Add regression test for #91949

src/test/ui/traits/issue-91949-hangs-on-recursion.rs

@@ -0,0 +1,30 @@
+// build-fail
+// error-pattern: overflow evaluating the requirement `(): Sized`
+// error-pattern: function cannot return without recursing
+
+// Regression test for #91949.
+// This hanged *forever* on 1.56, fixed by #90423.
+
+#![recursion_limit = "256"]
+
+struct Wrapped<T>(T);
+
+struct IteratorOfWrapped<T, I: Iterator<Item = T>>(I);
+
+impl<T, I: Iterator<Item = T>> Iterator for IteratorOfWrapped<T, I> {
+    type Item = Wrapped<T>;
+    fn next(&mut self) -> Option<Wrapped<T>> {
+        self.0.next().map(Wrapped)
+    }
+}
+
+fn recurse<T>(elements: T) -> Vec<char>
+where
+    T: Iterator<Item = ()>,
+{
+    recurse(IteratorOfWrapped(elements).map(|t| t.0))
+}
+
+fn main() {
+    recurse(std::iter::empty());
+}