Handle zero-valued oversized bit shifts

crates/functions/src/engineering.rs

@@ -175,6 +175,9 @@ fn bitxor(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
 fn shift(a: &[Arg], left: bool) -> R<Value> {
     let n = bit_operand(a, 0)?;
     let s = num(a, 1)?.trunc();
+    if n == 0 {
+        return Ok(Value::Number(0.0));
+    }
     if s.abs() > 53.0 {
         return Err(CellError::Num);
     }
@@ -446,6 +449,14 @@ mod tests {
         is_err(ev("BITAND", vec![n(1.5), n(5.0)]), CellError::Num);
         is_err(ev("BITLSHIFT", vec![n(1.0), n(54.0)]), CellError::Num);
         is_err(ev("BITLSHIFT", vec![n(2f64.powi(47)), n(1.0)]), CellError::Num);
+        for f in ["BITLSHIFT", "BITRSHIFT"] {
+            for shift in [-100.0, -54.0, 54.0, 100.0] {
+                close(ev(f, vec![n(0.0), n(shift)]), 0.0);
+            }
+            is_err(ev(f, vec![n(1.0), n(54.0)]), CellError::Num);
+        }
+        is_err(ev("BITLSHIFT", vec![n(0.0), t("bad")]), CellError::Value);
+        is_err(ev("BITRSHIFT", vec![n(-1.0), n(100.0)]), CellError::Num);
     }
 
     #[test]

docs/function-parity.md

@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-27 · **Change:** minor (ECMA.CEILING listed with USDOLLAR as not on Microsoft's list; earlier: IMSQRT reproduces the stray real part Excel gives for negative reals, and the other IM* differences from Excel are listed; earlier: the LAMBDA row lists the recursion limit and its `#NUM!`, and the error a defined name holding an error gives when called; earlier: number text with accounting parentheses coerces like Excel 365; INDEX with one index on a 2-D reference is #REF! as in Excel; earlier: SUBTOTAL and AGGREGATE cap the cells they visit, #2175) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-31 · **Last updated:** 2026-10-31 · **Change:** minor (BITLSHIFT and BITRSHIFT return zero for zero operands with shifts beyond 53 bits, as observed in Excel; earlier: ECMA.CEILING listed with USDOLLAR as not on Microsoft's list; IMSQRT reproduces Excel's stray real part for negative reals) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Excel's worksheet functions, category by category, against GridCraft's function library. Part of
 [target-app-parity.md](target-app-parity.md); gaps feed [gaps.md](gaps.md).
@@ -29,6 +29,8 @@ Excel's worksheet functions, category by category, against GridCraft's function
 
 ## Coverage
 
+| 2026-10-31 | minor | BITLSHIFT and BITRSHIFT return `0` for zero operands shifted beyond 53 bits in either direction; nonzero operands retain the `#NUM!` limit, matching Excel |
+
 **Measured: 513 / 523 by name (98.1%); 504 / 523 working (96.4%).**
 
 | Category | Excel | Ours | Working | % working | Missing |