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A general description is here.
R now supports custom binary types which are of the form ‘"<system>.binary.<build>"’ where ‘<system>’ is lower-case name (letters only) of the system and ‘<build>’ is the name of the build (alphanumeric and dashes). The corresponding paths returned by ‘contrib.url’ (which govern repository layout) with the above ‘type=’ have the form ‘bin/<system>/<build>/contrib/<x.y>’ where ‘<x.y>’ is the R version without patch level. This is a generalisation of the binary types previously supported on macOS. In addition, binary packages can have the extensions ‘.tar.bz2’, ‘.tar.xz’, ‘.tar.zst’ and ‘.tar.zstd’ with the corresponding compression support.
Asymmetric ‘toeplitz(x, r)’ is type consistent now, even when ‘r’ of length 0 or 1 is not used directly. Fixes PR#18996, thanks to Mikael Jagan's patch and test.