Pop Quiz what is wrong with this code?

[WebFreak001]

On Monday, 22 June 2020 at 12:42:26 UTC, Steven Schveighoffer 
wrote:
> On 6/20/20 11:22 PM, Danni Coy wrote:
>> On Sun, Jun 21, 2020 at 9:40 AM Avrina via Digitalmars-d
>> <digitalmars-d at puremagic.com> wrote:
>>>
>>> On Saturday, 20 June 2020 at 18:15:27 UTC, Steven 
>>> Schveighoffer
>>> wrote:
>>>> On 6/20/20 12:00 PM, Danni Coy wrote:
>>>>
>>>>> I tried explicitly making x and y ints and I got a
>>>>> depreciation warning.
>>>>
>>>> foreach(ptrdiff_t y, ref row; offsetMap)
>>>>
>>>> is that what you wanted?
>>>>
>>>> ptrdiff_t is the signed version of size_t. The complaint is 
>>>> not
>>>> that you are converting from unsigned to signed, but that you
>>>> are converting from 64-bit to 32-bit.
>
>>> Why isn't that deprecated as well? implicitly converting from
>>> ulong to long is an error as much as ulong to uint.
>> 
>> It turns out this line is the bigger problem.
>> 
>>   immutable SDL_FPair offset  = { center.x - x, y - center.y };
>> FPair is a
>> struct SDL_FPair
>> {
>>      float x;
>>      float y;
>> ...
>> if those values were ints.
>> dmd will not allow a downcast from ulong to int
>> (the compiler would have caught the issue for me)
>> but it will allow casting ulong to float which is why It took 
>> me a
>> couple of hours with a debugger
>> trying to find the bug.
>
> So this entire conversation is incorrect -- there is no 
> promotion to unsigned happening.
>
> What is the bug with size_t implicitly casting to float? I 
> would think it would work.
>
> -Steve

I think the issue is that it's converting the ulongs to float, 
which are actually negative and not positive: center.x - x and y 
- center.y might become negative and because it's a ulong it will 
be a huge number, then converting to float it will be a huge 
float number instead of some normal negative number.

so:

size_t centerX = 1;
size_t x = 3;
float f = centerX - x; // <- this is not -2, this is 1.84467e+19 
without any warnings or deprecations printed

Meanwhile an implicit cast to int is disallowed (error) but it 
would actually contain the correct result and nobody would have 
ever found it.