Add regression test

tests/ui/type-alias-impl-trait/nested_impl_trait_in_assoc_ty.rs

@@ -0,0 +1,37 @@
+// edition: 2021
+
+#![feature(impl_trait_in_assoc_type)]
+
+use std::future::Future;
+
+pub struct MemtableLocalStateStore {
+    mem_table: MemTable,
+}
+
+impl LocalStateStore for MemtableLocalStateStore {
+    type IterStream<'a> = impl Sized + 'a where Self: 'a;
+
+    fn iter(&self) -> impl Future<Output = Self::IterStream<'_>> + '_ {
+        async move { merge_stream(self.mem_table.iter()) }
+    }
+}
+
+trait LocalStateStore {
+    type IterStream<'a>
+    where
+        Self: 'a;
+
+    fn iter(&self) -> impl Future<Output = Self::IterStream<'_>> + '_;
+}
+
+struct MemTable;
+
+impl MemTable {
+    fn iter<'a>(&'a self) -> impl Iterator<Item = &'a ()> {
+        std::iter::empty()
+    }
+}
+
+pub(crate) async fn merge_stream<'a>(mem_table_iter: impl Iterator<Item = &'a ()>) {}
+
+fn main() {}

tests/ui/type-alias-impl-trait/nested_impl_trait_in_assoc_ty.stderr

@@ -0,0 +1,15 @@
+error[E0792]: non-defining opaque type use in defining scope
+  --> $DIR/nested_impl_trait_in_assoc_ty.rs:15:9
+   |
+LL |         async move { merge_stream(self.mem_table.iter()) }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument `'_` is not a generic parameter
+   |
+note: for this opaque type
+  --> $DIR/nested_impl_trait_in_assoc_ty.rs:35:1
+   |
+LL | pub(crate) async fn merge_stream<'a>(mem_table_iter: impl Iterator<Item = &'a ()>) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0792`.