Correctly allocate logical of length 0 in rowwise edge case (#7749)

Author: DavisVaughan

NEWS.md

@@ -1,5 +1,7 @@
 # dplyr (development version)
 
+* Empty `rowwise()` list-column elements now resolve to `logical()` rather than a random logical of length 1 (#7710).
+
 * `last_dplyr_warnings()` no longer prevents objects from being garbage collected (#7649).
 
 * Progress towards making dplyr conformant with the public C API of R (#7741).

src/chop.cpp

@@ -26,7 +26,7 @@ void dplyr_lazy_vec_chop_grouped(SEXP chops_env, SEXP rows, SEXP data, bool roww
           SET_VECTOR_ELT(column, 0, ptype);
         } else {
           // i.e. `vec_ptype_finalise(unspecified())` (#6369)
-          SET_VECTOR_ELT(column, 0, Rf_allocVector(LGLSXP, 1));
+          SET_VECTOR_ELT(column, 0, Rf_allocVector(LGLSXP, 0));
         }
         SET_PRCODE(prom, column);
         UNPROTECT(2);

tests/testthat/test-mutate.R

@@ -332,6 +332,14 @@ test_that("mutate preserves class of zero-row rowwise (#4224, #6303)", {
   out <- mutate(rf, x2 = identity(x), x3 = x)
   expect_equal(out$x2, logical())
   expect_equal(out$x3, logical())
+
+  # with the empty list case, `x` is `logical()`, not a random logical of length
+  # 1 that happens to get recycled to length 0 (#7710)
+  rf <- rowwise(tibble(x = list()))
+  out <- mutate(rf, x2 = {
+    expect_identical(x, logical())
+    x
+  })
 })
 
 test_that("mutate works on empty data frames (#1142)", {