std.math.abs and shared/const/immutable BigInts

[Joseph Rushton Wakeling]

Hello all,

I recently discovered this issue with std.bigint.BigInt and std.math.abs:
http://d.puremagic.com/issues/show_bug.cgi?id=11188

In short, the following code:

   import std.bigint, std.math, std.typetuple;

   auto foo(T)()
   {
       T n = -3;
       return std.math.abs(n);
   }

   void main()
   {
       foreach (T; TypeTuple!(byte, short, int, long, BigInt))
       {
           assert(foo!T() == 3);             // works
           assert(foo!(shared(T)) == 3);     // fails for BigInt
           assert(foo!(const(T)) == 3);      // fails for BigInt
           assert(foo!(immutable(T)) == 3);  // fails for BigInt
       }
   }

... will fail to compile for the shared, const and immutable variants of BigInt, 
with the compiler error message being:

/opt/dmd/include/d2/std/math.d(303):        std.math.abs(Num)(Num x) if 
(is(typeof(Num.init >= 0)) && is(typeof(-Num.init)) && !(is(Num* : 
const(ifloat*)) || is(Num* : const(idouble*)) || is(Num* : const(ireal*))))
/opt/dmd/include/d2/std/math.d(314):        std.math.abs(Num)(Num z) if (is(Num* 
: const(cfloat*)) || is(Num* : const(cdouble*)) || is(Num* : const(creal*)))
/opt/dmd/include/d2/std/math.d(322):        std.math.abs(Num)(Num y) if (is(Num* 
: const(ifloat*)) || is(Num* : const(idouble*)) || is(Num* : const(ireal*)))
bigabs.d(6): Error: template std.math.abs(Num)(Num x) if (is(typeof(Num.init >= 
0)) && is(typeof(-Num.init)) && !(is(Num* : const(ifloat*)) || is(Num* : 
const(idouble*)) || is(Num* : const(ireal*)))) cannot deduce template function 
from argument types !()(shared(BigInt))

... and similarly for const and immutable.

What I can't see is why there is a failure to deduce the template function.  In 
fact some checks show that it's the very first of these conditions:

     is(typeof(Num.init >= 0))

... that fails.  This is rather unintuitive.  Doesn't a BigInt -- even an 
immutable one -- default to 0?

Advice very much appreciated, as I'm feeling stumped.

Thanks & best wishes,

     -- Joe