1 files changed, 37 insertions, 0 deletions
diff --git a/library/std/src/error/tests.rs b/library/std/src/error/tests.rs
new file mode 100644
index 00000000000..66d6924f34d
--- /dev/null
+++ b/library/std/src/error/tests.rs
@@ -0,0 +1,37 @@
+use super::Error;
+use crate::fmt;
+
+#[derive(Debug, PartialEq)]
+struct A;
+#[derive(Debug, PartialEq)]
+struct B;
+
+impl fmt::Display for A {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "A")
+    }
+}
+impl fmt::Display for B {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "B")
+    }
+}
+
+impl Error for A {}
+impl Error for B {}
+
+#[test]
+fn downcasting() {
+    let mut a = A;
+    let a = &mut a as &mut (dyn Error + 'static);
+    assert_eq!(a.downcast_ref::<A>(), Some(&A));
+    assert_eq!(a.downcast_ref::<B>(), None);
+    assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
+    assert_eq!(a.downcast_mut::<B>(), None);
+
+    let a: Box<dyn Error> = Box::new(A);
+    match a.downcast::<B>() {
+        Ok(..) => panic!("expected error"),
+        Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
+    }
+}