Unification of function return types may involve the Void type and requires a clear definition of what unifies with Void. With Void describing the absence of a type, it is not assignable to any other type, not even Dynamic. This means that if a function is explicitly declared as returning Dynamic, it cannot return Void.
The opposite applies as well: if a function declares a return type of Void, it cannot return Dynamic or any other type. However, this direction of unification is allowed when assigning function types:
var func:Void->Void = function() return "foo";
The right-hand function is clearly of type Void->String, yet we can assign it to the variable func of type Void->Void. This is because the compiler can safely assume that the return type is irrelevant, given that it could not be assigned to any non-Void type.