The minimum is g++ 4.9.3 or clang 6.0 with the exception of Windows where clang is not supported.
I think everything still runs for clang 5.0 or 4.0, but we test with 6.0 on Mac/Linux so we should encourage 6.0+ on Mac/Linux (latest version is 10.0).
clang or msvc on Windows do not work with Stan ATM, though we are very close with making clang work.
Bummer, yeah I guess it depends then. At least at one point I was able to get it working on a laptop with just the command line tools and not all of xcode, but if there’s any chance it won’t work we should just recommend installing all of xcode.