Language Service doesn't work with custom generics

I’m using a new Angular 12 project that was generated with the CLI with the defaults strict modes.

Consider the following component:

@Directive({
  selector: '[appFoo]'
})
export class FooDirective<T extends any[]> {
  @Input() appFoo!: T;
}

This will work as expected and infer the correct type for T:

But if we change it to a custom type, it will infer it as any:

@Directive({
  selector: '[appFoo]'
})
export class FooDirective<T extends { id: number }> {
  @Input() appFoo!: T;
}

[JoostK]

Okay, most of these limitations have been resolved in #42492 which just landed for the next release. Any usage of ambient types within a generic type constraint will still not work (using types like Partial, Array, Pick, Readonly, etc) as the compiler is unaware that they don’t need an import. This can now be mitigated by adding an exported type-alias for the type, at which point the compiler is able to use that type-alias.

I will go ahead and close this, as the reported issue and most of the limitations have been addressed.

[JoostK]

So the reason it’s limited is that this used to be done for performance reasons only. The actual compiler has a fallback scenario if a generic type is not supported, but the language service cannot use this fallback due to how source files are managed in a live editing context. However, that means that the kinds of generic types we support now directly affect the editing experience in the language service, whereas it used to be invisible.

So yes this is something we want to fix but it’s not a top priority at the moment.