Template mixins and struct constructors

[Daniel Kozak via Digitalmars-d-learn]

On Wednesday, 2 March 2016 at 12:27:04 UTC, Adrian Matoga wrote:
> I can do this:
>
> struct Foo {
> 	int a;
> 	string b;
> 	this(int a) { this.a = a; }
> 	this(Args...)(string b, auto ref Args args) { this.b = b; 
> this(args); }
> }
>
> unittest {
> 	auto foo1 = Foo(5);
> 	auto foo2 = Foo("foo", 15);
> }
>
> However, the following code is invalid:
>
> mixin template AddField(T) {
> 	T b;
> 	this(Args...)(T b, auto ref Args args)
> 	{
> 		this.b = b;
> 		this(args);
> 	}
> }
>
> struct Bar {
> 	mixin AddField!string;
> 	int a;
> 	this(int a) { this.a = a; }
> }
>
> unittest {
> 	auto bar1 = Bar(5);
> 	auto bar2 = Bar("bar", 15);  // line 31
> }
>
> sctor.d(31): Error: constructor sctor.Bar.this (int a) is not 
> callable using argument types (string, int)
>
> Is it by design or is it a bug?
> And, if it is by design, what is the reason for that?

You can use string mixins:

template AddField(T) {
         enum AddField = T.stringof ~ `  b;
         this(Args...)(` ~ T.stringof ~ ` b, auto ref Args args)
         {
                 this.b = b;
                 this(args);
         }`;
}

struct Bar {
         mixin(AddField!string);
         int a;
         this(int a) { this.a = a; }
}

unittest {
         auto bar1 = Bar(5);
         auto bar2 = Bar("bar", 15);  // line 31
}