Cannot parse enum value refer to previously defined enum value

Created originally on Bitbucket by mozbugbox (Mozbugbox)

Refer to an enumeration value defined earlier in the same enumeration failed to parse.

This code failed:

import cffi
ffi = cffi.FFI()
ffi.cdef("enum {a, b, c=b, d};")

because of c=b in the enum definition.