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A general description is here.
The ‘Rcomplex’ definition (in header ‘R_ext/Complex.h’) has been extended to prevent possible mis-compilation when interfacing with Fortran (PR#18430). The new definition causes compiler warnings with static initializers such as ‘{1, 2}’, which can be changed to ‘{.r=1, .i=2}’.
Using the new definition from C++ depends on compiler extensions supporting C features that have not been incorporated into the C++ standards but are available in ‘g++’ and ‘clang++’: this may result in C++ compiler warnings but these have been worked around for recent versions of common compilers (GCC, Apple/LLVM clang, Intel).
It is intended to change the inclusion of header ‘R_ext/Complex.h’ by other R headers, so C/C++ code files which make use of ‘Rcomplex’ should include that header explicitly.