avoid_futureor_void

Don't use the type 'FutureOr'.

The analyzer produces this diagnostic when the type FutureOr<void> is used as the type of a result (to be precise: it is used in a position that isn't contravariant). The type FutureOr<void> is problematic because it may appear to encode that a result is either a Future<void>, or the result should be discarded (when it is void). However, there is no safe way to detect whether we have one or the other case because an expression of type void can evaluate to any object whatsoever, including a future of any type.

It is also conceptually unsound to have a type whose meaning is something like "ignore this object; also, take a look because it might be a future".

An exception is made for contravariant occurrences of the type FutureOr<void> (e.g., for the type of a formal parameter), and no warning is emitted for these occurrences. The reason for this exception is that the type does not describe a result, it describes a constraint on a value provided by others. Similarly, an exception is made for type alias declarations, because they may well be used in a contravariant position (e.g., as the type of a formal parameter). Hence, in type alias declarations, only the type parameter bounds are checked.

import 'dart:async';
​
FutureOr<void> m() => null;

A replacement for the type FutureOr<void> which is often useful is Future<void>?. This type encodes that a result is either a Future<void> or it is null, and there is no ambiguity at run time since no object can have both types.

It may not always be possible to use the type Future<void>? as a replacement for the type FutureOr<void>, because the latter is a supertype of all types, and the former is not. In this case it may be a useful remedy to replace FutureOr<void> by the type void.