add known-bug test for unsound issue 84366

tests/ui/closures/static-closures-with-nonstatic-return.rs

@@ -0,0 +1,15 @@
+// check-pass
+// known-bug: #84366
+
+// Should fail. Associated types of 'static types should be `'static`, but
+// argument-free closures can be `'static` and return non-`'static` types.
+
+#[allow(dead_code)]
+fn foo<'a>() {
+    let closure = || -> &'a str { "" };
+    assert_static(closure);
+}
+
+fn assert_static<T: 'static>(_: T) {}
+
+fn main() {}