Re: Specialization Not Allowed for Deduced Parameter

[Ali Çehreli via Digitalmars-d-learn]

On 08/22/2014 02:22 PM, Ali Çehreli wrote:

> Admittedly, std.traits.isInstanceOf is the right tool to use
Answering myself: Yes, it is.

> but both of
> the following worked!

I figured that out.

>      auto opBinary(string op, That)(That rhs)
>          if (isInstanceOf!(UnitDef, That) &&

Note that UnitDef above means the template instance UnitDef!unitString. 
(There is such a shortcut in D and C++.)

Since I've been using the same type in my test code, UnitDef!unitString 
and That were the same type. (Hm. Does that mean that one is the 
instance of the other? Will have to test that separately.)

So, the correct check should use std.traits.TemplateOf first:

    auto opBinary(string op, That)(That rhs)
        if (isInstanceOf!(TemplateOf!UnitDef, That) &&
            op == "*") {
        return UnitDef!(unitString ~ " " ~ rhs.US)();
    }

Now, that's correct and allows different instances:

import std.traits;

public struct UnitDef(string unitString) {
    alias US = unitString;

    auto opBinary(string op, That)(That rhs)
        if (isInstanceOf!(TemplateOf!UnitDef, That) &&
            op == "*") {
            pragma(msg, typeof(this));
            pragma(msg, That);
        return UnitDef!(unitString ~ " " ~ rhs.US)();
    }
}

void main()
{
    auto u = UnitDef!"hello"();
    auto v = UnitDef!"world"();
    auto result = u * v;
    pragma(msg, result.US);
}

Ali