Syntax ideas for `ref` returning delegate and FP parameters

[Dukc]

On Saturday, 10 April 2021 at 14:27:31 UTC, Q. Schroll wrote:
> Currently in D, one cannot directly express that a function 
> parameter's type is a delegate or function pointer that returns 
> by `ref`. If you write
> ```D
> void f(ref int delegate() dg) { .. }
> ```
> then `dg` will be bound by reference which is probably not what 
> the user likely intended. It is equivalent to:
> ```D
> alias DG = int delegate();
> void f(ref DG dg) { .. }
> ```

Question to somebody more familiar with grammar specs: is this 
behaviour enforced by the spec, or is it implementation-defined 
behaviour?