So, if you distribute the source code, you need to do so with the copyright notice, list of conditions and disclaimers. That gives the original project the right to take the improved code and use it under the BSD style license.Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimers
By the rules of the BSD style license, you're not required to distribute the source code along with binaries derived from that source code, but GPL programs obviously do. So, when libstdc++ uses files from libc++, any modifications will be distributed in source code form, and they will be licensed BSD style.