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A general description is here.
The deprecated legacy S-compatibility macros ‘PROBLEM’, ‘MESSAGE’, ‘ERROR’, ‘WARN’, ‘WARNING’, ‘RECOVER’, ... are no longer defined by default in ‘R_exts/RS.h’ (included by ‘R.h’). Replace these by calls to ‘Rf_error’ and ‘Rf_warning’.
For the time being the definitions can be reinstated by defining macro ‘R_LEGACY_S_DEFS’ to ‘1’ _before_ including any R header. Header ‘R_ext/RS.h’ will no longer include ‘R_ext/Error.h’ unless ‘R_LEGACY_S_DEFS’ is set.
[Early warning: removal planned for Oct 8.]
The legacy S-compatibility macros ‘PROBLEM’, ‘MESSAGE’, ‘ERROR’, ‘WARN’, ‘WARNING’, ‘RECOVER’, ... are deprecated and will be hidden in R 4.2.0. R's native interface of ‘Rf_error’ and ‘Rf_warning’ has long been preferred.
‘type.convert()’ now warns when its ‘as.is’ argument is not specified, as the help file always said it _should_. In that case, the default is changed to ‘TRUE’ in line with its change in ‘read.table()’ (related to ‘stringsAsFactors’) in R 4.0.0.
‘names()’ now works for ‘DOTSXP’ objects. On the other hand, in ‘R-lang’, the R language manual, we now warn against relying on the structure or even existence of such dot-dot-dot objects.
‘str()’ now works correctly for ‘DOTSXP’ and related exotics, even when these are doomed.
Additionally, it no longer fails for ‘list’s with a ‘class’ and “irregular” method definitions such that e.g. ‘lapply(*)’ will necessarily fail, as currently for different ‘igraph’ objects.
Too long lines in environment files (e.g., ‘Renviron’) no longer crash R. This limit has been increased to 100,000 bytes. (PR#18001.)
Defunct functions ‘mem.limits()’, ‘.readRDS()’, ‘.saveRDS()’, ‘.find.package()’, and ‘.path.package()’ from package ‘base’ and ‘allGenerics()’, ‘getAccess()’, ‘getAllMethods()’, ‘getClassName()’, ‘getClassPackage()’, ‘getExtends()’, ‘getProperties()’, ‘getPrototype()’, ‘getSubclasses()’, ‘getVirtual()’, ‘mlistMetaName()’, ‘removeMethodsObject()’, ‘seemsS4Object()’, ‘traceOff()’, and ‘traceOn()’ from ‘methods’ have been removed.