analyzer throws "type_argument_not_matching_bounds" on extension type about sdk

[lrhn]

It's intended.
The only reason implements Object looks weird is that you're not used to needing to implement Object, because all classes (other than Null) are automatically subtypes of Object, and all record and function types are subtypes of `Object.

There are very few types in the pre-extension-type type system that don't automatically implement Object.
The one exception is type variables, a foo<T>() { ... T ...} introduces a type which is not a subtype of Object. To make sure that it is, you have to write <T extends Object>.
You can sometimes think of extension types in the same way. Extension types don't automatically become subtypes of Object, because they are types, not classes. And if you want them to, you have to add implements Object.
And you can only do that if the representation type is itself a subtype of Object, which it doesn't have to be.

An extension type like:

extension type const Opt<T>._(({T value})? _) {
  const Opt.value(T value) : this._((value: value));
  const Opt.none() : this._(null);
}

has a nullable representation type, and cannot implement Object.

[iapicca]

An extension type like:

extension type const Opt<T>._(({T value})? _) {
  const Opt.value(T value) : this._((value: value));
  const Opt.none() : this._(null);
}

has a nullable representation type, and cannot implement Object.

yeah, makes sense thank you for your answer @lrhn