__traits(getAttributes) with string mixin

[Jacob Carlborg]

I have some code looking like this. I don't understand why I can use a 
string mixin together with __traits(getAttributes) when it doesn't work 
if I use the template "getAttributes".

template Tuple (T...)
{
    alias T Tuple;
}
struct Foo
{
    @(3) int a;
}

template getAttributes (alias s)
{
    alias Tuple!(__traits(getAttributes, s)) getAttributes;
}

void main ()
{
    Foo foo;
    writeln(__traits(getAttributes, mixin("foo.a")));

    // Error: variable foo cannot be read at compile time       
    // writeln(getAttributes!(mixin("foo.a")));

    writeln(getAttributes!(foo.a));
    mixin(`writeln(getAttributes!(foo.a));`);
}

--
/Jacob Carlborg