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: I think the field names also have to match.

Consider:

```d
struct S(bool swapped,T1,T2){
     static if(swapped){
         T2 second;
         T1 first;
     }else{
         T1 first;
         T2 second;
     }
}

void main(){
     import std.stdio;
     auto s1=S!(true,int,int)(1,2);
     S!(false,int,int) s2=s1;
     writeln(s1.first," ",s1.second); // 1 2
     writeln(s2.first," ",s2.second); // 2 1
}
```