track optional function arguments

compiler/ml/error_message_utils.ml

@@ -527,6 +527,7 @@ let type_clash_context_for_function_argument type_clash_context sarg0 =
                Some txt
              | _ -> None);
          })
+  | None -> Some (FunctionArgument {optional = false})
   | type_clash_context -> type_clash_context
 
 let type_clash_context_maybe_option ty_expected ty_res =

compiler/ml/typecore.ml

@@ -3657,7 +3657,9 @@ and type_application ~context total_app env funct (sargs : sargs) :
                    env sarg0 ty ty0
                else fun () ->
                  option_some
-                   (type_argument ~context env sarg0
+                   (type_argument
+                      ~context:(Some (FunctionArgument {optional = true}))
+                      env sarg0
                       (extract_option_type env ty)
                       (extract_option_type env ty0))) )
       in

tests/build_tests/super_errors/expected/optional_fn_argument_pass_option.res.expected

@@ -0,0 +1,18 @@
+
+  We've found a bug for you!
+  /.../fixtures/optional_fn_argument_pass_option.res:5:18
+
+  3 │ let t = Some(1)
+  4 │ 
+  5 │ let f = optFn(~x=t)
+  6 │ 
+
+  This has type: option<int>
+  But this optional function argument is expecting: int
+
+  You're passing an optional value into an optional function argument.
+  Optional function arguments expect you to pass the concrete value, not an option, when passed directly.
+
+  Possible solutions: 
+  - Unwrap the option and pass a concrete value directly
+  - If you really do want to pass the optional value, prepend the value with ? to show you want to pass the option, like: ?t

tests/build_tests/super_errors/fixtures/optional_fn_argument_pass_option.res

@@ -0,0 +1,5 @@
+let optFn = (~x: option<int>=?) => x
+
+let t = Some(1)
+
+let f = optFn(~x=t)