Add test for rust-lang#60564.

src/test/ui/existential_types/issue-60564.rs

@@ -0,0 +1,26 @@
+#![feature(existential_type)]
+
+trait IterBits {
+    type BitsIter: Iterator<Item = u8>;
+    fn iter_bits(self, n: u8) -> Self::BitsIter;
+}
+
+existential type IterBitsIter<T, E, I>: std::iter::Iterator<Item = I>;
+//~^ ERROR could not find defining uses
+
+impl<T, E> IterBits for T
+where
+    T: std::ops::Shr<Output = T>
+        + std::ops::BitAnd<T, Output = T>
+        + std::convert::From<u8>
+        + std::convert::TryInto<u8, Error = E>,
+    E: std::fmt::Debug,
+{
+    type BitsIter = IterBitsIter<T, E, u8>;
+    fn iter_bits(self, n: u8) -> Self::BitsIter {
+    //~^ ERROR type parameter `E` is part of concrete type but not used
+        (0u8..n)
+            .rev()
+            .map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())
+    }
+}

src/test/ui/existential_types/issue-60564.stderr

@@ -0,0 +1,25 @@
+error[E0601]: `main` function not found in crate `issue_60564`
+   |
+   = note: consider adding a `main` function to `$DIR/issue-60564.rs`
+
+error: type parameter `E` is part of concrete type but not used in parameter list for existential type
+  --> $DIR/issue-60564.rs:20:49
+   |
+LL |       fn iter_bits(self, n: u8) -> Self::BitsIter {
+   |  _________________________________________________^
+LL | |
+LL | |         (0u8..n)
+LL | |             .rev()
+LL | |             .map(move |shift| ((self >> T::from(shift)) & T::from(1)).try_into().unwrap())
+LL | |     }
+   | |_____^
+
+error: could not find defining uses
+  --> $DIR/issue-60564.rs:8:1
+   |
+LL | existential type IterBitsIter<T, E, I>: std::iter::Iterator<Item = I>;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0601`.