Improve default argument type mismatch errors (#8389)

* Regenerate super errors test snapshots

* Improve error for generated optional default match

Signed-off-by: Christoph Knittel <ck@cca.io>

* CHANGELOG

---------

Signed-off-by: Christoph Knittel <ck@cca.io>

Author: cknitt

CHANGELOG.md

@@ -39,6 +39,7 @@
 - Improve error message for dependency without `rescript.json`. https://github.com/rescript-lang/rescript/pull/8292
 - Resolve workspace dependencies in editor analysis. https://github.com/rescript-lang/rescript/pull/8392
 - Improve deprecated attribute extraction and support record form. https://github.com/rescript-lang/rescript/pull/8396
+- Improve default argument type mismatch errors. https://github.com/rescript-lang/rescript/pull/8389
 
 # 12.2.0
 

compiler/ml/typecore.ml

@@ -2433,6 +2433,7 @@ and type_expect_ ?deprecated_context ~context ?in_function ?(recarg = Rejected)
     in
     let smatch =
       Exp.match_ ~loc:sloc
+        ~attrs:[(mknoloc "#optional_arg_default", PStr [])]
         (Exp.ident ~loc (mknoloc (Longident.Lident "*opt*")))
         scases
     in
@@ -2521,13 +2522,15 @@ and type_expect_ ?deprecated_context ~context ?in_function ?(recarg = Rejected)
     (* Note: val_caselist = [] and exn_caselist = [], i.e. a fully
        empty pattern matching can be generated by Camlp4 with its
        revised syntax.  Let's accept it for backward compatibility. *)
+    let has_attr name =
+      Ext_list.exists sexp.pexp_attributes (fun ({txt}, _) -> txt = name)
+    in
     let call_context =
-      if
-        Ext_list.exists sexp.pexp_attributes (fun ({txt}, _) ->
-            match txt with
-            | "let.unwrap" -> true
-            | _ -> false)
-      then `LetUnwrap
+      (* Optional-default lowering synthesizes a match expression, but from the
+         user's point of view this is still part of a function definition. Use
+         function-style diagnostics instead of reporting a phantom switch. *)
+      if has_attr "let.unwrap" then `LetUnwrap
+      else if has_attr "#optional_arg_default" then `Function
       else `Switch
     in
     let val_cases, partial =

tests/build_tests/super_errors/expected/optional_default_arg_type_mismatch.res.expected

@@ -0,0 +1,9 @@
+
+  We've found a bug for you!
+  /.../fixtures/optional_default_arg_type_mismatch.res:1:29-30
+
+  1 │ let f = (~test: option<int>=42) => ()
+  2 │ 
+
+  This has type: int
+  But it's expected to have type: option<int>

tests/build_tests/super_errors/fixtures/optional_default_arg_type_mismatch.res

@@ -0,0 +1 @@
+let f = (~test: option<int>=42) => ()