libstdc++ span not ABI compatible with nostd::span since GCC 11

[johanpel]

I was debugging some segfaults that occurred when I was building the OTel libraries with WITH_STL=ON, statically linking them to an application. I wasn't using an explicit CMAKE_CXX_STANDARD for OTel which caused it to build with C++20, while the application is built with C++17. This caused a segfault in the InMemorySpanExporter as that was using a different implementation of nostd::span because its included through a header-only file.

The solution was to just explicitly set the same CMAKE_CXX_STANDARD for the OTel build so the same nostd::span implementation is selected, but I did notice the following.

There is a comment here hinting that it is desirable to have ABI compatibility with libstdc++'s implementation of std::span:
https://github.com/open-telemetry/opentelemetry-cpp/blob/main/api/include/opentelemetry/nostd/span.h#L243-L247

However, that implementation has since GCC 11 swapped around the extent and data fields:
https://github.com/gcc-mirror/gcc/blob/releases/gcc-11.3.0/libstdc++-v3/include/std/span#L392-L393

This was causing my segfaults.

Of course my problem was fixed by properly setting the CMAKE_CXX_STANDARD for OTel the same as my application, but I just wanted to mention that here in case this has any implications I am not aware of.

[lalitb]

Option to build with Standard library is only provided for environment where ABI compatibility is not required. No ABI compatibility is guaranteed between nostd implementations and standard library - https://github.com/open-telemetry/opentelemetry-cpp/blob/main/docs/building-with-stdlib.md

[johanpel]

No ABI compatibility is guaranteed between nostd implementations and standard library.

Understood. Perhaps the title of this issue is a bit misleading of my intentions, but it's mainly about the comment in the code. Feel free to close this if that comment should not be updated.

[marcalff]

No ABI compatibility is guaranteed between nostd implementations and standard library.

Understood. Perhaps the title of this issue is a bit misleading of my intentions, but it's mainly about the comment in the code. Feel free to close this if that comment should not be updated.

@johanpel

Thanks for the detailed investigations.

The comment is indeed misleading, and should be removed.