I would say it’s not an error. If there are ever experimental C library features, they could be enabled by it, with any additional flag required to make it happen (such as linking against another library) being added by the driver implicitly.
In the current implementation, the flag can be sniffed by inspecting __has_feature(experimental_library), and I think that’s exactly what we want. In particular, defining a libc++ specific macro would tie the flag to libc++, which would be an artificial self-imposed limitation.