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A general description is here.
The behaviour of ‘unlink()’ for reparse points (including junctions and
symbolic links) has changed. It no longer follow links (deleting the
contents of the link target rather than the link), and it is able to
delete the link itself (given sufficient permissions).
The installer has many fewer options with more files always being installed, as a few Mb of file space is nowadays no longer an issue.
‘Rcmd INSTALL --merge-multiarch’ will work (but do a normal install) if only one architecture is installed.
As the current toolchain uses only 32-bit executables, it is possible to compile a package for both architectures on 32-bit Windows provided both architectures of R have been installed: use ‘Rcmd INSTALL --compile-both’.