TypeScript: getByRef() error when using generic type param as Ref value

[frank-weindel]

A user on the forums asked a question about how to create a subclassable base Component that takes a generic parameter that is used to inform the type of a Ref value. In diving into a potential solution I discovered that the type definitions for getByRef() are not allowing for a genericized Ref value type, while the definitions for tag() are allowing it just fine.

Example code:

import { Lightning } from '@lightningjs/sdk'

class Header extends Lightning.Component {}

export interface IPageTemplateSpec<
  T extends Lightning.Component.Constructor = Lightning.Component.Constructor,
> extends Lightning.Component.TemplateSpec {
  Header: typeof Header
  Content: T
}

export interface IPageTypeConfig extends Lightning.Component.TypeConfig {
  IsPage: true
}

export class Page<T extends Lightning.Component.Constructor = Lightning.Component.Constructor>
  extends Lightning.Component<IPageTemplateSpec<T>, IPageTypeConfig>
  implements Lightning.Component.ImplementTemplateSpec<IPageTemplateSpec<T>>
{
  static override _template(): Lightning.Component.Template<IPageTemplateSpec> {
    return {
      w: (w: number) => w,
      h: (h: number) => h,
      rect: true,
      color: 0xff0e0e0e,

      Header: {
        type: Header,
      },
      Content: undefined,
    }
  }

  protected Content = this.getByRef('Content')!
  //                                ^-------^ Error!!!
}

The error is:

Argument of type 'string' is not assignable to parameter of type 'keyof TemplateSpecRefs<IPageTemplateSpec<T>>'.
  Type '"Content"' is not assignable to type '"Header" | (TransformPossibleElement<P, IPageTemplateSpec<T>[P], never> extends never ? never : P) | ((T extends Constructor<...> ? InstanceType<...> : Element<...>) extends never ? never : "Content")'.ts(2345)

Example Repo: https://github.com/frank-weindel/com.domain.app.LngForum546Subclassable