libc++-dev 1:9.0-49~exp2 (amd64 binary) in ubuntu focal

 libc++ is another implementation of the C++ standard library
 .
 Features and Goals
 .
   * Correctness as defined by the C++ standards.
   * Fast execution.
   * Minimal memory use.
   * Fast compile times.
   * ABI compatibility with gcc's libstdc++ for some low-level features such
     as exception objects, rtti and memory allocation.
   * Extensive unit tests.
 This is a dependency package providing the default LLVM C++ Standard library
 development files.