Match T DIST zero degree errors by mode

crates/functions/src/stat/distributions.rs

@@ -173,8 +173,14 @@ fn df_arg(a: &[Arg], i: usize) -> R<f64> {
 
 pub(super) fn t_dist(a: &[Arg], _: C) -> R<Value> {
     let x = num(a, 0)?;
-    let v = df_arg(a, 1)?;
+    let v = num(a, 1)?.trunc();
     let cum = boolean(a, 2)?;
+    if v == 0.0 {
+        return Err(if cum { CellError::Num } else { CellError::Div0 });
+    }
+    if v < 0.0 {
+        return Err(CellError::Num);
+    }
     num_val(if cum { t_cdf(x, v) } else { t_pdf(x, v) })
 }
 

crates/functions/src/stat/tests.rs

@@ -405,9 +405,9 @@ fn t_chi_f() {
 }
 
 #[test]
-fn t_dist_degrees_truncating_below_one_return_num() {
+fn t_dist_zero_degrees_error_depends_on_mode() {
     for df in [0.0, 0.9, -0.1] {
-        is_err(ev("T.DIST", vec![n(0.0), n(df), b(false)]), CellError::Num);
+        is_err(ev("T.DIST", vec![n(0.0), n(df), b(false)]), CellError::Div0);
         is_err(ev("T.DIST", vec![n(0.0), n(df), b(true)]), CellError::Num);
     }
     is_err(ev("T.DIST", vec![n(0.0), n(-1.0), b(false)]), CellError::Num);

docs/function-parity.md

@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-31 · **Last updated:** 2026-10-31 · **Change:** minor (T.DIST gives `#DIV/0!` when truncated degrees of freedom are zero; 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 (T.DIST gives `#DIV/0!` for density mode and `#NUM!` for cumulative mode when truncated degrees of freedom are zero; 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
 
 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).
@@ -123,7 +123,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 
 | Date | Change | Summary |
 |---|---|---|
-| 2026-10-31 | minor | T.DIST returns `#DIV/0!` when degrees truncate to zero; negative integer degrees remain `#NUM!` |
+| 2026-10-31 | minor | T.DIST returns `#DIV/0!` in density mode and `#NUM!` in cumulative mode when degrees truncate to zero; negative integer degrees remain `#NUM!` |
 | 2026-10-25 | minor | LAMBDA row: a LAMBDA that calls itself more than 140 times, and a formula nested past the depth limit, give `#NUM!` (#1809; nesting gave `#CALC!` before). `=Foo(1)` where the defined name Foo holds `#N/A` gives `#N/A`, not `#NAME?` |
 | 2026-10-23 | minor | Number text coercion (VALUE, arithmetic, typed entry): a dollar sign before accounting parentheses (`$(1,234)`, `$ (5)`) and a percent sign after them (`(5)%`) read as negatives; `(-5)`, `((5))`, `($-5)`, `$5%`, `$(5)%` and dates or times in parentheses are `#VALUE!`, as observed in Excel 365 |
 | 2026-10-23 | minor | INDEX with only a row index on a reference of several rows and columns (`INDEX(A1:C3,2)`, `INDEX(A1:C3,0)`) is `#REF!`, as observed in Excel, so it also fails as a range endpoint and inside other functions; an empty column argument (`INDEX(A1:C3,2,)`) still gives the row. Checked that a column array with one index stays a 1×1 array (`TYPE` 64), which is what Excel returns |