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A general description is here.
‘untar()’ used with an external ‘tar’ command assumes this supports decompression including ‘xz’ and automagically detecting the compression type. This has been true of all mainstream implementations since 2009 (for GNU ‘tar’, since version 1.22): older implementations are still supported _via_ the new argument ‘support_old_tars’ whose default is controlled by environment variable ‘R_SUPPORT_OLD_TARS’. (It looks like NetBSD and OpenBSD have ‘older’ ‘tar’ commands for this purpose.)
New functions ‘errorCondition()’ and ‘warningCondition()’ provide a convenient way to create structured error and warning objects.
‘.Deprecated()’ now signals a warning of class ‘deprecatedWarning’, and ‘.Defunct()’ now signals an error of class ‘defunctError’.
Many ‘package not found’ errors are now signaled as errors of class ‘packageNotFoundError’.
As an experimental feature, when ‘loadNamespace()’ fails because the requested package is not available the error is initially signaled with a ‘retry_loadNamespace’ restart available. This allows a calling handler to try to install the package and continue.