Merge PR #3003

# Conflicts:
#	crates/functions/src/stat/tests.rs
#	docs/function-parity.md

crates/functions/src/stat/distributions.rs

@@ -629,6 +629,9 @@ pub(super) fn weibull_dist(a: &[Arg], _: C) -> R<Value> {
     if x < 0.0 || al <= 0.0 || be <= 0.0 {
         return Err(CellError::Num);
     }
+    if x == 0.0 {
+        return num_val(0.0);
+    }
     let t = (x / be).powf(al);
     num_val(if cum { -(-t).exp_m1() } else { al / be.powf(al) * x.powf(al - 1.0) * (-t).exp() })
 }

crates/functions/src/stat/tests.rs

@@ -536,6 +536,17 @@ fn gamma_density_at_zero_shape_one_is_num() {
     is_err(ev("GAMMA.DIST", vec![n(-0.1), n(1.0), n(2.0), b(false)]), CellError::Num);
 }
 
+#[test]
+fn weibull_density_at_zero_is_zero() {
+    for shape in [0.5, 1.0, 2.0] {
+        close_tol(ev("WEIBULL.DIST", vec![n(0.0), n(shape), n(1.0), b(false)]), 0.0, 0.0);
+        close_tol(ev("WEIBULL.DIST", vec![n(0.0), n(shape), n(1.0), b(true)]), 0.0, 0.0);
+    }
+    close_tol(ev("WEIBULL.DIST", vec![n(1.0), n(1.0), n(1.0), b(false)]), (-1.0f64).exp(), 1e-15);
+    close_tol(ev("WEIBULL.DIST", vec![n(1.0), n(1.0), n(1.0), b(true)]), 1.0 - (-1.0f64).exp(), 1e-15);
+    is_err(ev("WEIBULL.DIST", vec![n(-0.1), n(1.0), n(1.0), b(false)]), CellError::Num);
+}
+
 #[test]
 fn aggregate_helper() {
     let vals = vec![nv(1.0), nv(2.0), tv("x"), Value::Empty, Value::Bool(true), nv(6.0)];

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 (GAMMA.DIST density at x=0 and shape 1 returns `#NUM!` as in Excel; earlier: T.INV returns `#DIV/0!` at probability zero; earlier: 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 (WEIBULL.DIST density is zero at x=0, including shape below one, as in Excel; earlier: GAMMA.DIST density at x=0 and shape 1 returns `#NUM!` as in Excel; earlier: 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
 
 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).
@@ -137,6 +137,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 | 2026-10-31 | minor | Student-t density uses an asymptotic log-gamma ratio for large degrees of freedom, avoiding cancellation while preserving bounded work; checked against Excel at 100 million and 10 billion degrees |
 | 2026-10-31 | minor | T.INV probability zero returns `#DIV/0!`, while other out-of-range probabilities and invalid degrees remain `#NUM!`; T.INV.2T keeps its separate boundary behavior |
 | 2026-10-31 | minor | GAMMA.DIST density at x=0 and shape 1 returns `#NUM!` for all positive scales; the cumulative branch and other shape boundaries are unchanged |
+| 2026-10-31 | minor | WEIBULL.DIST returns zero at x=0 in both density and cumulative modes, including shape below one; negative x remains `#NUM!` |
 | 2026-10-27 | minor | ECMA.CEILING added to the functions that are in our library but not on Microsoft's list (with USDOLLAR), so the counts don't change |
 | 2026-10-26 | minor | IMSQRT uses Excel's polar route with its cosine reduction, so `IMSQRT("-4")` is `1.22514845490862E-16+2i` as in Excel (exact `2i` before); the rows list the IM* differences still open |
 | 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?` |