The main advantages

The main advantages of choosing <outcome/experimental> over default Outcome:

1. Codegen is tighter and less verbose¹, sometimes remarkably so.

2. Build time impact is markedly lower, barely above the inclusion of naked <basic_result.hpp>, as the STL allocator machinery and std::string et al is not dragged into inclusion by including <system_error>. Note that <outcome/experimental/status_outcome.hpp> bring in <exception>, however <outcome/experimental/status_result.hpp> brings in no extra system headers.

3. More discipline is imposed on your use of Outcome, leading to less ambiguous code which is easier to optimise by the compiler, lower cost to maintain, and lower cognitive load to audit code based on experimental Outcome for correctness.

4. Code written to <outcome/experimental> can be fairly easily dual targeted, with just a few switching type aliases, to default Outcome. This author has several Outcome-based libraries with identical source code which can target either configuration of Outcome. The experimental Outcome build regularly beats the default Outcome build in benchmarks by 2-3%, and the dual target source code, being written to tighter discipline, is faster and more deterministic in the default target than it was before the (non-trivial) port to <outcome/experimental>.

5. If you want ‘official’ C support, experimental Outcome is able to provide that in a way not possible for default Outcome which cannot make sufficiently strong C compatibility assumptions about std::error_code.

If you are building a codebase on top of Outcome expecting long term maintenance, the author’s personal recommendation is that you design, write, test and optimise it for <outcome/experimental>. What you ship to your customers ought to be targeted at default Outcome however, so employ type aliases and macros as appropriate to switch the build configuration for production releases. This is what the Outcome author does himself, to date with great success, despite the fundamental architectural differences between <system_error> and proposed <system_error2>.