delegate passed in annotation struct cannot be invoked.

[Alexandru Ermicioi via Digitalmars-d-learn]

Given code below:
import std.stdio;

struct Annotation {
     public int delegate(int) dg;
}

void main() {
	import std.traits;
	
	__traits(getAttributes, Cls)[0].dg(20).writeln;
}

@Annotation(delegate int(int d) {
     return d;
})
class Cls {

     void method() {

     }
}

Dmd will complain with following statement for the delegate 
passed in annotation:
src/app.d(13,13): Error: delegate app.__dgliteral6 is a nested 
function and cannot be accessed from D main.

GDC will just throw internal compiler exception:
src/app.d: In function ‘D main’:
src/app.d:20:2: internal compiler error: in get_frame_for_symbol, 
at d/d-codegen.cc:3981
   __traits(getAttributes, Cls)[0].dg(20).writeln;
   ^

LDC will not argue, and compile it flawlessly, and return 20 as 
expected.

So the question, is the dmd error correct behavior?
If so, how the delegate is nested, and what is context that it is 
nested in?

DMD version is: v2.072.1
LDC version is: v1.1.0 based on v2.071.2 and LLVM 3.8.1
GDC version is: 6.2.1 20161215