Using opApply and Attributes

[Q. Schroll via Digitalmars-d-learn]

When using functions with delegate (or function ptr) parameters 
which should comply with some attributes, I cannot call the 
delegate in the function what makes it pretty useless (I haven't 
done research, but I claim that generally most functions taking 
delegate parameters call them).

   void f1(void delegate(int) dg) // cannot be pure ...
   {
     dg(1); // ... due to this call.
   }

   void main() pure
   {
     f(x => 2*x); // Error, f is impure.
   }

One has to overload the function like
   auto f(delegate(Arg arg) @attrib dg) @attrib;
   auto f(delegate(Arg arg)         dg);
for each combination of attributes available (today 4, worst case 
is 16 overloads), could be more in the future.

Even worse (due to bug 15859, see [1]) overload resolution on 
opApply does not work properly. Making opApply a template is a 
real solution for various reasons (virtual functions, no type 
inference on the delegate, etc.). For opApply, using the range 
interface (empty, front, popFront) is not a real solutions either 
because opApply can be truly recursive. Those can then only have 
a range interface through generators via fibers which have an 
overhead (and make them non- at nogc).

How can I have relative- at attrib functions without unnecessary 
manual overloading?
(relative: if all called parameter delegates have it, then the 
function has it)

Generally, are relative attributes worth an enhancement?

[1] https://issues.dlang.org/show_bug.cgi?id=15859