Testing implicit conversion to template instance with is() expression
via Digitalmars-d-learn
digitalmars-d-learn at puremagic.com
Sun Mar 15 09:53:33 PDT 2015
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?
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