• R CMD check skips vignette re-building (with a warning) if the
  VignetteBuilder package(s) are not available.

BUG FIXES:

• Paths with non-ASCII characters caused problems for package
  loading on Windows PR#17833.

• Using tcltk widgets no longer crashes R on Windows.

• source(*, echo=TRUE) no longer fails in some cases with empty
  lines; reported by Bill Dunlap in PR#17769.

• on.exit() now correctly matches named arguments, thanks to
  PR#17815 (including patch) by Brodie Gaslam.

• regexpr(*, perl=TRUE) no longer returns incorrect positions into
  text containing characters outside of the Unicode Basic
  Multilingual Plane on Windows.