Trying to understand something about UDAs.

[Kevin Balbas via Digitalmars-d-learn]

I'm trying to figure out how to test whether a UDA struct was 
initialized with arguments (and change the behavior of the 
surrounding code if it was.  While I was tinkering around with 
test files, I came up with this little gem:

import std.traits;

struct Attr
{
     string str;
}

@Attr("test")
void foo(){}

@Attr
void bar(){}

void main()
{
     alias UDA = getUDAs!(bar, Attr)[0];
     static if (__traits(hasMember, UDA, "str"))
     {
         static if (__traits(compiles, __traits(getMember, UDA, 
"str")))
         {
             __traits(getMember, UDA, "str"); // Compile error! 
need 'this' for 'str' of type 'string'
         }
     }
}

I can rationalize what the error means -- the compiler wasn't 
given a string so it requires a concrete object to get the member 
from.  What I don't understand is why *both* of those static ifs 
passed.  If it has the member and I can't get it, and if getting 
the member compiles yet at the same time it doesn't, I'm not sure 
how I can actually make this test work.

It would make sense if the attribute over bar wasn't semantically 
valid, but I can use it in every other way *except* getting the 
string field out of it.

Am I missing something obvious here?