[Bug c++/114663] New: Several contracts test cases fail with -fsanitize=undefined -fsanitize-trap

["iains at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114663

            Bug ID: 114663
           Summary: Several contracts test cases fail with
                    -fsanitize=undefined -fsanitize-trap
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: testsuite-fail, wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iains at gcc dot gnu.org
                CC: jason at gcc dot gnu.org
  Target Milestone: ---

I found this while working on -funreachable-traps (but the failure equally
occurs with -fsanitize=undefined -fsanitize-trap)

FAIL: g++.dg/contracts/contracts10.C   execution test
FAIL: g++.dg/contracts/contracts18.C   execution test
FAIL: g++.dg/contracts/contracts19.C   execution test
FAIL: g++.dg/contracts/contracts2.C   execution test

Initial analysis is that somehow the lowering of the contracts code is
exploiting UB [which has a large measure of irony if true] to make these cases
pass, for example contracts2.C optimised tree dump contains:

;; Function main (main, funcdef_no=0, decl_uid=2531, cgraph_uid=1,
symbol_order=0)

int main ()
{
  int x;
  int D.2551;
  const struct  D.2542;
  int _2;

  <bb 2> :
  x_1 = 1;
  if (x_1 < 0)
    goto <bb 4>; [INV]
  else
    goto <bb 3>; [INV]

  <bb 3> :
  __builtin_unreachable ();

  <bb 4> :
  if (x_1 <= 0)
    goto <bb 5>; [INV]
  else
    goto <bb 6>; [INV]

  <bb 5> :

=====

When (default) the __builtin_unreachable () is replaced with nothing (i.e. it
falls though) the test case passes.

When we replace the __builtin_unreachable () with a trap (either using the
ubsan or -funreachable-traps style) the test case fails with the trap.

This seems to be unlikely to be what was intended (or if it was intended it's
terribly fragile); I'm labelling it wrong code for now.

Similar code patterns exist in the other cases mentioned.