seq

Flag colon ranges from 1 up to a length—1:length(x), 1:nrow(x), or 1:n—which silently count down when that length is zero (1:0 is c(1L, 0L), not an empty sequence), a classic off-by-one bug for loops over possibly-empty input. seq_along(x) and seq_len(n) return a zero-length sequence instead, and agree with the colon form everywhere else.

The length/nrow/ncol/NROW/NCOL forms fire only when the callee resolves to base R; a redefinition is left alone. Literal ranges (1:10) and computed bounds (1:(n - 1)) are not flagged.

Ranges over a vector’s indices and up to a count:

for (i in 1:length(x)) print(x[i])
for (j in 1:n) f(j)

warning: seq
 --> example.R:1:11
  |
1 | for (i in 1:length(x)) print(x[i])
  |           ^^^^^^^^^^^ `1:length(x)` counts down (`1:0`) when the length is zero; `seq_along(x)` handles empty input
  = help: Use `seq_along(x)`.
warning: seq
 --> example.R:2:11
  |
2 | for (j in 1:n) f(j)
  |           ^^^ `1:n` counts down (`1:0`) when the length is zero; `seq_len(n)` handles empty input
  = help: Use `seq_len(n)`.

After applying the fix:

for (i in seq_along(x)) print(x[i])
for (j in seq_len(n)) f(j)