[llvm-bugs] [Bug 39519] New: clang incorrectly implements inc/dec?

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39519

            Bug ID: 39519
           Summary: clang incorrectly implements inc/dec?
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

https://reviews.llvm.org/D53949#1282884
@regehr
The C99 standard (6.5.3.1.2) appears to be very clear on this point: "The
expression ++E is equivalent to (E+=1)."

[17:52:50] <TNorthover> LebedevRI: John Regehr is right.
[17:53:32] <TNorthover> It also says "See the discussions of additive operators
for information on [...] conversions [...]".
[17:53:37] <LebedevRI> TNorthover: so clang is implementing inc/dec incorrectly
[17:54:25] <AaronBallman> LebedevRI: yeah, I agree with John on that

https://godbolt.org/z/qd-QV4
clang clearly does not do promotions in the c++; case.

The question at hand is:
* signed char c = 127; c++;
  Is this UB? UBSan does not catch it.
* unsigned char c = 255; c++;
  Is this (not ub!) unsigned integer overflow?
  Integer sanitizer does not catch it.

If these aren't ub/overflows, then the computation should be done in some
promoted type?
If the computation is not done in promoted type, then conversion sanitizer will
not catch it either.

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