In file included from /Users/omochi/work/swift-source/llvm/include/llvm/ADT/STLExtras.h:20: /Users/omochi/work/swift-source/llvm/include/llvm/ADT/Optional.h:141:3: error: unknown type name 'constexpr' constexpr Optional() {} ^ /Users/omochi/work/swift-source/llvm/include/llvm/ADT/Optional.h:141:13: error: constructor cannot have a return type constexpr Optional() {} ^~~~~~~~