Yes, constraints can have helpful error messages!

[Marvin Hannott]

On Friday, 15 April 2022 at 10:07:10 UTC, Atila Neves wrote:
> Have you looked at https://github.com/atilaneves/concepts ?

I have, but, ehm, to be honest, I don't quite understand how this 
is different from just not using constraints at all. Referring to 
your example:

```D
import concepts.models: models;

void checkFoo(T)()
{
     T t = T.init;
     t.foo();
}

enum isFoo(T) = is(typeof(checkFoo!T));

@models!(Foo, isFoo) //as a UDA
struct Foo
{
     void foo() {}
     static assert(models!(Foo, isFoo)); //as a static assert
}

// can still be used as a template constraint:
void useFoo(T)(auto ref T foo) if(isFoo!T) {

}
```

I mean, where's the difference between `checkFoo(T)` failing or 
`useFoo(T)`? And to check that a type satisfies a constraint with 
`static assert` can be done with only `checkFoo(T)`, letting it 
return `true`.

Being able to implement compile-time interfaces is really cool, 
though.

And I apologize if I am getting this completely wrong. Sometimes 
it can be difficult to immediately see why something is useful.