Error(s):

source_file.cpp:35:7: error: use of overloaded operator '=' is ambiguous (with operand types 'base' and 'typename remove_reference<base &>::type' (aka 'base'))
    a = std::move(b);
    ~ ^ ~~~~~~~~~~~~
source_file.cpp:18:11: note: candidate function
    base& operator=(base other) {
          ^
source_file.cpp:23:11: note: candidate function
    base& operator=(base&& other) {
          ^
1 error generated.