Re: new DIP40: Template parameter deduction for constructors

[deadalnix]

On Tuesday, 14 May 2013 at 09:56:14 UTC, Timon Gehr wrote:

DMD's strategy is roughly: use the first eponymous declaration that can be found without analysing the template body for IFTI, then use the first eponymous declaration in the analyzed template body to resolve the eponymous declaration after instantiation.

---
import std.stdio;
template fun(T){
        int fun(double arg){ return 1; }
        int fun(T arg){ return 0; }
}
void main(){ writeln(fun(2)); } // error

---
import std.stdio;
template fun(T){
        int fun(T arg){ return 0; }
        int fun(double arg){ return 1; }
}
void main(){ writeln(fun(2)); } // ok

---

This has funny implications, as the compiler may decide to resolve to a different declaration than was used for IFTI later, without doing any kind of overload resolution within the template body.

---
template fun(T){
        int fun(T arg){ return 0; }
        static if(true) int fun(double arg){ return 1; }
}
pragma(msg, fun(2)); // 0
---
template fun(T){
        static if(true) int fun(double arg){ return 1; }
        int fun(T arg){ return 0; }
}
pragma(msg, fun(2)); // 1
---

In the second case, instantiation is performed with the second function, so T is resolved to 'int', but in the end, the 'double' overload is called with an implicit conversion.

That creative. But completely b0rken IMO