Yes, definitely looks like a bug.
Are you able to submit it to bugs.r-project.org?
Duncan Murdoch
On 05/04/2024 8:15 a.m., June Choe wrote:
There seems to be a bug in out-of-bounds assignment of list objects to an
expression() vector. Tested on release and devel. (Many thanks to folks
over at Mastodon for the help narrowing down this bug)
When assigning a list into an existing index, it correctly errors on
incompatible type, and the expression vector is unchanged:
```
x <- expression(a,b,c)
x[[3]] <- list() # Error
x
#> expression(a, b, c)
```
When assigning a list to an out of bounds index (ex: the next, n+1 index),
it errors the same but now changes the values of the vector to NULL:
```
x <- expression(a,b,c)
x[[4]] <- list() # Error
x
#> expression(NULL, NULL, NULL)
```
Curiously, this behavior disappears if a prior attempt is made at assigning
to the same index, using a different incompatible object that does not
share this bug (like a function):
```
x <- expression(a,b,c)
x[[4]] <- base::sum # Error
x[[4]] <- list() # Error
x
#> expression(a, b, c)
```
That "protection" persists until x[[4]] is evaluated, at which point the
bug can be produced again:
```
x[[4]] # Error
x[[4]] <- list() # Error
x
#> expression(NULL, NULL, NULL)
```
Note that `x` has remained a 3-length vector throughout.
Best,
June
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