[Bug c++/55643] New: g++ 4.7 gives "warning: variable ‘myVar’ set but not used [-Wunused-but-set-variable]" when an "enum class"-typed variable is cast to double before use

[dholbert at cs dot stanford.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55643

             Bug #: 55643
           Summary: g++ 4.7 gives "warning: variable ‘myVar’ set but not
                    used [-Wunused-but-set-variable]" when an "enum
                    class"-typed variable is cast to double before use
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: dholb...@cs.stanford.edu


When I compile the attached C++ source file with g++ 4.7.2, I get this build
warning:
> gcc-warning-test.cpp: In function ‘int main()’:
> gcc-warning-test.cpp:19:10: warning: variable ‘myVar’ set but
>  not used [-Wunused-but-set-variable]

I believe this is incorrect, because I _am_ using the variable -- I'm printing
out its value like so:
> printf("%f\n", (double)myVar);

If I instead cast it to an int or an unsigned value, I get no warning -- that
apparently counts as "using" it, from GCC's perspective.  But when I use
(float)myVar or (double)myVar, GCC apparently doesn't count that as "using".

NOTE: This only happens with "enum class" (a C++11 feature). If I instead just
use "enum", then I don't get a warning.

For comparison, clang yields expected results. (No warning -- and it does warn
if I remove the printf statements, indicating that it is doing some checking)

SYSTEM INFO:
 g++-4.7.real (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2
 OS: Ubuntu 12.10
 Kernel: Linux 3.5.0-19-generic x86_64