Implement suggestions

ocaml/parsing/lexer.mll

@@ -80,6 +80,7 @@ let keyword_table =
     "once_", ONCE;
     "open", OPEN;
     "or", OR;
+    (* CR uniqueness: Check that there are no variables named overwrite_ in Jane *)
     "overwrite_", OVERWRITE;
 (*  "parser", PARSER; *)
     "private", PRIVATE;

ocaml/parsing/pprintast.ml

@@ -1117,6 +1117,12 @@ and expression ?(jane_syntax_parens = false) ctxt f x =
           (expression ctxt) body
     | Pexp_extension e -> extension ctxt f e
     | Pexp_unreachable -> pp f "."
+    | Pexp_overwrite (x, e) ->
+        (* Similar to the case of [Pexp_stack] *)
+        pp f "@[<hov2>overwrite_@ %a@ with@ %a@]"
+          longident_loc x
+          (expression2 reset_ctxt) e
+    | Pexp_hole -> pp f "_"
     | _ -> expression1 ctxt f x
 
 and expression1 ctxt f x =

ocaml/testsuite/tests/parsetree/source_jane_street.ml

@@ -1032,3 +1032,34 @@ module type S' =
     module M : sig val foo : 'a -> 'a end
   end
 |}]
+
+(***************)
+(* Overwriting *)
+
+
+let overwrite_tuple = function
+    (a, b) as t -> overwrite_ t with (b, _)
+
+type record = { a : int; b : int }
+
+let overwrite_record = function
+    { a; b } as t -> overwrite_ t with { b = a; a = _ }
+
+let overwrite_record = function
+    { a; b } as t -> overwrite_ t with { b = a }
+
+type constructor = C of { a : int; b : int }
+
+let overwrite_constructor = function
+    C { a; b } as t -> overwrite_ t with C { b = a; a = _ }
+
+let overwrite_constructor = function
+    C { a; b } as t -> overwrite_ t with C { b = a }
+[%%expect{|
+Line 2, characters 19-43:
+2 |     (a, b) as t -> overwrite_ t with (b, _)
+                       ^^^^^^^^^^^^^^^^^^^^^^^^
+Alert : Overwrite not implemented.
+Uncaught exception: File "ocaml/parsing/location.ml", line 1106, characters 2-8: Assertion failed
+
+|}]