(ARM §5.16) The compiler does not apply reference conversions to the second and third expressions of the conditional operator. In other words, unless the second and third expressions are numeric types, they must be the same type.

class base { };
class derived : public base { };

static void foo(derived i)
{
  base      &a = i;
  derived   &b = i, c;
c = (sizeof(0) ? a:b); // ERROR: b is not converted to (base &)
c = (sizeof(0) ? a:(base &)b)    // OK, typecast
}