First Draft: Implicit Conversion of Template Instantiations

[Timon Gehr]

On 3/16/24 16:50, Walter Bright wrote:
> https://github.com/WalterBright/documents/blob/9dba63a4c2887bdb2b988c354ebb0d6bb44c4968/templatecast.md
> 
> DConf: https://dconf.org/2024/online/index.html#walterb

Issue: Subtyping under indirections remains magic.

```d
struct Mutability(bool mutable,T){
     static if(mutable){
         T payload;
     }else{
         const(T) payload;
     }
}

void main(){
     int[] a1 = [1,2,3];
     const(int)[] a2 = a1; // ok

     auto m1=[Mutability!(true,int)(1),
              Mutability!(true,int)(2),
              Mutability!(true,int)(3)];
     Mutability!(false,int)[] m2=m1; // error
}
```

(With variance annotations, this example would need some kind of support 
for variance annotations for template value parameters.)