Treat foo(. ) as empty application if all arguments are optional.

When all the processed arguments to the function are ignored (hence optional), and no arguments are ignored (so no mandatory labelled), it means the uncurried function only has optional arguments.

- If the uncurried type of the function had an unlabelled arg, then it would be caught by the unit application, so it would be a non-ignored argument. But this is not possible as all the argiments are ignored.
- If it had a labelled mandatory argument, then either it would be non-ignored, or omitted, but both cases are excluded.
- It follows that the type only has optional arguments, and that the unit argument was the only argument supplied.

The new mechanism does not kick in when some legit argument is passed alongside the unit. And noes not interfere with cases where the function expects a legitimate unit argument.

jscomp/ml/typecore.ml

@@ -3019,7 +3019,7 @@ and type_application uncurried env funct (sargs : sargs) : targs * Types.type_ex
     match has_uncurried_type t with
     | Some (arity, _) ->
       let newarity = arity - nargs in
-      let fully_applied = newarity = 0 in
+      let fully_applied = newarity <= 0 in
       if uncurried && not fully_applied then
         raise(Error(funct.exp_loc, env,
           Uncurried_arity_mismatch (t, arity, List.length sargs)));
@@ -3046,6 +3046,10 @@ and type_application uncurried env funct (sargs : sargs) : targs * Types.type_ex
           | _ -> collect_args ())
         else
           collect_args ()
+    | [(Nolabel, {pexp_desc = Pexp_construct ({txt = Lident "()"}, None)})]
+      when uncurried && omitted = [] && List.length args = List.length !ignored ->
+      (* foo(. ) treated as empty application if all args are optional (hence ignored) *)
+        type_unknown_args max_arity args omitted ty_fun []
     | (l1, sarg1) :: sargl ->
         let (ty1, ty2) =
           let ty_fun = expand_head env ty_fun in

jscomp/test/uncurried_default.args.js

@@ -20,11 +20,42 @@ var partial = Curry._1((function (param) {
 
 var total = withOpt(10, 3)(4, 11);
 
+function foo1(xOpt, y) {
+  var x = xOpt !== undefined ? xOpt : 3;
+  return x + y | 0;
+}
+
+var x = 3;
+
+var r1 = x + 11 | 0;
+
+function foo2(y, xOpt, zOpt) {
+  var x = xOpt !== undefined ? xOpt : 3;
+  var z = zOpt !== undefined ? zOpt : 4;
+  return (x + y | 0) + z | 0;
+}
+
+var r2 = foo2(11, undefined, undefined);
+
+function foo3(xOpt, yOpt) {
+  var x = xOpt !== undefined ? xOpt : 3;
+  var y = yOpt !== undefined ? yOpt : 4;
+  return x + y | 0;
+}
+
+var r3 = foo3(undefined, undefined);
+
 var StandardNotation = {
   withOpt: withOpt,
   testWithOpt: testWithOpt,
   partial: partial,
-  total: total
+  total: total,
+  foo1: foo1,
+  r1: r1,
+  foo2: foo2,
+  r2: r2,
+  foo3: foo3,
+  r3: r3
 };
 
 function withOpt$1(xOpt, y) {
@@ -45,30 +76,40 @@ var partial$1 = Curry._1((function (param) {
 
 var total$1 = withOpt$1(10, 3)(4, 11);
 
-function foo1(xOpt, y) {
+function foo1$1(xOpt, y) {
   var x = xOpt !== undefined ? xOpt : 3;
   return x + y | 0;
 }
 
-var x = 3;
+var x$1 = 3;
 
-var r1 = x + 11 | 0;
+var r1$1 = x$1 + 11 | 0;
 
-function foo2(y, xOpt, zOpt) {
+function foo2$1(y, xOpt, zOpt) {
   var x = xOpt !== undefined ? xOpt : 3;
   var z = zOpt !== undefined ? zOpt : 4;
   return (x + y | 0) + z | 0;
 }
 
-var r2 = foo2(11, undefined, undefined);
+var r2$1 = foo2$1(11, undefined, undefined);
+
+function foo3$1(xOpt, yOpt) {
+  var x = xOpt !== undefined ? xOpt : 3;
+  var y = yOpt !== undefined ? yOpt : 4;
+  return x + y | 0;
+}
+
+var r3$1 = foo3$1(undefined, undefined);
 
 exports.StandardNotation = StandardNotation;
 exports.withOpt = withOpt$1;
 exports.testWithOpt = testWithOpt$1;
 exports.partial = partial$1;
 exports.total = total$1;
-exports.foo1 = foo1;
-exports.r1 = r1;
-exports.foo2 = foo2;
-exports.r2 = r2;
+exports.foo1 = foo1$1;
+exports.r1 = r1$1;
+exports.foo2 = foo2$1;
+exports.r2 = r2$1;
+exports.foo3 = foo3$1;
+exports.r3 = r3$1;
 /* testWithOpt Not a pure module */

jscomp/test/uncurried_default.args.res

@@ -3,6 +3,15 @@ module StandardNotation = {
   let testWithOpt = withOpt(. 3)(. 4)
   let partial = withOpt(~x=10)(3)(~z=4)(11)
   let total = withOpt(. ~x=10, 3)(. ~z=4, 11)
+
+  let foo1 = (. ~x=3, ~y) => x+y
+  let r1 = foo1(. ~y=11)
+
+  let foo2 = (. ~y, ~x=3, ~z=4) => x+y+z
+  let r2 = foo2(. ~y=11)
+
+  let foo3 = (. ~x=3, ~y=4) => x+y
+  let r3 = foo3(. )
 }
 
 @@uncurried
@@ -19,3 +28,6 @@ let r1 = foo1(~y=11)
 
 let foo2 = (~y, ~x=3, ~z=4) => x+y+z
 let r2 = foo2(~y=11)
+
+let foo3 = (~x=3, ~y=4) => x+y
+let r3 = foo3()