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]

It is worth turning on C++ 17 if you can, as there are many usability and performance improvements. If your compiler implements the Concepts TS, it is worth turning support on. Support 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.
GCC’s 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. GCC 6 also seems to be more reliable. clang and Visual Studio work correctly, if you need an alternative.
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.