Function hijack on selective import

[rumbu via Digitalmars-d-learn]

Is there anyway to extend an existing function to accept custom 
data types?


****************
Option 1 - global import of std.math


import std.math;

struct Custom {}

int signbit(T)(T x) if (is(T == Custom))
{
    return 0;
}

Custom c;
assert(signbit(c) == 0);
assert(signbit(-1.0) == 1);

Error: template main.signbit cannot deduce function from argument 
types !()(double), candidates are:
main.signbit(T)(T x) if (is(T == Custom))


*******************
Option 2 - selective import of std.math

import std.math : signbit;

struct Custom {}

int signbit(T)(T x) if (is(T == Custom))
{
    return 0;
}

Custom c;
assert(signbit(c) == 0);
assert(signbit(-1.0) == 1);


main.signbit called with argument types (Custom) matches both:
\..\src\phobos\std\math.d(5721):     
std.math.signbit!(Custom).signbit(Custom x)
and:
main.signbit!(Custom).signbit(Custom x)