A valid function with no return type?

[Max Haughton]

https://issues.dlang.org/show_bug.cgi?id=12638

https://run.dlang.io/is/jkEIiI

ref foo() {}

void main()
{
     import std.traits;
     pragma(msg, ReturnType!foo);
     foo();
}

Assuming this is supposed to be allowed by the standard, 
replacing void with ref (i.e. ref main { ) does not compile which 
suggests something isn't right.