From decdb5e6301c75f77239343e2ee5c3dfb3ff496f Mon Sep 17 00:00:00 2001
From: Andrew Gallant <jamslam@gmail.com>
Date: Sun, 21 Dec 2025 09:09:15 -0500
Subject: [PATCH] error: add `Error::is_range` predicate

I'm somewhat concerned that this doesn't cover all cases, but I think
this should be a good start.
---
 src/civil/date.rs  |  2 +-
 src/error/civil.rs |  2 --
 src/error/mod.rs   | 24 ++++++++++++++++++++++++
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/src/civil/date.rs b/src/civil/date.rs
index 5d53c6c..e56f66f 100644
--- a/src/civil/date.rs
+++ b/src/civil/date.rs
@@ -1059,7 +1059,7 @@ impl Date {
 
         let nth = t::SpanWeeks::try_new("nth weekday", nth)?;
         if nth == C(0) {
-            Err(Error::from(E::NthWeekdayNonZero))
+            Err(Error::slim_range("nth weekday"))
         } else if nth > C(0) {
             let nth = nth.max(C(1));
             let weekday_diff = weekday.since_ranged(self.weekday().next());
diff --git a/src/error/civil.rs b/src/error/civil.rs
index 4f548b8..40fce64 100644
--- a/src/error/civil.rs
+++ b/src/error/civil.rs
@@ -13,7 +13,6 @@ pub(crate) enum Error {
     InvalidISOWeekNumber,
     OverflowDaysDuration,
     OverflowTimeNanoseconds,
-    NthWeekdayNonZero,
     RoundMustUseDaysOrBigger { unit: Unit },
     RoundMustUseHoursOrSmaller { unit: Unit },
 }
@@ -68,7 +67,6 @@ impl core::fmt::Display for Error {
             OverflowTimeNanoseconds => {
                 f.write_str("adding duration to time overflowed")
             }
-            NthWeekdayNonZero => f.write_str("nth weekday cannot be `0`"),
             RoundMustUseDaysOrBigger { unit } => write!(
                 f,
                 "rounding the span between two dates must use days \
diff --git a/src/error/mod.rs b/src/error/mod.rs
index 8cdc895..14ae0c1 100644
--- a/src/error/mod.rs
+++ b/src/error/mod.rs
@@ -93,6 +93,23 @@ impl Error {
     pub fn from_args<'a>(message: core::fmt::Arguments<'a>) -> Error {
         Error::from(ErrorKind::Adhoc(AdhocError::from_args(message)))
     }
+
+    /// Returns true when this error originated as a result of a value being
+    /// out of Jiff's supported range.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use jiff::civil::Date;
+    ///
+    /// assert!(Date::new(2025, 2, 29).unwrap_err().is_range());
+    /// assert!("2025-02-29".parse::<Date>().unwrap_err().is_range());
+    /// assert!(Date::strptime("%Y-%m-%d", "2025-02-29").unwrap_err().is_range());
+    /// ```
+    pub fn is_range(&self) -> bool {
+        use self::ErrorKind::*;
+        matches!(*self.root().kind(), Range(_) | SlimRange(_) | ITimeRange(_))
+    }
 }
 
 impl Error {
@@ -218,6 +235,13 @@ impl Error {
         }
     }
 
+    /// Returns the root error in this chain.
+    fn root(&self) -> &Error {
+        // OK because `Error::chain` is guaranteed to return a non-empty
+        // iterator.
+        self.chain().last().unwrap()
+    }
+
     /// Returns a chain of error values.
     ///
     /// This starts with the most recent error added to the chain. That is,