Fix the build failure with C++20 standard

[BewareMyPower]

Motivation

When building the project with the -DCMAKE_CXX_STANDARD=20 option and GCC 11.3, it failed. There are two main reasons.

One is the ObjectPool.h, see http://eel.is/c++draft/diff.cpp17.class#2

In short, see the code below:

template <typename T>
struct A {
  // A<T>() {}  // error: simple-template-id not allowed for constructor
  A() {}        // OK, injected-class-name used
};

The other reason is deeply hidden and OS-specific. When building the target for the unit test, the lib/ directory is added into the include directories. So for #include "Semaphore.h", the Semaphore.h header will be looked up first in the lib/ directory. However, C++20 introduced a <semaphore> header, which finds the POSIX semaphore header semaphore.h in the system path.

For example, the include order in ubuntu:22.04 arm64 container is:

• $PROJECT_DIR/lib/ (where Semaphore.h is)
• ...
• /usr/lib/gcc/aarch64-linux-gnu/11/include (where semaphore.h is)

The C++ header might be case insensitive so the lib/Semaphore.h will be included by the <semaphore> header, which is implicitly included by <thread>. Our own Semaphore.h does not have the POSIX semaphore struct definitions so the build failed.

Modifications

• Fix the semantics error in ObjectPool.h
• Remove the lib/ directory from the included directories of the unit test and include lib/xxx.h for header in lib/ directory.
• Add a workflow to verify now it can be built with C++20