Argument S to typeof is not an expression

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I tried defining opAdd for S as following:

auto opAdd(alias rhs)()
if (is(typeof(rhs) == S))
{
     return Test!(n + rhs.n);
}

Which I know is kind of silly, since I can't use opAdd to do a + 
b anymore, but it fails to compile anyway. The compiler says that 
b (of type Test) does not match a's version of opAdd, which 
doesn't seem right to me.

Removing the template constraint doesn't help either, as I run 
into "local variable argument to non-global template" bug, and I 
can't get the structs to be evaluated at compile time, so I guess 
that's a lost cause.