void is not supported as a generic method type argument

[jmesserly]

Reported by @jacob314 over at dart-archive/dev_compiler#496

---

Repro:

wrapZone/*<E, R>*/(/*=R*/ callback(/*=E*/ evt)) {

}
...
wrapZone/*<Event, void>*/(someCallback).

[jmesserly]

CC @leafpetersen

It'd be nice to have some consistency around void -- this was something I always found annoying about generic function types in C# (and our func.dart package has a similar problem)

That said this seems like a Dart 2 issue, rather than something we can address in the short term.

(Future<void> is a similar one, we get that style question a lot from folks)

[zoechi]

Shouln't it be Null instead of void or is this about support void as alternative to Null?

[lrhn]

In Dart, void is not a type expression. It's something you can write instead of a type expression as the return type of a function. So, you can't use it as a type parameter, even if the type parameter is going to be used as a return type.

In this particular case, I'd consider parameterizing the class with the function type instead of with the parameter and return type:

wrapZone/*<F extends UnaryFunction>*/(/*=F*/ callback)

Persoanlly, I'd accept void as a type anywhere. It doesn't make a lot of sense as a parameter type, but if you really want it, I don't have a problem with that.

(The "void" type represents the empty type, a type with no value, hence the name. It doesn't make sense in places other than return types because you can never produce a value of that type - and for return types it only makes sense because null is assignable to any type, even void - with non-nullable types, a void! function would be one that never returns, e.g., always throws).

[jmesserly]

CC @leafpetersen ... does it make sense to kick this to area-language instead of strong-mode. I don't think there's anything actionable we can do in the implementation

[jmesserly]

I'm going to reclassify this as area-language, I don't think it's something we can address via strong mode

[leafpetersen]

Under consideration for the language.

[munificent]

De-duping with newer but more active other issue: #27504.