hasDataMember and mixin inconsistency

[Olivier Grant]

Hi,

First of all, I am very new to D as I've just been playing around 
with it for the last week, so I might be missing something very 
obvious.

I'm trying to write a template that would allow me to determine 
if a struct or class has a data member with a specific name 
(similar to what the hasMember template does, but only for data 
members).

I originally wrote the following template :

[code]
template hasDataMember( T, string M )
{
    enum hasDataMember = __traits(
       compiles,
       mixin("( ref T x, ref T y ){ x." ~ M ~ " = y." ~ M ~ "; }")
    );
}

unittest
{
    struct Test
    {
       struct A { }
       void B( ) { }

       @property A C( ) const { return F; }
       @property long D( ) const { return E; }
       @property void D( long e ) { E = e; }

       long E;
       A F;
    }

    assert(!hasDataMember!(int,"A"));
    assert(!hasDataMember!(int,"init"));

    //assert(!hasDataMember!(Test,"init")); // This fails, why?
    assert(!hasDataMember!(Test,"A"));
    assert(!hasDataMember!(Test,"B"));
    assert(!hasDataMember!(Test,"C"));
    assert( hasDataMember!(Test,"D"));
    assert( hasDataMember!(Test,"E"));
    assert( hasDataMember!(Test,"F"));
}
[/code]

And it works pretty well, but it gets the wrong result for the 
following test case which is commented out. Originally, I thought 
maybe you were allowed to write to the .init member of 
structures, but writing the same code directly without relying on 
a mixin actually yields the right result (all the asserts pass):

[code]
struct A { int b_; }
assert(!__traits(compiles,( ref int x, ref int y ){ x.init = 
y.init; }));
assert(!__traits(compiles,( ref Test0 x, ref Test0 y ){ x.init = 
y.init; }));
assert( __traits(compiles,( ref Test0 x, ref Test0 y ){ x.b_ = 
y.b_; }));
[/code]

So I have two questions :
1) Why is there this inconsistency between the mixin and direct 
version ?
2) Is there a better way to check for the existence of a data 
member ?

Best regards,

Olivier.