Unable to use std::complex

I am unable to compile the following snippet that uses std::complex, getting the error "qualified name is not allowed". The version of the compiler  (15.0.6) seems pretty up to date and compatible with the existing gcc installation (5.1.0). Am I missing anything???

#include <cmath>
#include <complex>
namespace tensor {
  typedef std::complex<double> cdouble;
}

[nnp05@cierzo ~]$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/cm/local/apps/gcc/5.1.0/libexec/gcc/x86_64-unknown-linux-gnu/5.1.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-5.1.0/configure --prefix=/cm/local/apps/gcc/5.1.0 --enable-languages=c,c++,fortran --with-gmp-include=/root/rpmbuild/BUILD/gcc-5.1.0-obj/../gcc-5.1.0/our-gmp --with-gmp-lib=/root/rpmbuild/BUILD/gcc-5.1.0-obj/../gcc-5.1.0/our-gmp/.libs --with-mpc-include=/root/rpmbuild/BUILD/gcc-5.1.0-obj/../gcc-5.1.0/our-mpc/src --with-mpc-lib=/root/rpmbuild/BUILD/gcc-5.1.0-obj/../gcc-5.1.0/our-mpc/src/.libs --with-mpfr-include=/root/rpmbuild/BUILD/gcc-5.1.0-obj/../gcc-5.1.0/our-mpfr/src --with-mpfr-lib=/root/rpmbuild/BUILD/gcc-5.1.0-obj/../gcc-5.1.0/our-mpfr/src/.libs
Thread model: posix
gcc version 5.1.0 (GCC)
[nnp05@cierzo ~]$ icpc -v
icpc version 15.0.6 (gcc version 5.1.0 compatibility)
[nnp05@cierzo ~]$ icc -v
icc version 15.0.6 (gcc version 5.1.0 compatibility)