better_completer_record_covariant.dart

import 'dart:async';

typedef BetterCompleter<T> = ({
  void Function(T) complete,
  void Function(Object, [StackTrace?]) completeError,
  bool Function() isCompleted
});

BetterCompleter<T> fromCompleter<T>(Completer<T> c) => (
      complete: c.complete,
      completeError: c.completeError,
      isCompleted: () => c.isCompleted
    );

void completeBetterCompleterWithInt(BetterCompleter<int> c) {
  c.complete(1);
}

void main() {
  final Completer<int> intCompleter = Completer<Never>();
  final betterIntCompleter = fromCompleter(intCompleter);

  completeBetterCompleterWithInt(betterIntCompleter); // Compiles, but throws at run time.
}