From fc2f00bd501a1ffa6fc8500eb3c1609c2bc2c2b1 Mon Sep 17 00:00:00 2001
From: nihilist-narwhal <240798405+nihilist-narwhal@users.noreply.github.com>
Date: Sat, 10 Oct 2026 04:24:42 -0500
Subject: [PATCH] Recalc: a range ending in INDEX depends on the cells it spans

=SUM(C3:INDEX(B9:C11,2,2)) adds C3:C10, but only C3 and B9:C11 were
recorded as its precedents, so typing into C5 left the total unchanged
until a full recalculation. A range whose ends are function results now
depends on the box spanned by the references inside it, which contains
any cell those functions can return.
---
 crates/calc/src/eval.rs    | 14 ++++++++++++++
 crates/engine/src/tests.rs |  8 ++++++++
 2 files changed, 22 insertions(+)

diff --git a/crates/calc/src/eval.rs b/crates/calc/src/eval.rs
index abef54ffd..c010020a6 100644
--- a/crates/calc/src/eval.rs
+++ b/crates/calc/src/eval.rs
@@ -1337,6 +1337,20 @@ pub fn precedents(wb: &Workbook, sheet: usize, e: &Expr) -> (Vec<Area>, bool) {
         }
         Expr::Name(_) | Expr::Struct(_) => dynamic = true,
         Expr::Call(n, _) if gridcraft_functions::lookup(n).is_none() && !is_special(n) => dynamic = true,
+        // A range ending in a function's result (`A1:INDEX(B1:B9,n)`) covers the cells between
+        // the references inside it, not only those references.
+        Expr::Binary(BinOp::Range, a, b) if !matches!((&**a, &**b), (Expr::Ref(_), Expr::Ref(_))) => {
+            let mut inner = precedents(wb, sheet, a).0;
+            inner.extend(precedents(wb, sheet, b).0);
+            let mut boxes: Vec<Area> = Vec::new();
+            for area in inner {
+                match boxes.iter_mut().find(|x| x.sheet == area.sheet) {
+                    Some(x) => x.range = x.range.union(&area.range),
+                    None => boxes.push(area),
+                }
+            }
+            out.extend(boxes);
+        }
         _ => {}
     });
     (out, dynamic)
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 361fbce48..9fe3fa8fa 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -165,6 +165,14 @@ fn replacing_a_spilling_formula_clears_its_spill() {
     assert_eq!((v(&s, "A2"), v(&s, "B1")), (Value::Empty, Value::Number(0.0)));
 }
 
+#[test]
+fn range_ending_in_index_recalculates() {
+    let mut s = s();
+    s.execute("cell.set", json!({"cell": "B6", "input": "=SUM(C3:INDEX(B9:C11,2,2))"})).unwrap();
+    s.execute("cell.set", json!({"cell": "C5", "input": "2"})).unwrap();
+    assert_eq!(v(&s, "B6"), Value::Number(2.0));
+}
+
 #[test]
 fn every_command_survives_empty_params() {
     let mut s = Session::new();