Allow "void" as a type argument

[munificent]

The language will change to allow void as a type argument in generics (and as a type annotation in general).

This comes up in cases like:

• A function that returns nothing asynchronously could return Future<void>. Today, code either uses Future, which doesn't give you great static checking of the result, or Future<Null>, which is kind of unfamiliar to most users.
• Generic visitor classes that are parametric on their visit methods' return types. If you want to create a visitor that returns nothing, it would be nice to be able to explicitly extend AstVisitor<void>.

We also allow void to be the type annotation for locals, parameters, fields, ...

In Dart 1, void should simply be treated like Object, except as a return type for function types (where the old behavior should be kept).

In strong mode, an additional "voidness" rule will disallow assignments from void to Object when the assignment is statically visible. We will update the bug when we have a document ready.

• specification Spec: allow void as a type argument #28944
• VM VM: allow void as a type argument #28945
• dart2js dart2js: allow void as a type argument #28946
• DDC DDC: allow void as a type argument. #28947
• dartfmt Ensure that void as type annotation (and type argument) works dart_style#588

[lrhn]

A void function can actually return anything (or more precisely: a function assignable to a function type with void return type, can return anything because of the way "more specific" is defined on functions).

That has led to errors where someone does

void foo() => delegate.foo();

but the delegate type's foo overrides the expected type with a function returning non-void and returns a non-null value. In checked mode that's an error.

The fix for that will likely be to allow anything to be returned by a void function. That makes void act like Object (and at runtime, it will likely be Object) except that you can't use a value with static type void for anything.

[floitschG]

Status update: we need to do #27727 for 1.50, but can implement this issue at a later point.

[dskloetg]

Another use case could be that raw generic types could implicitly have void as their generic type parameter if the user has disabled implicit dynamic.