better_completer.dart

import 'dart:async';

typedef Inv<X> = X Function(X);
typedef BetterCompleter<X> = _BetterCompleter<X, Inv<X>>;

extension type _BetterCompleter<X, Invariance extends Inv<X>>
    .fromCompleter(Completer<X> _) {
  void complete(X value) => _.complete(value);
  void completeError(Object error, [StackTrace? stackTrace]) =>
      _.completeError(error, stackTrace);
  bool get isCompleted => _.isCompleted;
}

void completeCompleter(Completer<Object?> c) {
  c.complete(1.5);
}

void completeBetterCompleter(BetterCompleter<Object?> c) {
  c.complete(1.5);
}

void completeBetterCompleterWithInt(BetterCompleter<int> c) {
  // `c.complete(1.5)` is a compile-time error.
  c.complete(1);
}

void main() {
  final objectCompleter = Completer<Object?>();
  final intCompleter = Completer<int>();
  final betterObjectCompleter = BetterCompleter.fromCompleter(objectCompleter);
  final betterIntCompleter = BetterCompleter.fromCompleter(intCompleter);

  completeBetterCompleter(betterObjectCompleter); // OK.
  // completeBetterCompleter(betterIntCompleter); // Compile-time error.
  completeBetterCompleterWithInt(betterIntCompleter); // OK.
}