fix: set descriptor generic as covariant (out) on typedApi

Closes #580
microsoft/TypeScript#10717

Author: voliva

packages/client/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - `cli`: Commands `add`, `remove` and `update` run codegen by default after finishing [#577](https://github.com/polkadot-api/polkadot-api/pull/577).
 
+### Fixed
+
+- `typedApi` not being assignable to union of multiple chains.
+
 ## 0.11.2 - 2024-07-19
 
 ### Fixed

packages/client/src/compatibility.ts

@@ -20,8 +20,10 @@ import { Observable, combineLatest, filter, firstValueFrom, map } from "rxjs"
 import { ChainDefinition } from "./descriptors"
 
 export class CompatibilityToken<D = unknown> {
+  private constructor() {}
+
   // @ts-ignore
-  private constructor(protected _descriptors: D) {}
+  protected _phantom(value: D) {}
 }
 
 interface CompatibilityTokenApi {