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/////////////// Background ///////////////
/*

(The test case that appears below may do a better job of explaning than this wall of text :)

I recently noticed this calculation while investigating some framework code:
https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart#L235

There's an associated test case, and though it makes sense, I think it misses a potential issue:
https://github.com/flutter/flutter/blob/master/packages/flutter/test/widgets/slivers_evil_test.dart#L255

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/*    Copyright 2009 10gen Inc.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,

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import 'package:flutter/material.dart';

class RootOverlayInjector extends Overlay {
  final Widget child;

  const RootOverlayInjector({Key key, this.child})
      : super(key: key, initialEntries: const []);

  @override
  _RootOverlayInjectorState createState() => _RootOverlayInjectorState();

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class Built<T extends Built<T, S>, S extends Builder<T, S>> {
  
}

class Builder<T extends Built<T, S>, S extends Builder<T, S>> {
  
}

class StatefulWidget {
  

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class Pair<T extends Pair<T, S>, S extends Pair<T, S>> {}

mixin BrokenMixin<T extends Pair<T, S>, S extends Pair<T, S>> {
  void broken<V extends Pair<V, W>, W extends Pair<V, W>>([void Function(T, V) func]) {}
}

class Example with BrokenMixin<Null, Null> {}

main() {
  Example().broken<Null, Null>();

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class Identifiable<T> {
  Id<T> id;
}

class Id<T> {}

class Person extends Identifiable<Person> {}

class Animal extends Identifiable<Animal> {}