Add test for issue-51506

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);
+}

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`.