use 2-argument static_assert form for older compilers
The previous form fails with gcc version 8.5.0 20210514 (Red Hat 8.5.0-22.0.1) (GCC)
on EL8 (in my case, Oracle Linux 8).
The previous form fails with gcc version 8.5.0 20210514 (Red Hat 8.5.0-22.0.1) (GCC)
on EL8 (in my case, Oracle Linux 8).