I'm pretty sure that "folly requires gcc 5.1+" is meant to be a statement about the minimum GCC version, and not that GCC is the only compiler that will work.

If you poke around the github issues, you'll note that several folks are successfully compiling folly with Clang.

Don't know why people downvoted you. The intended interpretation was "if gcc, must be 5.1+", but I can see how it might be misunderstood as it was written.

The readme was updated and now reads

> folly supports gcc (5.1+), clang, or MSVC. It should run on Linux (x86-32, x86-64, and ARM), iOS, macOS, and Windows (x86-64). The CMake build is only tested on some of these platforms; at a minimum, we aim to support macOS and Linux (on the latest Ubuntu LTS release or newer.)

folly is used in FB opensource projects on Windows, macOS, and Linux (e.g. watchman) and Android and iOS (React Native, though that might not include the whole library.)

The highlighted sentence gives the minimum version of gcc which folly broadly supports.

One motivation for this is so that a user may know whether folly supports the version of the gcc c++ compiler which is preinstalled on their system, since many systems ship with a preinstalled gcc. Likewise for systems which ship with a blessed package repo.

But folly also supports recent versions of clang and msvc, as other commenters here note.

It seems like a pretty mature cross-platform library.

It could be their statement "folly requires gcc 5.1+" meant gcc 5.1 or greater, where clang might be greater?

Could you elaborate on that? What aspects of using clang as opposed to gcc were so beneficial?

I could be wrong, but, a quick Googling / wiki shows clang is from 2007, whilst facebook was founded 2004, and even though clang was available in 2007, it takes years before people consider using newer tools like clang over proven technology, especially in the C space.