Fix arity error message with optional args.

compiler/ml/typecore.ml

@@ -3514,13 +3514,14 @@ and type_application ?type_clash_context total_app env funct (sargs : sargs) :
     | Some arity ->
       let newarity = arity - nargs in
       let fully_applied = newarity <= 0 in
-      if total_app && not fully_applied then
-        raise
-          (Error
-             ( funct.exp_loc,
-               env,
-               Uncurried_arity_mismatch
-                 (funct.exp_type, arity, List.length sargs) ));
+      (if total_app && not fully_applied then
+         let required_args = List.length sargs in
+         raise
+           (Error
+              ( funct.exp_loc,
+                env,
+                Uncurried_arity_mismatch
+                  (funct.exp_type, required_args + newarity, required_args) )));
       let new_t =
         if fully_applied then new_t
         else
@@ -4463,7 +4464,7 @@ let report_error env ppf error =
       "@ @[It is applied with @{<error>%d@} argument%s but it requires \
        @{<info>%d@}.@]@]"
       args
-      (if args = 0 then "" else "s")
+      (if args = 1 then "" else "s")
       arity
   | Field_not_optional (name, typ) ->
     fprintf ppf "Field @{<info>%s@} is not optional in type %a. Use without ?"

tests/build_tests/super_errors/expected/arity_mismatch.res.expected

@@ -7,4 +7,4 @@
   3 │ 
 
   This function has type (~f: 'a => 'a, unit) => int
-  It is applied with 1 arguments but it requires 2.
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/method_arity_mismatch.res.expected

@@ -9,4 +9,4 @@
   5 │ 
 
   This function has type (int, int) => unit
-  It is applied with 1 arguments but it requires 2.
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/moreArguments1.res.expected

@@ -7,4 +7,4 @@
   3 │ 
 
   This function has type (~a: int, ~b: int) => int
-  It is applied with 1 arguments but it requires 2.
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/moreArguments2.res.expected

@@ -7,4 +7,4 @@
   3 │ 
 
   This function has type (int, int) => int
-  It is applied with 1 arguments but it requires 2.
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/moreArguments3.res.expected

@@ -7,4 +7,4 @@
   3 │ 
 
   This function has type (int, int, 'a, 'b) => int
-  It is applied with 1 arguments but it requires 4.
+  It is applied with 1 argument but it requires 4.

tests/build_tests/super_errors/expected/moreArguments4.res.expected

@@ -7,4 +7,4 @@
   3 │ 
 
   This function has type (int, ~b: int, ~c: 'a, ~d: 'b) => int
-  It is applied with 1 arguments but it requires 4.
+  It is applied with 1 argument but it requires 4.

tests/build_tests/super_errors/expected/moreArguments5.res.expected

@@ -8,4 +8,4 @@
   6 │ 
 
   This function has type (int, 'a, 'b, 'c) => Sub.a
-  It is applied with 1 arguments but it requires 4.
+  It is applied with 1 argument but it requires 4.

tests/build_tests/super_errors/expected/opt_args_arity.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/opt_args_arity.res:2:9
+
+  1 │ let f = (~a=0, b, c) => a + b + c
+  2 │ let x = f(42)
+  3 │ 
+
+  This function has type (~a: int=?, int, int) => int
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/recursive_type.res.expected

@@ -10,4 +10,4 @@
 
   This function has type
     ((option<'a>, ([> #List(list<'b>)] as 'b)) => 'c, 'd) => 'c
-  It is applied with 1 arguments but it requires 2.
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/expected/warnings1.res.expected

@@ -9,4 +9,4 @@
   5 │ }
 
   This function has type (int, int) => int
-  It is applied with 1 arguments but it requires 2.
+  It is applied with 1 argument but it requires 2.

tests/build_tests/super_errors/fixtures/opt_args_arity.res

@@ -0,0 +1,2 @@
+let f = (~a=0, b, c) => a + b + c
+let x = f(42)