defect when using 'alias this' in a struct?

[Kieron Brown]

I have used alias this and found odd behaviour in complex code.

I tracked the problem down and was able to replicate it
with a much simpler case as given below.

I'm new to D (but have read TDPL).
Is this my mis-understanding or a defect?

Many thanks, Kieron

----- self contained d sample starts -----
import std.stdio, std.math;

void main()
{
    auto p = Test(0.5, 0.5);
    assert(0.5 == p.a); // ok
    assert(0.5 == p.b); // ok
    Test q;
//  q = Test(0.5, 0.5);
    //
    // the above line gave same
    // error when subst for
    // the line below
    q = p;
    if (isNaN(q.b))
        printf("problem replicated\n");
    assert(0.5 == q.a); // ok
    assert(0.5 == q.b); // this fails
}

struct Test
{
    double a;
    double b;
    alias a this;
}
----- end of file -----