Testing implicit conversion to template instance with is() expression

[via Digitalmars-d-learn]

On Sunday, 15 March 2015 at 16:53:34 UTC, Marc Schütz wrote:
> On Sunday, 15 March 2015 at 16:44:14 UTC, Ali Çehreli wrote:
>> On 03/15/2015 08:47 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= 
>> <schuetzm at gmx.net>" wrote:
>>
>> > Should this work?
>> >
>> >     struct V(string s) {
>> >     }
>> >
>> >     struct S(int U) {
>> >         V!"xyz" x;
>> >         alias x this;
>> >     }
>> >
>> >     void main() {
>> >         S!10 a;
>> >         static assert(is(a : V!Args, Args...));
>> >     }
>> >
>> > With DMD Git master, the static assert() fails. Should it? Am
>> I doing
>> > something wrong? How can I test whether something is
>> implicitly
>> > convertible to any instance of a particular template?
>>
>> There is no way other than checking for compile-time duck 
>> typing (see the implementations of isInputRange and others).
>>
>> One reason is that the compiler does not have the concept of 
>> "an instance of a template". Templates are for code generation 
>> and only the end-result (i.e. S!10) lives as a concept when 
>> compiling.
>
> The code contained a small mistake, I forgot a `typeof()`:
>
>     // static assert(is(a : V!Args, Args...));
>     // should be:
>     static assert(is(typeof(a) : V!Args, Args...));
>
> This still fails, but it works when I change it to:
>
>     static assert(is(typeof(a) : S!Args, Args...));
>
> This means I can indeed test whether something _is_ an instance 
> of a template. It just doesn't take the `alias this` into 
> account. So I guess that's a bug?

Ok, now I'm pretty sure:

     class V(string s) {
     }

     class S(int U) : V!"xyz" {
     }

     void main() {
         S!10 a;
         static if(is(typeof(a) : V!Args, Args...))
             pragma(msg, Args);
     }

This works, and it even correctly infers `Args` to be 
`tuple("xyz")`. As `alias this` is supposed to be interchangeable 
with subtyping, it must be a bug.