Curious effect with traits, meta, and a foreach loop ... mystifies me.

[james.p.leblanc via Digitalmars-d-learn]

Dear All,

In playing with some reflection and meta programming, this 
curiosity
appeared.
Does someone understand what is happening?  I would appreciate 
learning
about it if possible.  Enclosed code snippet tells the story:

```d
import std.stdio;
import std.traits;
import std.meta;

struct  S0{ double[3] x; };
struct  S1{ double junk0 ; double[3] x; };

union U { S0 s0;  S1 s1; }

void main(){

   U u;

   double* ptr;

   u.s0.x = [ 0.0, 0.1, 0.3 ];
   u.s1.x = [ 1.0, 1.1, 1.3 ];

   writeln("typeid( u.tupleof): ",    typeid( u.tupleof ) );
   writeln("typeid( u.tupleof[0]): ", typeid( u.tupleof[0] ) );
   writeln("typeid( u.tupleof[0].x): ", typeid( u.tupleof[0].x ) 
);

   writeln();

   // indeed both tuples exist
   writeln("u.tupleof[0].x.ptr: ", u.tupleof[0].x.ptr  );
   writeln("u.tupleof[1].x.ptr: ", u.tupleof[1].x.ptr  );

   // this is fine (notice that 'val' is never used
   foreach( i, val ; u.tupleof ){
      ptr = u.tupleof[i].x.ptr;
      writeln("ptr: ", ptr);
   }

   // this fails with: "Error: variable 'i' cannot be read at 
compile time
   //
   // foreach( i ; 0 .. 3 ){
   //    ptr = u.tupleof[i].x.ptr;
   //    writeln("ptr: ", ptr);
   // }
}

```

Best Regards,
James