fix non c++20 code
fix non c++20 code

see: https://eel.is/c++draft/diff.cpp17.class#2

google/or-tools,or-tools/or-tools,google/or-tools,google/or-tools,google/or-tools,google/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,or-tools/or-tools,google/or-tools