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A general description is here.
Header ‘R_ext/Boolean.h’ now ensures a ‘bool’ type is available either as a keyword (C23 and C++) or by including the C99 header ‘stdbool.h’. This is used in places in R itself to replace ‘Rboolean’ by ‘bool’.
There are a new functions ‘asRboolean’ and ‘asBool’, variannts of ‘asLogical’ more suited to converting logical arguments to ‘Rboolean’ or to ‘bool’.