# Prerequisites

Outcome is a header-only C++ 14 library known to work well on the latest point releases of these compiler-platform combinations or better:

- clang 4.0.1 (LLVM) [FreeBSD, Linux, OS X]
- GCC 6.5 [Linux]
- Visual Studio 2017.9 [Windows]
- XCode 9 [MacOS]

For non-Windows non-POSIX platforms (typically embedded systems), Outcome is usable in its Outcome.Experimental form with the macro `BOOST_OUTCOME_SYSTEM_ERROR2_NOT_POSIX` defined.

It is worth turning on C++ 17 or C++ 20 if you can, as there are many usability and performance improvements. Any Concepts TS or Coroutines TS implemented by your compiler is automatically detected and used.

Partially working compilers (this was last updated January 2019):

- clang 3.5 - 3.9 can compile varying degrees of the test suite, the problem is lack of complete and unbuggy C++ 14 language support.
- Older point releases of GCCs 7 and 8 have internal compiler error bugs in their constexpr implementation which tend to be triggered by using Outcome in constexpr. If you don’t use Outcome in constexpr, you won’t see these problems. If you need your GCC to not ICE, upgrade to the very latest point release, the constexpr ICE has been since fixed.
- Early editions of Visual Studio 2017 have many corner case problems. The latest point release, VS2017.9, only has a few known problems, and should be relatively unsurprising for most use cases.

“C++ 14” compilers which do not work, and will not work until their maintainers fix them:

- GCC 5, due to a bug in nested template variables parsing which was fixed in GCC 6. I appreciate that this upsets a lot of users. Please raise your upset at https://gcc.gnu.org/bugzilla/. In the meantime, you can get fairly far in Outcome with even clang 3.5.
- Any compiler which uses the libstdc++ version which comes with GCC 5, as it does not implement enough of the C++ 14 standard library for Outcome to compile.

Last revised: February 25, 2020 at 11:15:13 UTC