Fix letops tuples handling (fixes #1683 and ocaml/ocaml-lsp#1182) (#1684

)

from voodoos/letop-tuples-issue-1683

CHANGES.md

@@ -1,3 +1,11 @@
+merlin 4.12
+===========
+unreleased
+
+  + merlin binary
+    - Fix issue with let operators and tuples (#1684, fixes #1683, fixes
+      ocaml/ocaml-lsp#1182)
+
 merlin 4.11
 ===========
 Thu Sep 24 18:01:42 CEST 2023

src/ocaml/merlin_specific/browse_raw.ml

@@ -380,14 +380,18 @@ let of_expression_desc loc = function
   | Texp_unreachable | Texp_extension_constructor _ ->
     id_fold
   | Texp_letop { let_; ands; body; _ } ->
-    let rec flatten_patterns acc pat =
+    (* let+ ..pat1 and pat2 and ... are represented as pattern couples:
+       [pat1; [pat2; ...]]. The following function flattens these couples.
+       Keeping track of the known size of the tuple prevent wrongly flattening
+       the patterns patN when they are tuples themselves. *)
+    let rec flatten_patterns ~size acc pat =
       match pat.pat_desc with
-      | Tpat_tuple [ tuple; pat ] ->
-           flatten_patterns (pat :: acc) tuple
+      | Tpat_tuple [ tuple; pat ] when size > 0 ->
+           flatten_patterns ~size:(size - 1) (pat :: acc) tuple
       | _ -> List.rev (pat :: acc)
     in
     let bindops = let_ :: ands in
-    let patterns = flatten_patterns [] body.c_lhs in
+    let patterns = flatten_patterns ~size:(List.length ands) [] body.c_lhs in
     let of_letop (pat, bindop) = of_bop bindop ** of_pattern pat in
     list_fold of_letop (List.combine patterns bindops) **
     of_expression body.c_rhs

tests/test-dirs/issue1683-ill-formed-letop.t

@@ -0,0 +1,35 @@
+  $ cat >main.ml <<EOF
+  > let (let+) x f = f x
+  > let (and+) x f = f x
+  > 
+  > let _ =
+  >   let+ x, _ = 42, 43
+  >   and+ y = 36 and+ _ = 37 in 
+  >   x + y
+  > EOF
+
+Calling `holes` on the code fragment was raising an exception as
+described in issue #1683.
+  $ $MERLIN single holes -filename main.ml <main.ml
+  {
+    "class": "return",
+    "value": [],
+    "notifications": []
+  }
+
+
+  $ $MERLIN single type-enclosing \
+  > -position 5:7 \
+  > -filename main.ml <main.ml | jq '.value[0]'
+  {
+    "start": {
+      "line": 5,
+      "col": 7
+    },
+    "end": {
+      "line": 5,
+      "col": 8
+    },
+    "type": "int",
+    "tail": "no"
+  }