does not trigger it.
export class MyElement extends HTMLElement implements WebComponentInterface {
  location?: RouterLocation<RouteExt>;
}

🙁 Actual behavior

I get this error:

Type instantiation is excessively deep and possibly infinite.

But the information isn't very useful to know what to do exactly in this case.

🙂 Expected behavior

Ideally no error, otherwise some more pointers about how to fix this.

Additional information about the issue

When running with --extendedDiagnostics:

typescript 5.9.3:

     Files:                         259
     Types:                      203241
     Instantiations:            2593860

typescript 6.0.2:

     Files:                         259
     Types:                      217322
     Instantiations:            5379836

Start with the linked Playground and compare TypeScript 5.9.3 with 6.0.2 using --extendedDiagnostics, focusing on the implements declaration and concrete RouterLocation property. Trace the type-instantiation path responsible for TS2589 and validate the result against the provided reproduction; done means the example no longer reports the error or provides more actionable guidance.