I'm not sure if implicit conversions are applied to type deductions. It could be that GUID has a constructor that takes in an integer, which would mean that if

I'm not sure if implicit conversions are applied to type deductions. It could be that GUID has a constructor that takes in an integer, which would mean that if the function were declared with a return type of "GUID", returning -1 would be fine. I don't know if the same applies to functions with "auto" as the return type, but the example with the int and the double suggests not. So, this is a long-winded way of saying "yes, I think it's a bug".