Does `is` expression with template alias need fixing.

[FeepingCreature]

On Wednesday, 22 March 2023 at 12:34:26 UTC, FeepingCreature 
wrote:
> On Wednesday, 22 March 2023 at 12:30:46 UTC, FeepingCreature 
> wrote:
>> On Wednesday, 22 March 2023 at 10:43:15 UTC, Elfstone wrote:
>>> I don't know how C++ compilers resolve `template using` just 
>>> fine.
>>>
>>> ```C++
>>> template <typename T>
>>> using Vector3 = Matrix<T, 3, 1>;
>>> ```
>>>
>>> I declare an alias and I use it everywhere. It acts as a 
>>> natural constraint. I never needed `isVector3` with my old 
>>> C++ code. I expected D could do the same, and was really 
>>> frustrated when I found out it couldn't. Even more frustrated 
>>> when I read Steven's reply, that the bug with `is` has been 
>>> there for 16 years.
>>>
>>> When the compiler allows people to use template aliases as 
>>> template function parameters, it should make it work, or 
>>> reject it aloud. Maybe some future system can save the day, 
>>> but does it mean the old `alias` will just be left bugged?
>>
>> You can perfectly fine use `alias Vector3(T) = Matrix!(T, 3, 
>> 1)` in D. You cannot *constrain a type to be Vector3* in D, 
>> but you cannot in C++ either. (I don't think so? I'm poking at 
>> C++20 concepts, but I don't see a way to do it.)
>
> Oh wait, correction: you *can* do it! Neat! I wonder how that 
> works.
>
> Probably it's because `using` is more syntactically constrained?
>
> ```
> template <typename T, int M, int N>
> struct Matrix {
> };
>
> template <typename T>
> using Vector3 = Matrix<T, 3, 1>;
>
> template<typename T>
> void baz(Vector3<T>& vector)
> {
> }
>
> int main() {
>   auto v = Vector3<int>();
>   baz(v);
> }
> ```

Looks like it substitutes through simple cases but fails with 
more complex ones?

```
#include <type_traits>

template <typename T, int M, int N>
struct Matrix {
};

template <typename T>
using Vector3 = typename std::conditional<true, Matrix<T, 3, 1>, 
void>::type;

template<typename T>
void baz(Vector3<T>& vector)
{
}

int main() {
   auto v = Vector3<int>();
   /*
   test.cpp:17:6: error: no matching function for call to 
‘baz(Matrix<int, 3, 1>&)’
      17 |   baz(v);
         |   ~~~^~~
   test.cpp:11:6: note: candidate: ‘template<class T> void 
baz(Vector3<T>&)’
      11 | void baz(Vector3<T>& vector)
         |      ^~~
   test.cpp:11:6: note:   template argument deduction/substitution 
failed:
   test.cpp:17:6: note:   couldn’t deduce template parameter ‘T’
   */
   baz(v);
}
```

That much we can probably implement in D, granted: specifically 
recognize trivial aliases in the template resolution code.

Do file a bug for that.