From 43ef554b6ab72c4cc3c56dab8e9b61021c63b3b7 Mon Sep 17 00:00:00 2001
From: Yuki Okushi <huyuumi.dev@gmail.com>
Date: Tue, 23 Jun 2020 17:52:26 +0900
Subject: [PATCH] Add test for issue-51506

---
 src/test/ui/never_type/issue-51506.rs     | 41 +++++++++++++++++++++++
 src/test/ui/never_type/issue-51506.stderr | 14 ++++++++
 2 files changed, 55 insertions(+)
 create mode 100644 src/test/ui/never_type/issue-51506.rs
 create mode 100644 src/test/ui/never_type/issue-51506.stderr

diff --git a/src/test/ui/never_type/issue-51506.rs b/src/test/ui/never_type/issue-51506.rs
new file mode 100644
index 00000000000..d0fe6a0f59a
--- /dev/null
+++ b/src/test/ui/never_type/issue-51506.rs
@@ -0,0 +1,41 @@
+#![feature(never_type, specialization)]
+#![allow(incomplete_features)]
+
+use std::iter::{self, Empty};
+
+trait Trait {
+    type Out: Iterator<Item = u32>;
+
+    fn f(&self) -> Option<Self::Out>;
+}
+
+impl<T> Trait for T {
+    default type Out = !; //~ ERROR: `!` is not an iterator
+
+    default fn f(&self) -> Option<Self::Out> {
+        None
+    }
+}
+
+struct X;
+
+impl Trait for X {
+    type Out = Empty<u32>;
+
+    fn f(&self) -> Option<Self::Out> {
+        Some(iter::empty())
+    }
+}
+
+fn f<T: Trait>(a: T) {
+    if let Some(iter) = a.f() {
+        println!("Some");
+        for x in iter {
+            println!("x = {}", x);
+        }
+    }
+}
+
+pub fn main() {
+    f(10);
+}
diff --git a/src/test/ui/never_type/issue-51506.stderr b/src/test/ui/never_type/issue-51506.stderr
new file mode 100644
index 00000000000..73865a9b5a0
--- /dev/null
+++ b/src/test/ui/never_type/issue-51506.stderr
@@ -0,0 +1,14 @@
+error[E0277]: `!` is not an iterator
+  --> $DIR/issue-51506.rs:13:5
+   |
+LL |     type Out: Iterator<Item = u32>;
+   |     ------------------------------- required by `Trait::Out`
+...
+LL |     default type Out = !;
+   |     ^^^^^^^^^^^^^^^^^^^^^ `!` is not an iterator
+   |
+   = help: the trait `std::iter::Iterator` is not implemented for `!`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.