Exhaustiveness failure on Dart map patterns

main.dart

import 'dart:collection';

void main() {
  print(test({})); // -> unknown
  print(test(BadMap2(false, 5))); // -> double
  print(test(BadMap())); // -> oops! this map must be a sneaky one
}

String test(Map<bool, int> map) => switch (map) {
    {} => 'unknown',
    {false: _} => 'false',
    {true: _} => 'true',
    {false: _, true: _} => 'contradiction',
    {false: _, false: _} => 'double',
    _ => 'oops! this map must be a sneaky one',
};

class BadMap<K, V> extends UnmodifiableMapBase<K, V> {
  @override
  int get length => 1;
  
  @override
  List<K> get keys => [];
  
  @override
  V? operator[](Object? key) => null;
}

class BadMap2<K, V> extends UnmodifiableMapBase<K, V> {
  BadMap2(this._key, this._value);

  K _key;
  V _value;

  @override
  int get length => 2;
  
  @override
  List<K> get keys => [_key];
  
  @override
  V? operator[](Object? key) => key == _key ? _value : null;
}