add known-bug test for unsound issue 84533

tests/ui/fn/fn-item-lifetime-bounds.rs

@@ -0,0 +1,37 @@
+// check-pass
+// known-bug: #84533
+
+// Should fail. Lifetimes are checked correctly when `foo` is called, but NOT
+// when only the lifetime parameters are instantiated.
+
+use std::marker::PhantomData;
+
+#[allow(dead_code)]
+fn foo<'b, 'a>() -> PhantomData<&'b &'a ()> {
+    PhantomData
+}
+
+#[allow(dead_code)]
+#[allow(path_statements)]
+fn caller<'b, 'a>() {
+    foo::<'b, 'a>;
+}
+
+// In contrast to above, below code correctly does NOT compile.
+// fn caller<'b, 'a>() {
+//     foo::<'b, 'a>();
+// }
+
+// error: lifetime may not live long enough
+//   --> src/main.rs:22:5
+//   |
+// 21 | fn caller<'b, 'a>() {
+//   |           --  -- lifetime `'a` defined here
+//   |           |
+//   |           lifetime `'b` defined here
+// 22 |     foo::<'b, 'a>();
+//   |     ^^^^^^^^^^^^^^^ requires that `'a` must outlive `'b`
+//   |
+//   = help: consider adding the following bound: `'a: 'b`
+
+fn main() {}