Add test for issue-38591

Author: JohnTitor

src/test/ui/issues/issue-38591.rs

@@ -0,0 +1,10 @@
+// run-pass
+
+struct S<T> {
+    t : T,
+    s : Box<S<fn(u : T)>>
+}
+
+fn f(x : S<u32>) {}
+
+fn main () {}