But when you recompile, it still uses `c++11`. The reason for this is that default options are only looked at when you are setting up a build directory for the very first time. After that the setting is considered to have a value and thus the default value is ignored. To change an existing build dir to `c++14`, either reconfigure your build dir with `mesonconf` or delete the build dir and recreate it from scratch.
But when you recompile, it still uses `c++11`. The reason for this is that default options are only looked at when you are setting up a build directory for the very first time. After that the setting is considered to have a value and thus the default value is ignored. To change an existing build dir to `c++14`, either reconfigure your build dir with `mesonconfigure` or delete the build dir and recreate it from scratch.