DIP66 - Multiple alias this

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On Tuesday, 28 October 2014 at 20:09:07 UTC, IgorStepanov wrote:
> And please comment my way to resolving "is" expression via 
> alias-this:
> http://forum.dlang.org/thread/ubafmwvxwtolhmnxbrsf@forum.dlang.org?page=5

Something else related to the discussion about `is` from this 
thread: 
http://forum.dlang.org/post/tulvmydnogbgebnxybfk@forum.dlang.org. 
The following code:

import std.math;
import std.traits;

struct S(T)
if(isFloatingPoint!T)
{
     T val;
     alias val this;
}

void main()
{

     auto s = S!float();
     assert(isNaN(s));
     s = 10.0;
     assert(!isNaN(s));
}

Current fails to compile with this error:

Error: template std.math.isNaN cannot deduce function from
argument types !()(S!float), candidates are:

std/math.d(4171):        std.math.isNaN(X)(X x) if
(isFloatingPoint!X)

Is this a problem with the current implementation of `alias 
this`, or should isFloatingPoint be changed so that it also 
accepts types that alias a floating point type?