[Rd] Intermittent crashes with inset `[<-` command

[Brian Montgomery]

The following code crashes after about 300 iterations on my x86_64-w64-mingw32 machine on R 3.5.2 --vanilla.  
Others have duplicated this (see https://github.com/tidyverse/magrittr/issues/190 if necessary), but I don't know how machine/OS-dependent it may be.  
If it doesn't crash for you, please try increasing the length of the x vector.

Substituting the commented-out line for the one below it works correctly (prints out 1:1000 and ends normally) every time.

x <- 1:200000
y <- rep(letters[1:5], length(x) / 5L)
for (i in 1:1000) {
  # x[y == 'a'] <- x[y == 'b']
  x <- `[<-`(x, y == 'a', x[y == 'b'])
  cat(i, '')
}
cat('\n')

The point of using this syntax is to make it work better with pipes, but the errors occur without pipes or magrittr.

Thank you for your help!