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A general description is here.
The use of .required by require/detach has been replaced by .Depends which is set from the 'Depends' field of a package (even in packages with name spaces). By default detach() prevents such dependencies from being detached in interactive use, and warns otherwise. [The default is intended to be changed before release.]
Placeholder.
Using with(), eval() etc with a list with some unnamed elements now works. (PR#14035)
intToUtf8() could fail when asked to produce 10Mb or more strings, something it was never intended to do: unfortunately Windows crashed R (other OSes reported a lack of resources). (PR#14068)