Accept signed literal seq steps

Author: t-kalinowski

R/r2f-iterables-helpers.R

@@ -375,7 +375,8 @@ seq_like_r2f <- function(
 # Used by: seq_like_r2f() (subscript context)
 check_subscript_range_bounds <- function(info, from, to, by_f, hoist, scope) {
   lit <- function(e) {
-    if (is_wholenumber(e)) as.integer(e) else NA_integer_
+    e <- unwrap_parens(e)
+    if (is_scalar_integerish(e)) as.integer(e) else NA_integer_
   }
   from_lit <- lit(info$from)
   to_lit <- lit(info$to)

tests/testthat/test-subscript-validation.R

@@ -196,6 +196,12 @@ test_that("literal in-range bounds emit no guard; bad literals error at compile
 })
 
 test_that("x[seq(a, b, by)] requires a literal step and guards wrong signs", {
+  fdesc <- function(x) {
+    declare(type(x = double(5)))
+    x[seq(5L, 1L, by = -1L)]
+  }
+  expect_quick_identical(fdesc, list(as.double(1:5)))
+
   # the result length divides by the step, and the C bridge evaluates it
   # before any runtime guard could run -> compile error
   fn <- function(x, k) {