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A general description is here.
The S4 pseudo-class "single" has been removed.
["double" is deprecated, but remains until a bug in the
Matrix package is fixed.]
The S4 pseudo-classes "single" and double have been removed. (The S4 class for a REALSXP is "numeric": for back-compatibility as(x, "double") coerces to "numeric".)
There is a new public interface to the encoding info stored on CHARSXPs, getCharCE and mkCharCE using the enumeration type cetype_t.
dump() and friends now warn that all S4 objects (even those based on vectors) are not source()able, with a stronger wording.