Add a regression test for rust-lang#53457

Author: varkor

src/test/ui/impl-trait/issue-53457.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(existential_type)]
+
+existential type X: Clone;
+
+fn bar<F: Fn(&i32) + Clone>(f: F) -> F {
+    f
+}
+
+fn foo() -> X {
+    bar(|x| ())
+}
+
+fn main() {}