Where does "U" in Rebindable.Rebindable come from?

[Ali Çehreli]

On 03/29/2012 01:51 PM, Tove wrote:
> On Thursday, 29 March 2012 at 20:15:43 UTC, simendsjo wrote:
>> If you look at the struct, it uses the type U in, among others, the
>> union.
>>
>> The only place I could see U referenced is in the first static if, but
>> I cannot recreate this behavior:
>> void main() {
>> static if(is(int X == const(U), U))
>> {
>> }
>> else
>> {
>> static assert(is(typeof(U))); // false, U not defined
>> }
>> }
>>
>
> magic pattern matching...
>
> import std.stdio;
>
> void main()
> {
> static if(is(int x == const(U), U))
> writeln("Mutable: " ~ U.stringof);
> else static if(is(const int x == const(U), U))
> writeln("Const: " ~ U.stringof);
> }
>

Some uses of the 'is' expression have been the most magical (read: 
difficult) part of D for me. :) I am still not sure that I understand it 
correctly. ;)

   http://dlang.org/expression.html#IsExpression

Ali