Add test.

tests/ui/mir/debug-ref-undef.rs

@@ -0,0 +1,57 @@
+// run-pass
+// compile-flags: -g -O -Zmir-opt-level=0 -Zinline-mir=y -Zmir-enable-passes=+ReferencePropagation
+
+#![allow(dead_code)]
+
+use std::marker::PhantomData;
+
+struct RawTable<T> {
+    marker: PhantomData<T>,
+}
+
+impl<T> RawTable<T> {
+    fn iter(&self) -> RawIter<T> {
+        RawIter { marker: PhantomData }
+    }
+}
+
+struct RawIter<T> {
+    marker: PhantomData<T>,
+}
+
+impl<T> Iterator for RawIter<T> {
+    type Item = ();
+    fn next(&mut self) -> Option<()> {
+        None
+    }
+}
+
+struct HashMap<T> {
+    table: RawTable<T>,
+}
+
+struct Iter<T> {
+    inner: RawIter<T>, // Removing this breaks the reproducer
+}
+
+impl<T> IntoIterator for &HashMap<T> {
+    type Item = T;
+    type IntoIter = Iter<T>;
+    fn into_iter(self) -> Iter<T> {
+        Iter { inner: self.table.iter() }
+    }
+}
+
+impl<T> Iterator for Iter<T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        None
+    }
+}
+
+pub fn main() {
+    let maybe_hash_set: Option<HashMap<()>> = None;
+    for _ in maybe_hash_set.as_ref().unwrap_or(&HashMap { table: RawTable { marker: PhantomData } })
+    {
+    }
+}