auto add(T)(T a,T b)
{
    return to!RetureType(a+b);
}

void fun(T)(T x)
{

}
fun!double(add(1,2));
fun!int(add(1,2));

just add a implicit RetureType,not need visible,background syntax tree like this

auto add(RetureType,T)(T a,T b)

but we can see ,the type reduce logical changed from down-up to up-down
template SelectFrom(T)
{
 alias SelectFrom=T;
}
auto add(T=SelectFrom!(ReturnType))(T a,T b)

these will change the language' template more magic when mix use with opCast,opDispacth...act