[Bug c++/70744] New: preincrements possibly double-evaluated in GNU ternaries

[donald.chai at synopsys dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70744

            Bug ID: 70744
           Summary: preincrements possibly double-evaluated in GNU
                    ternaries
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: donald.chai at synopsys dot com
  Target Milestone: ---

GCC, in C++ mode, appears to evaluate pre-increments twice in GNU ternaries:

$ gcc-5 --version
gcc-5 (Ubuntu 5.2.1-23ubuntu1~12.04) 5.2.1 20151031
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat test.c
int main() {
    int x = 1;
    ++x ?: 1337;
    return x;
}
$ gcc-5 -x c test.c; ./a.out; echo $?
2
$ gcc-5 -x c++ test.c; ./a.out; echo $? 
3

Clang appears to work:

$ clang --version
Ubuntu clang version 3.4-1ubuntu3~precise2 (tags/RELEASE_34/final) (based on
LLVM 3.4)
Target: x86_64-pc-linux-gnu
Thread model: posix

$ clang -x c++ test.c; ./a.out; echo $?
test.c:3:5: warning: expression result unused [-Wunused-value]
    ++x ?: 1337;
    ^~~~~~~~~~~
1 warning generated.
2