Fix parsing of expressions with underscore _ and comments.

Fixes #661

Author: cristianoc

CHANGELOG.md

@@ -30,6 +30,7 @@
 - Fix printing of comments inside empty blocks https://github.com/rescript-lang/syntax/pull/647
 - Fix location issue in error messages with JSX V4 where the multiple props types are defined https://github.com/rescript-lang/syntax/pull/655
 - Fix location issue in make function in JSX V4 that breaks dead code elimination https://github.com/rescript-lang/syntax/pull/660
+- Fix parsing (hence pretty printing) of expressions with underscore `_` and comments.
 
 ## ReScript 10.0
 

src/res_core.ml

@@ -510,11 +510,13 @@ let processUnderscoreApplication args =
     | _ -> arg
   in
   let args = List.map check_arg args in
-  let wrap exp_apply =
+  let wrap (exp_apply : Parsetree.expression) =
     match !exp_question with
     | Some {pexp_loc = loc} ->
       let pattern =
-        Ast_helper.Pat.mk (Ppat_var (Location.mkloc hidden_var loc)) ~loc
+        Ast_helper.Pat.mk
+          (Ppat_var (Location.mkloc hidden_var loc))
+          ~loc:Location.none
       in
       Ast_helper.Exp.mk (Pexp_fun (Nolabel, None, pattern, exp_apply)) ~loc
     | None -> exp_apply

tests/parsing/grammar/expressions/expected/underscoreApply.res.txt

@@ -14,4 +14,6 @@ let l1 =
     List.length
 let l2 =
   ([Some 1; None; Some 2] |> (List.map (fun __x -> optParam ?v:__x ()))) |>
-    List.length
+    List.length
+;;fun __x ->
+    underscoreWithComments (fun x -> ((something ())[@ns.braces ])) __x

tests/parsing/grammar/expressions/underscoreApply.res

@@ -19,3 +19,12 @@ let l1 =
 
 let l2 =
   list{Some(1), None, Some(2)} |> List.map(optParam(~v=?_, ())) |> List.length
+
+underscoreWithComments(
+  // Comment 1
+  x => {
+    // Comment 2
+    something()
+  },
+  _,
+)

tests/printer/expr/expected/underscoreApply.res.txt

@@ -61,3 +61,9 @@ Array.get(_, 1) + Array.get(_, 0)
 let f = Array.get(_, 0)
 
 let photo = pricedRoom["room"]["photos"] |> filterNone |> Array.get(_, 0)
+
+underscoreWithComments(// Comment 1
+x => {
+  // Comment 2
+  something()
+}, _)

tests/printer/expr/underscoreApply.res

@@ -63,3 +63,12 @@ Array.get(_, 1) + Array.get(_, 0)
 let f = Array.get(_, 0)
 
 let photo = pricedRoom["room"]["photos"] |> filterNone |> Array.get(_, 0)
+
+underscoreWithComments(
+  // Comment 1
+  x => {
+    // Comment 2
+    something()
+  },
+  _,
+)