Is it intended that #281 would make std::complex inaccessible? Should there be a MPI_CXX_COMPLEX, MPI_CXX_DOUBLE_COMPLEX, MPI_CXX_LONG_DOUBLE_COMPLEX, and (possibly) MPI_CXX_BOOL? Note that C99 complex is not a substitute because Microsoft does not implement it and it has different semantics regarding numerical stability. (Well, C99 semantics are specified and useful, std::complex stability is not specified by any C++ standard and in practice, is not implemented in a stable way.) This is not such a big deal for use with collectives because user-defined MPI_Ops can be used, but only predefined types can be used with one-sided operations, so presence of complex datatypes (or lack thereof) is relevant to applications.