"Non-Void" constraint for type parameters

I'd like to have a "Non-Void" constraint for type parameters.
Here is why:
```haxe
class Test {
static function main() {
var a = [1, 2, 3].map(function(i) i * 2); //Compiler does not complain that function does not returning any value.
trace(a); //null, null, null
}
}
```
https://try.haxe.org/#aa8D8

Start by reproducing the provided Haxe example and the linked try.haxe.org case. The issue names no source file or test path; done would mean defining and enforcing the proposed “Non-Void” type-parameter constraint so callbacks that return no value are rejected.

Written by the indexing model from the issue text.