Strange template instantiating behaviour

[ref2401]

public template rgba(uint value)
{
        enum Color rgba = Color(
                cast(ubyte)((value >> 24) & 0xFF),
                cast(ubyte)((value >> 16) & 0xFF),
                cast(ubyte)((value >> 8) & 0xFF),
                cast(ubyte)(value & 0xFF));
}
public struct Color
{
        public static immutable Color black = rgba!0xFF;


        public ubyte r = 0;

        public ubyte g = 0;

        public ubyte b = 0;

        public ubyte a = 0;


        public this(ubyte r, ubyte g, ubyte b, ubyte a)
        {
                this.r = r;
                this.g = g;
                this.b = b;
                this.a = a;
        }

        public string toString() const
        {
		return std.string.format("(r: %s, g: %s, b: %s, a: %s)", r, g, 
b, a);
        }
}

void main()
{
        Color templateValue = rgba!0x00_00_00_FF;
        Color ctorValue = Color(0, 0, 0, 255);

        writefln("[template result] %s", templateValue);
        writefln("[ctor. result] %s", ctorValue);
        readln();
}

Console output:
[template result] (r: 0, g: 0, b: 0, a: 255)
[ctor. result] (r: 0, g: 0, b: 0, a: 255)


Now, if we change fields order in the struct we will get strange 
template instantiating behaviour.

public struct Color
{
        public static immutable Color black = rgba!0xFF;


	public ubyte a = 0; // "a" is the last field in the previous 
example

        public ubyte r = 0;

        public ubyte g = 0;

        public ubyte b = 0;

        ...

}

...

Console output:
[template result] (r: 0, g: 0, b: 255, a: 0)
[ctor. result] (r: 0, g: 0, b: 0, a: 255)

if we change templateValue variable or Color.black value(or even 
comment it) all works correct.
is it a bug?