Warn about do nothing expressions?

[Justin Whear]

Someone is sure to ask:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.8.2-16' 
--with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-
languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-
suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/
lib --without-included-gettext --enable-threads=posix --with-gxx-include-
dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ 
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes 
--enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-
system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-
cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-
java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --
with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-
arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --
enable-objc-gc --enable-multiarch --with-arch-32=i586 --with-abi=m64 --
with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-
checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --
target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Debian 4.8.2-16)

$ clang -v
Debian clang version 3.3-16 (branches/release_33) (based on LLVM 3.3)
Target: x86_64-pc-linux-gnu
Thread model: posix