# 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.

Known compiler issues (this was last updated April 2023):

- 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. From VS2017.9 onwards there remain a number of usually untroublesome corner case issues, but use should be relatively unsurprising for most use cases. Be aware that only from Visual Studio 2022 onwards are almost all corner case problems fixed.

- Some point releases of GCC 10 with libstdc++ 10 can induce an infinite template instantiation, which fails the build for some rare use cases. Earlier or later GCCs or different point releases of the 10 series do not have this issue.

“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: April 18, 2023 at 20:57:50 +0100