Template mixins and struct constructors

[Adrian Matoga via Digitalmars-d-learn]

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?