Add tests for issue-58887

src/test/ui/existential-type/issue-58887.rs

@@ -0,0 +1,23 @@
+#![feature(existential_type)]
+
+trait UnwrapItemsExt {
+    type Iter;
+    fn unwrap_items(self) -> Self::Iter;
+}
+
+impl<I, T, E> UnwrapItemsExt for I
+where
+    I: Iterator<Item = Result<T, E>>,
+    E: std::fmt::Debug,
+{
+    existential type Iter: Iterator<Item = T>;
+    //~^ ERROR: could not find defining uses
+
+    fn unwrap_items(self) -> Self::Iter {
+    //~^ ERROR: type parameter `T` is part of concrete type
+    //~| ERROR: type parameter `E` is part of concrete type
+        self.map(|x| x.unwrap())
+    }
+}
+
+fn main() {}

src/test/ui/existential-type/issue-58887.stderr

@@ -0,0 +1,30 @@
+error: type parameter `T` is part of concrete type but not used in parameter list for existential type
+  --> $DIR/issue-58887.rs:16:41
+   |
+LL |       fn unwrap_items(self) -> Self::Iter {
+   |  _________________________________________^
+LL | |
+LL | |
+LL | |         self.map(|x| x.unwrap())
+LL | |     }
+   | |_____^
+
+error: type parameter `E` is part of concrete type but not used in parameter list for existential type
+  --> $DIR/issue-58887.rs:16:41
+   |
+LL |       fn unwrap_items(self) -> Self::Iter {
+   |  _________________________________________^
+LL | |
+LL | |
+LL | |         self.map(|x| x.unwrap())
+LL | |     }
+   | |_____^
+
+error: could not find defining uses
+  --> $DIR/issue-58887.rs:13:5
+   |
+LL |     existential type Iter: Iterator<Item = T>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+