[Issue 5710] cannot use delegates as parameters to non-global template

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=5710



--- Comment #29 from Kenji Hara <k.hara.pg at gmail.com> 2013-11-18 02:00:56 PST ---
(In reply to comment #27)
> (In reply to comment #26)
> > (In reply to comment #25)
> > > Good to see, that you already know this solution.
> > > I think the biggest hurdle to implement is teaching the IR/backend to get
> > > doStuff's this pointer from an array.
> > 
> > Unfortunately it is merely an idea, and it would break existing delegate ABI.
> 
> How would it break the existing delegate ABI?

Sorry, it was not good words. It will break some existing assumptions around
delegate. For example:

class C {
  void foo() {}
  void bar(alias fun)() {}
}
void test()
{
  C c = new C();
  auto dg1 = &c.foo;

/* currently upsupported case.

    void nested() {}
    auto dg2 = &c.bar!nested;
    dg2();

   #1. dg2.ptr cannot become c.
   Instead, it should point to the "context pointer list".
   But, if dg2 is returned from the function 'test', the list
   should be allocated on heap.
 */
}

#1 is necessary for the multiple-context delegates. But applying same rule for
existing one-context delegates will add hidden runtime cost against existing
code.

To avoid the cost, we should introduce the second delegate type for the
multiple-context delegates. Then, the typeof(dg1) and typeof(dg2) will become
incompatible. It will make D's type system more complicated.

Therefore, I had withdrawn my idea. It won't add enough benefits compared with
the necessary complexity.

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