Problem when building the code: function is not a member in std

[guanghuhappysf128]

Hi,

I got a new problem when I tried to build the code with the command ./build.py -pd:

g++ -o _build/prod/src/atom.os -c -Wall -pedantic -std=c++14 -Wno-deprecated-register -O3 -DNDEBUG -isystem/usr/local/include -isystem/home/ghu1/local/include -fPIC -Isrc -I/home/ghu1/projects/code/lapkt-installation/include _build/prod/src/atom.cxx
In file included from src/relaxed_state.hxx:4:0,
                 from _build/prod/src/relaxed_state.cxx:2:
src/fs_types.hxx:76:24: error: ‘function’ in namespace ‘std’ does not name a template type
  using Function = std::function<ObjectIdx(const ValueTuple&)>;

This problem has never occurred before. And it can simply be fixed by adding #include <functional> in file fs/src/fs_types.hxx

Not sure why this happened. And the GCC version I used is: gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0