From 3377c3c43faba7bfc446402d1c6d31b119bca50f Mon Sep 17 00:00:00 2001
From: Philippe-Cholet <phcholet@orange.fr>
Date: Sun, 26 Nov 2023 12:21:47 +0100
Subject: [PATCH 1/2] `ExactlyOneError::fold`

---
 src/exactly_one_err.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/exactly_one_err.rs b/src/exactly_one_err.rs
index e24d33fc5..47c734c1a 100644
--- a/src/exactly_one_err.rs
+++ b/src/exactly_one_err.rs
@@ -63,6 +63,21 @@ where
     fn size_hint(&self) -> (usize, Option<usize>) {
         size_hint::add_scalar(self.inner.size_hint(), self.additional_len())
     }
+
+    fn fold<B, F>(self, mut init: B, mut f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        match self.first_two {
+            Some(Either::Left([first, second])) => {
+                init = f(init, first);
+                init = f(init, second);
+            }
+            Some(Either::Right(second)) => init = f(init, second),
+            None => {}
+        }
+        self.inner.fold(init, f)
+    }
 }
 
 impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}

From 5835a90f726e4e9a42a82f7d6b20aef8ac85f740 Mon Sep 17 00:00:00 2001
From: Philippe-Cholet <phcholet@orange.fr>
Date: Fri, 15 Dec 2023 19:06:04 +0100
Subject: [PATCH 2/2] Test `ExactlyOneError` specializations

---
 tests/specializations.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tests/specializations.rs b/tests/specializations.rs
index abe41fe92..e14b1b669 100644
--- a/tests/specializations.rs
+++ b/tests/specializations.rs
@@ -330,6 +330,17 @@ quickcheck! {
         test_specializations(&it);
         test_double_ended_specializations(&it);
     }
+
+    fn exactly_one_error(v: Vec<u8>) -> TestResult {
+        // Use `at_most_one` would be similar.
+        match v.iter().exactly_one() {
+            Ok(_) => TestResult::discard(),
+            Err(it) => {
+                test_specializations(&it);
+                TestResult::passed()
+            }
+        }
+    }
 }
 
 quickcheck! {