Weird interaction of design choices: Duration + sum

[jmh530]

On Friday, 13 October 2017 at 19:28:03 UTC, Steven Schveighoffer 
wrote:
>> 
>> This wouldn't help, sum is in phobos, Duration in druntime.
>> 
>> The answer is that sum shouldn't be too clever. IMO, 
>> initializing to 0 should be only done if the type's .init 
>> value doesn't match 0. Since only floating points and 
>> char/wchar/dchar do this, just do the Seed(0) for those, else 
>> use Seed.init.
>
> I take the last part back a bit. Some custom types may default 
> to NaN-like values, so Seed(0) is necessary.
>
> What should happen is that sum should test if Seed(0) is 
> viable, and if not, use Seed.init.
>
> -Steve

Ah, I see. Something like:

private enum bool canZeroConstruct(R) = is(typeof(R(0)) == R);

auto sum(R)(R r)
     if (isInputRange!R && !isInfinite!R && is(typeof(r.front + 
r.front)))
{
     import std.algorithm : sum;
     alias E = Unqual!(ElementType!R);
     static if (isFloatingPoint!E)
         alias Seed = typeof(E.init + 0.0); //biggest of 
double/real
     else
         alias Seed = typeof(r.front + r.front);
     static if (canZeroConstruct!E)
         return sum(r, Unqual!Seed(0));
     else
         return sum(r, Unqual!Seed.init);
}