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A general description is here.
The included configuration code for 'libintl' has been updated to that from 'gettext' version 0.19.5.1 - this should only affect how an external library is detected. (Wish of PR#16464.)
(Windows) Compatibility information has been added to the manifests for 'Rgui.exe', 'Rterm.exe' and 'Rscript.exe'. This should allow 'win.version()' and 'Sys.info()' to report the actual Windows version up to Windows 10.
'close()' now reports the status of a 'pipe()' connection opened with an explicit 'open' argument. (PR#16481)
'detach()'ing the 'methods' package keeps '.isMethodsDispatchOn()' true, as long as the methods namespace is not unloaded.
Support for 'https://' URLs is available on Windows, and on other platforms if support for 'libcurl' was compiled in and if that supports the 'https' protocol (system installations can be expected to do). So 'https://' support can be expected except on rather old OSes (an example being OS X 'Snow Leopard', where a non-system version of 'libcurl' can be used).
(Windows only) The default method for accessing URLs _via_ 'download.file()' and 'url()' has been changed to be '"wininet"' using Windows API calls. This changes the way proxies need to be set and security settings made: there have been some reports of 'ftp:' sites being inaccessible under the new default method (but the previous methods remain available).
Thus by default the '"internal"' method for 'download.file()' and 'url()' uses the '"wininet"' method: to revert to the previous default use 'setInternet2(FALSE)'.
This means that 'https://' URLs can be read by default by 'download.file()' (they have been readable by 'file()' and 'url()' since R 3.2.0).
There are implications for how proxies need to be set (see '?download.file').