atomic_shared_ptr.h doesn't compile with gcc-9.3 / C++20

[maxgolov]

Implementation suggests that it's C++20 compatible here:

opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/atomic_shared_ptr.h

Line 14 in 9e5924c

* This wrapper relies on std::atomic for C++20, a mutex for gcc 4.8, and

However, if the project is built with

set(CMAKE_CXX_STANDARD 20)

Then the build is broken here:

opentelemetry-cpp/sdk/include/opentelemetry/sdk/trace/tracer.h

Line 47 in 9e5924c

opentelemetry::sdk::AtomicSharedPtr<SpanProcessor> processor_;

With the following error:

[ 50%] Building CXX object sdk/src/trace/CMakeFiles/opentelemetry_trace.dir/tracer_provider.cc.o
In file included from /mnt/c/work/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/atomic_shared_ptr.h:3,
                 from /mnt/c/work/opentelemetry-cpp/sdk/include/opentelemetry/sdk/trace/tracer.h:3,
                 from /mnt/c/work/opentelemetry-cpp/sdk/include/opentelemetry/sdk/trace/tracer_provider.h:9,
                 from /mnt/c/work/opentelemetry-cpp/sdk/src/trace/tracer_provider.cc:1:
/usr/include/c++/9/atomic: In instantiation of ‘struct std::atomic<std::shared_ptr<opentelemetry::v0::sdk::trace::SpanProcessor> >’:
/mnt/c/work/opentelemetry-cpp/sdk/include/opentelemetry/sdk/common/atomic_shared_ptr.h:33:35:   required from ‘class opentelemetry::v0::sdk::AtomicSharedPtr<opentelemetry::v0::sdk::trace::SpanProcessor>’
/mnt/c/work/opentelemetry-cpp/sdk/include/opentelemetry/sdk/trace/tracer.h:47:54:   required from here
/usr/include/c++/9/atomic:191:21: error: static assertion failed: std::atomic requires a trivially copyable type
  191 |       static_assert(__is_trivially_copyable(_Tp),

[maxgolov]

I was evaluating what happens when we build on latest Ubuntu release with its stock compiler in a project that is targeting C++20. The build is not gonna succeed. Workaround is to avoid using std::atomic if compiling with C++20:

#if 0 /* __cplusplus > 201703L */

[pyohannes]

I can look into this. The related template specialization is supposed to be part of C++20.

[maxgolov]

Fixed by #114