Add a test for #55731

src/test/ui/issues/issue-55731.rs

@@ -0,0 +1,52 @@
+use std::marker::PhantomData;
+
+trait DistributedIterator {
+    fn reduce(self)
+    where
+        Self: Sized,
+    {
+        unreachable!()
+    }
+}
+
+trait DistributedIteratorMulti<Source> {
+    type Item;
+}
+
+struct Connect<I>(PhantomData<fn(I)>);
+impl<I: for<'a> DistributedIteratorMulti<&'a ()>> DistributedIterator for Connect<I> where {}
+
+struct Cloned<Source>(PhantomData<fn(Source)>);
+impl<'a, Source> DistributedIteratorMulti<&'a Source> for Cloned<&'a Source> {
+    type Item = ();
+}
+
+struct Map<I, F> {
+    i: I,
+    f: F,
+}
+impl<I: DistributedIteratorMulti<Source>, F, Source> DistributedIteratorMulti<Source> for Map<I, F>
+where
+    F: A<<I as DistributedIteratorMulti<Source>>::Item>,
+{
+    type Item = ();
+}
+
+trait A<B> {}
+
+struct X;
+impl A<()> for X {}
+
+fn multi<I>(_reducer: I)
+where
+    I: for<'a> DistributedIteratorMulti<&'a ()>,
+{
+    DistributedIterator::reduce(Connect::<I>(PhantomData))
+}
+
+fn main() {
+    multi(Map { //~ ERROR implementation of `DistributedIteratorMulti` is not general enough
+        i: Cloned(PhantomData),
+        f: X,
+    });
+}

src/test/ui/issues/issue-55731.stderr

@@ -0,0 +1,12 @@
+error: implementation of `DistributedIteratorMulti` is not general enough
+  --> $DIR/issue-55731.rs:48:5
+   |
+LL |     multi(Map { //~ ERROR implementation of `DistributedIteratorMulti` is not general enough
+   |     ^^^^^
+   |
+   = note: Due to a where-clause on `multi`,
+   = note: `Map<Cloned<&()>, X>` must implement `DistributedIteratorMulti<&'0 ()>`, for any lifetime `'0`
+   = note: but `Map<Cloned<&()>, X>` actually implements `DistributedIteratorMulti<&'1 ()>`, for some specific lifetime `'1`
+
+error: aborting due to previous error
+