add tests, fix docs

Author: Qelxiros

library/core/src/option.rs

@@ -1965,8 +1965,23 @@ impl<T> Option<T> {
     /// Reduces two options into one, using the provided function if both are `Some`.
     ///
     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
-    /// Otherwise, if one of `self` and `other` is `Some`, that one is returned.
-    /// Otherwise, `None` is returned.
+    /// Otherwise, if only one of `self` and `other` is `Some`, that one is returned.
+    /// If both `self` and `other` are `None`, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_reduce)]
+    ///
+    /// let x = Some(12);
+    /// let y = Some(17);
+    /// let z = None;
+    /// let f = |a, b| a + b;
+    ///
+    /// assert_eq!(x.reduce(y, f), Some(29));
+    /// assert_eq!(x.reduce(z, f), Some(12));
+    /// assert_eq!(z.reduce(z, f), None);
+    /// ```
     #[unstable(feature = "option_reduce", issue = "144273")]
     pub fn reduce<U, R, F>(self, other: Option<U>, f: F) -> Option<R>
     where

library/coretests/tests/lib.rs

@@ -77,6 +77,7 @@
 #![feature(never_type)]
 #![feature(next_index)]
 #![feature(numfmt)]
+#![feature(option_reduce)]
 #![feature(pattern)]
 #![feature(pointer_is_aligned_to)]
 #![feature(portable_simd)]

library/coretests/tests/option.rs

@@ -584,3 +584,15 @@ fn as_slice() {
     assert_eq!(A, &[44]);
     assert_eq!(B, &[]);
 }
+
+#[test]
+fn reduce_options() {
+    let x = Some(10);
+    let y = Some(7);
+    let z = None::<i32>;
+
+    assert_eq!(x.reduce(y, |x, y| x + y), Some(17));
+    assert_eq!(x.reduce(z, |x, y| x + y), Some(10));
+    assert_eq!(y.reduce(z, |x, y| x + y), Some(7));
+    assert_eq!(z.reduce(z, |x, y| x + y), None);
+}