UFCS on forward reference

[Timon Gehr]

On 05/15/2012 11:18 AM, Jens Mueller wrote:
> Timon Gehr wrote:
>> On 05/15/2012 07:44 AM, Ali Çehreli wrote:
>>> On 05/14/2012 10:02 PM, Timon Gehr wrote:
>>>> On 05/15/2012 04:28 AM, John Belmonte wrote:
>>>>> C API's often use a opaque struct pointer as a handle. Mapping such a
>>>>> struct to D using a forward declaration, I noticed that UFCS doesn't
>>>>> work:
>>>>>
>>>>> struct State;
>>>>> ...
>>>>> State* s = new_state();
>>>>> foo(s); // ok
>>>>> s.foo(); // compile error
>>>>>
>>>>> Error detail:
>>>>>
>>>>> Error: struct State is forward referenced when looking for 'foo'
>>>>> Error: struct State is forward referenced when looking for 'opDot'
>>>>> Error: struct State is forward referenced when looking for 'opDispatch'
>>>>>
>>>>> I'm wondering if anything would be harmed by removing this restriction.
>>>>>
>>>>> As a workaround I can use "struct State {}", but that feels wrong.
>>>>>
>>>>
>>>> This is a compiler bug. You can report it here:
>>>> http://d.puremagic.com/issues/
>>>
>>> I would expect the compiler to need to see the definition of S to know
>>> that it really does not have a matching foo() member function.
>>>
>>> Ali
>>>
>>
>> S is opaque. It does not have any visible member functions.
>
> How should the compiler infer that S is opaque? How does it know when
> you write "struct State;" that State has no members? Is opaqueness
> implied when I do a forward declaration?
>
> Jens

This is a compile time error:
struct S;
struct S{}