Is this a bug in return type inference?

[Ali Çehreli via Digitalmars-d]

On 04/26/2015 12:32 PM, Meta wrote:
> import std.random;
>
> auto test(int n)
> {
>      if (n >= 0 && n < 33)
>      {
>          return int(0);
>      }
>      else if (n >= 33 && n < 66)
>      {
>          return float(0);
>      }
>      else
>      {
>          return real(0);
>      }
> }
>
> void main()
> {
>      auto n = uniform(0, 100);
>      auto res = test(n);
>      //Prints "float"
>      pragma(msg, typeof(res));
> }
>
> I expected the result to be real. Why is the return type of test
> inferred as float instead? I would expect it to choose real, as both int
> and float can be converted to real without precision loss, but the
> opposite is not true. Is this a bug?

Yes, a bug for floating types only. It seems that not the common type 
but the first type is used among floating point types. I wrote a short 
program to prove it to myself:

import std.traits;
import std.typetuple;
import std.format;

auto foo(A, B)(int n)
{
     if (n) {
         return A(0);

     } else {

         return B(0);
     }
}

void main()
{
     alias types = TypeTuple!(float, double, real);

     foreach (A; types) {
         foreach (B; types) {
             alias ReturnType = typeof(foo!(A, B)(0));

             pragma(msg, format("%s %s -> %s%s",
                                A.stringof, B.stringof,
                                ReturnType.stringof,
                                (is (ReturnType == CommonType!(A, B))
                                 ? ""
                                 : " <-- BUG")));
         }
     }
}

float float -> float
float double -> float <-- BUG
float real -> float <-- BUG
double float -> double
double double -> double
double real -> double <-- BUG
real float -> real
real double -> real
real real -> real

Ali