Nesting Generics doesn't work as expected.

[eximius313]

While having sucha a class with Generics is perfectly fine:

abstract class Response<TYPE extends Enum> {
  TYPE get type;
}

the compiler does not allow for "catching" the argument. Neither this:

class Processor<R extends Response<TYPE>> {
  TYPE handle(R response) => response.type;
}

nor this:

class Processor<R extends Response<TYPE extends Enum>> {
  TYPE handle(R response) => response.type;
}

works.

---

In your issue, please include:

• Dart version and tooling diagnostic info (dart info) -> - Dart 3.0.0 (stable) (Thu May 4 01:11:00 2023 -0700) on "windows_x64"
• Whether you are using Windows, macOS, or Linux (if applicable) -> Windows 10
• Whether you are using Chrome, Safari, Firefox, Edge (if applicable) -> N/A

[julemand101]

Would the following work for you?

class Processor<TYPE extends Enum, R extends Response<TYPE>> {
  TYPE handle(R response) => response.type;
}

[eximius313]

Would the following work for you?

class Processor<TYPE extends Enum, R extends Response<TYPE>> {
  TYPE handle(R response) => response.type;
}

I know that this compiles, but this is a workaround. Processor should be able to take one param. The TYPE is nested in Response

[eernstg]

Cf. dart-lang/language#620.

[lrhn]

Correct, Dart does not allow you to nest generics, meaning declaring a new type variable inside the bound of another.
That would be nice, either as the language issue mentioned above, or as an extension to patterns (case List<var T>(...):, and treating type parameter bounds as type patterns: <E extends Foo<var T>>.

It's not a bug in the implementation, and I believe there are issues for the possible language changes, so closing this.