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A general description is here.
The transitional support for installing packages without namespaces (required since R 2.14.0) has been removed. ‘R CMD build’ will still add a namespace, but a ‘.First.lib()’ function will need to be converted.
‘R CMD INSTALL’ no longer adds a namespace, and a ‘.First.lib()’ function in a package will be ignored (with an installation warning for now).
Packages without a ‘R’ directory and no ‘NAMESPACE’ file can still be installed.