[llvm-bugs] [Bug 47566] New: -Wcomma flags Boost.Array code incorrectly

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47566

            Bug ID: 47566
           Summary: -Wcomma flags Boost.Array code incorrectly
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mclow.lists at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

The following file:
```
#include <boost/array.hpp>

int main () {
        boost::array<int, 5> arr;
        return arr.size();
        }
```

when compiled with a recent clang using `-Wcomma` gives a warning.

/Users/marshall/Sources/Boost/main/boost/array.hpp:185:107: warning: possible
misuse of comma operator here [-Wcomma]
            return i >= size() ? boost::throw_exception(std::out_of_range
("array<>: index out of range")), true : true;
                                                                               
                          ^
/Users/marshall/Sources/Boost/main/boost/array.hpp:185:34: note: cast
expression to void to silence warning
            return i >= size() ? boost::throw_exception(std::out_of_range
("array<>: index out of range")), true : true;
                                
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                 static_cast<void>(                            
                          )

The code that it is warning on is constexpr under C++11, and so must be a
single expression. It looks like this:

return i >= size()
           ? boost::throw_exception(...), true
           : true;

The ", true" is necessary to make the two halves of the ?: expression have the
same type.

I believe that the code is correct, and the warning is erroneous. Certainly the
suggested "fix" is a complete non-starter.

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