[Issue 16997] New: Real-life evidence for integer promotion of unary minus creating bugs when porting C code

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16997

          Issue ID: 16997
           Summary: Real-life evidence for integer promotion of unary
                    minus creating bugs when porting C code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: aliloko at gmail.com

Consider the following C++ program:


-----
#include <cstdio>

int main()
{
    signed char c = -128;
    printf("%d\n", (int)(-c));
    return 0;
}
-----

This prints "128".


Consider the supposedly identical D code:

-----
import core.stdc.stdio;
void main()
{
    byte c = -128;
    printf("%d\n", cast(int)(-c));       // -128
}
-----

Since the D unary minus doesn't promote byte to int, it yields a different
result.


This is a real case that costed ketmar 40 min, while porting working C code.
Porting to D silently broke the code due to subtly different integer promotion
rules. I'm pretty sure I would have given up becasue code that is translated is
often enough not understood yet.


After investigation:
  * Taking the unary minus of byte and short and then asting to int yield
different results for the lowest values: -128 and -32768
  * Taking the unary minus of ubyte and ushort and then casting to int
 yield different results for any value except 0

Why are the promotion rules for unary + and - different from C?

--