value-free return should be allowed in function which returns FutureOr<void>

[srawlins]

According to the current spec, only void, dynamic and Null allow for return;. I'd like to add FutureOr<void> (and FutureOr<void>??) to this list.

Don't be alarmed, my motivation is not for users making FutureOr APIs, etc. My motivation is Future.catchError, see dart-lang/sdk#35825.

The onError parameter of Future<T>.catchError is typed as Function, because the acceptable signatures of that function cannot be expressed right now. onError can either be FutureOr<T> Function(dynamic) or FutureOr<T> Function(dynamic, StackTrace). In either case the return type is FutureOr<T>.

I think it makes sense to be able to write a Future<void>.catchError handler with return; statements (Flutter engine does it in a few places).

More generally, and out of correctness, it seems inconsistent that FutureOr<void> f() {} is legal, a function body that falls off the edge without returning a value, but FutureOr<void> f() { return; } is illegal.

Related issues: At first I thought this was an analyzer bug (dart-lang/sdk#44480); other catchError discussion at dart-lang/sdk#34144

[munificent]

More generally, and out of correctness, it seems inconsistent that FutureOr<void> f() {} is legal, a function body that falls off the edge without returning a value, but FutureOr<void> f() { return; } is illegal.

+1.