Unimplemented tree?

[Don Bowman]

Save this file as 'a.cc'
--------------
template <class X, int slen = (256/sizeof(X))>
class auto_buffer
{
public:
    auto_buffer<X>& operator=(const auto_buffer<X>& r)
    {
    }
};
---------------
$ cc1plus.exe -version a.cc
GNU C++ version egcs-2.92.29 19981214 (gcc2 ss-980609 experimental)
(mips-wrs-vxworks) compiled by GNU C version egcs-2.91.57 19980901 (egcs-1.1
release).
options passed:
options enabled:  -fpeephole -ffunction-cse -fkeep-static-consts
 -fpcc-struct-return -fexceptions -fcommon -fgnu-linker -fargument-alias
 -mgas -meb -mcpu=3000
a.cc:5: sorry, not implemented: initializer contains unrecognized tree code
a.cc:5: sorry, not implemented: initializer contains unrecognized tree code
 class auto_buffer<X,(256 / {error})> & auto_buffer<X,slen>::operator
=(const cl
ass auto_buffer<X,(256 / {error})> &)

What is the unrecognised tree code?
How would I work around this?