Add a test illustrating wrong untyping

Patterns without named existentials are not correctly constructed. Issue introduced in ocaml#9584

testsuite/tests/compiler-libs/test_untypeast.ml

@@ -0,0 +1,17 @@
+(* TEST
+   flags = "-I ${ocamlsrcdir}/typing \
+    -I ${ocamlsrcdir}/parsing"
+   include ocamlcommon
+   * expect
+*)
+
+let res =
+  let s = {| match None with Some (Some _) -> () | _ -> () |} in
+  let pe = Parse.expression (Lexing.from_string s) in
+  let te = Typecore.type_expression (Env.initial_safe_string) pe in
+  let ute = Untypeast.untype_expression te in
+  Format.asprintf "%a" Pprintast.expression ute
+
+[%%expect{|
+val res : string = "match None with | Some (Some _) -> () | _ -> ()"
+|}]