From 535556ff2f93b757647eeca5e3348667223e62af Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Fri, 17 Mar 2023 11:10:28 +0100
Subject: [PATCH] Fix underscore in uncurried mode.

---
 CHANGELOG.md                    |  1 +
 jscomp/test/UncurriedAlways.js  | 20 ++++++++++++++++++++
 jscomp/test/UncurriedAlways.res |  6 ++++++
 res_syntax/src/res_core.ml      |  8 +++++---
 4 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0cb4c24650..321b36a47b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -57,6 +57,7 @@ These are only breaking changes for unformatted code.
 - Fix issue with error messages for uncurried functions where expected and given type were swapped https://github.com/rescript-lang/rescript-compiler/pull/5973
 - Fix issue with integer overflow check https://github.com/rescript-lang/rescript-compiler/pull/6028
 - Make internal encoding of locations aware of unicode https://github.com/rescript-lang/rescript-compiler/pull/6073
+- Fix issue where `foo(x,_)` in uncurried mode would generate a curried function https://github.com/rescript-lang/rescript-compiler/pull/6082
 
 #### :nail_care: Polish
 
diff --git a/jscomp/test/UncurriedAlways.js b/jscomp/test/UncurriedAlways.js
index ba84db9d57..7cfcf4dff8 100644
--- a/jscomp/test/UncurriedAlways.js
+++ b/jscomp/test/UncurriedAlways.js
@@ -27,6 +27,22 @@ function ptl(param) {
   return foo(10, param);
 }
 
+function foo2(x, y) {
+  return x + y | 0;
+}
+
+function bar2(__x) {
+  return __x + 3 | 0;
+}
+
+function foo3(x, y, z) {
+  return (x + y | 0) + z | 0;
+}
+
+function bar3(__x) {
+  return foo3(__x, 3, 4);
+}
+
 exports.foo = foo;
 exports.z = z;
 exports.bar = bar;
@@ -34,4 +50,8 @@ exports.b = b;
 exports.w = w;
 exports.a = a;
 exports.ptl = ptl;
+exports.foo2 = foo2;
+exports.bar2 = bar2;
+exports.foo3 = foo3;
+exports.bar3 = bar3;
 /*  Not a pure module */
diff --git a/jscomp/test/UncurriedAlways.res b/jscomp/test/UncurriedAlways.res
index 63d8e25899..5da60a0852 100644
--- a/jscomp/test/UncurriedAlways.res
+++ b/jscomp/test/UncurriedAlways.res
@@ -17,3 +17,9 @@ Js.log(a) // Test automatic uncurried application
 let _ = Js.Array2.map([1], (. x) => x+1)
 
 let ptl = @res.partial foo(10) // force partial application
+
+let foo2 = (x,y) => x+y
+let bar2: _ => _ = foo2(_, 3)
+
+let foo3 = (x,y,z) => x+y+z
+let bar3 : _ => _ = foo3(_, 3, 4)
diff --git a/res_syntax/src/res_core.ml b/res_syntax/src/res_core.ml
index ca3dc4275e..a08180a9e6 100644
--- a/res_syntax/src/res_core.ml
+++ b/res_syntax/src/res_core.ml
@@ -516,7 +516,7 @@ let wrapTypeAnnotation ~loc newtypes core_type body =
   * return a wrapping function that wraps ((__x) => ...) around an expression
   * e.g. foo(_, 3) becomes (__x) => foo(__x, 3)
   *)
-let processUnderscoreApplication args =
+let processUnderscoreApplication (p : Parser.t) args =
   let exp_question = ref None in
   let hidden_var = "__x" in
   let check_arg ((lab, exp) as arg) =
@@ -537,7 +537,9 @@ let processUnderscoreApplication args =
           (Ppat_var (Location.mkloc hidden_var loc))
           ~loc:Location.none
       in
-      Ast_helper.Exp.mk (Pexp_fun (Nolabel, None, pattern, exp_apply)) ~loc
+      let funExpr = Ast_helper.Exp.fun_ ~loc Nolabel None pattern exp_apply in
+      if p.uncurried_config = Legacy then funExpr
+      else Ast_uncurried.uncurriedFun ~loc ~arity:1 funExpr
     | None -> exp_apply
   in
   (args, wrap)
@@ -3667,7 +3669,7 @@ and parseCallExpr p funExpr =
     List.fold_left
       (fun callBody group ->
         let dotted, args = group in
-        let args, wrap = processUnderscoreApplication args in
+        let args, wrap = processUnderscoreApplication p args in
         let exp =
           let uncurried =
             p.uncurried_config |> Res_uncurried.fromDotted ~dotted