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A general description is here.
‘merge()’ is no longer allowed to create a data frame with duplicate column names (which confused PR#14786).
The internal ‘untar()’ (as used by default by ‘R CMD INSTALL’) now knows about some ‘pax’ headers which ‘bsdtar’ (e.g., the default ‘tar’ for Mac OS >= 10.6) can incorrectly include in ‘tar’ files, and will skip them with a warning.
PCRE has been upgraded to version 8.21: as well as bug fixes and greater Perl compatibility, this adds a JIT pattern compiler, about which PCRE's news says ‘large performance benefits can be had in many situations’.