[Issue 23416] Difference DMD/GDC: Error: non-constant expression `& foo`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23416

--- Comment #2 from kdevel <kdevel at vogtner.de> ---
This code should compile and pass its unittest:

```post2.d
import std.stdio;
struct S {
   void function () fp = &foo;
   void foo ()
   {
      __PRETTY_FUNCTION__.writeln;
      fp = (&bar).funcptr;
   }
   void bar ()
   {
      __PRETTY_FUNCTION__.writeln;
      fp = (&foo).funcptr;
   }
   auto fun () // invocation helper
   {
      void delegate () dg;
      dg.ptr = &this;
      dg.funcptr = fp;
      return dg ();
   }
}

unittest {
   S s;
   s.fun;
   s.fun;
}
```

$ dmd -O -unittest -main -run post2 
post2.d(3): Error: non-constant expression `& foo`

But this code should compile and pass:

$ gdc -O -funittest -fmain post2.d -o post2 && ./post2
void post2.S.foo()
void post2.S.bar()
1 modules passed unittests

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