this is possibly because you are using the downloadable toolchains from swift.org, which are far less reliable than the same version that ships with Xcode. do you get the same build failure when you compile with the default system toolchain?
this is possibly because you are using the downloadable toolchains from swift.org, which are far less reliable than the same version that ships with Xcode. do you get the same build failure when you compile with the default system toolchain?