Fix arity error message with optional args.

Fixes #6637

Author: cristianoc

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

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

@@ -0,0 +1,9 @@
+
+  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)
+
+  This function has type (~a: int=?, int, int) => int
+  It is applied with 1 arguments 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)