Exception thrown when calling `Completer.completeError` - dart

Question

Please compare the two following examples:
Example 1:
import 'dart:async';
Future<void> main() async {
final c = Completer<void>();
print(1);
c.completeError(Exception("hello"));
print(2);
await c.future;
}
which prints
1
2
Failed to load ".../x_test.dart": hello
Example 2:
import 'dart:async';
Future<void> main() async {
final c = Completer<void>();
print(1);
await (() async => c.completeError(Exception("hello")))();
print(2);
await c.future;
}
which prints
1
Failed to load ".../x_test.dart": hello
I thought that in the second example, the exception should be bound to the future of the Completer and hence also be thrown when awaiting for the future.
Is there a way to defer an exception with a Completer to a later point?

Answer 1

What I see when I run your second example in DartPad is
1
Uncaught Error: Exception: hello
2
Uncaught Error: Exception: hello
The error is uncaught twice. That matches what is actually happening:
You print 1.
You call () async { c.complete(Exception("hello")); }(). This schedules a microtask to
complete the completer's future with the exception. Then the call returns another future and schedules a microtask to complete that future with null.
Then you await the latter future.
Then the first microtask runs and completes the completer's future with an error. At this point there are no listeners on the future, so the error is considered uncaught and reported to the root zone's uncaught async error handler. This prints the first uncaught error line.
Then the second microtask runs and completers the other future. This makes the await complete.
Then you print 2.
Then you await the completer's future. Since the future is already complete, this schedules a microtask report this to the await.
Then that microtask runs and the second await completes by throwing the exception of the completer's future.
That throw is uncaught and is therefore also reported as an uncaught error.
It's no entirely clear what you want. Do you want the completer's future to complete later (if so, how much), or do you just not want the first completion to be considered uncaught?
The latter is easier, just do:
completer..future.catchError((_){})..complete(Exception("hello"));
If you pre-attach an error handler to the future, which handles (by ignoring) the error, then the future won't consider the error unhandled.
(Arguably, it would be nice to have a way to complete a completer with an error, and say up front that it shouldn't be considered unhandled).