WITH_STL shouldn't try importing gsl::span if std::span is not available.

[lalitb]

As discussed in #742:
If the WITH_STL cmake option is enabled, the build should be using std::span if available (for C++20), and fall back to nostd::span instead of gsl::span ( or perhaps fail the build). Use gsl::span only if developers explicitly specify the WITH_GSL build option.

[maxgolov]

I already added a fix for that here:

opentelemetry-cpp/api/include/opentelemetry/std/span.h

Line 30 in d6f5d32

# undef HAVE_GSL

Could you please take a look at my PR? #771

[maxgolov]

I added my comments to #742 here - #742 (comment)

[maxgolov]

I'll assign to myself. I believe I have a reasonable fix in my bigger variant PR. Feedback is welcomed, in case if we want to decouple STDLIB vs GSL. But it's been described in the document here that we fallback to gsl::span for C++17 , because std::span is only available starting from C++20.

[maxgolov]

I would like to test this on latest build. Although we enable the option by-default for STDLIB build - we turn it off in case if customer's build system does not have gsl header. So the issue is largely alleviated: that way we do not force the customers to use the header.

Perhaps we should've switched our nostd::span implementation (aliasing it) to gsl::span. Min bar for gsl::span, however, is C++14. So switching to GSL may not work too well for gcc-4.8.x . I have not personally tested if it'd work specifically with that super-old compiler. Need more time to research.

Main motivation of defaulting to GSL span rather than implementing our own nostd::span :

• GSL span is better tested (more flight-hours on that).
• GSL span is endorsed by CppCoreGuidelines.
• GSL span is approved in various secure lifecycle development processes.
• less maintenance headache (we do not want to maintain our own mini-STL library in OpenTelemetry).

[owent]

The gsl::span implementation in gsl-lite support gcc-4.8.x. Should we use it as gsl for old compilers?

[lalitb]

The gsl::span implementation in gsl-lite support gcc-4.8.x. Should we use it as gsl for old compilers?

What's the current behavior? I thought if gsl-lite is installed, and HAVE_GSL is enabled, api/sdk will use nostd::variant from gsl-lite. If this is not so, we can add support for that.

[github-actions]

This issue was marked as stale due to lack of activity. It will be closed in 7 days if no furthur activity occurs.

[maxgolov]

@owent @lalitb - could you please folks take over the ownership of this issue? My main focus is on another project domain at the moment, and my ability to contribute to OpenTelemetry is limited.

[lalitb]

Thanks, @maxgolov, have unassigned you and added a help-wanted tag. I will take it over if no one picks it. The immediate need for this issue would be not using gsl::span with WITH_STL flag, and then we can have a separate issue to introduce the WITH_GSL cmake flag.

[esigo]

@lalitb I can take it over.