Using std traits

[JN via Digitalmars-d-learn]

I decided it's time to learn how std traits work. I still find 
the whole compile time business a bit weird to deal with, so I 
decided to write a simple JSON serializer for struct that loops 
over member fields and outputs them.

import std.stdio;
import std.json;
import std.traits;
struct TestStruct
{
        @("noserialize") int x;
        int y;
        int z;
}

void serialize(T)(T obj)
{
        if (is(T == struct))
        {
                foreach (i, member; FieldNameTuple!T)
                {
                        if (!hasUDA!(member, "noserialize"))
                        {
                                writeln(member);
                        }
                }
        } else {
                assert(0, "Not a struct");
        }
}

void main()
{
        TestStruct ts;
        ts.x = 1;
        ts.y = 2;
        ts.z = 3;
        serialize(ts);
}

here's a runnable version: https://ideone.com/U4ROAT


I expected it to output "y" "z", but "x" is also present. What am 
I doing wrong with hasUDA?