Improve default argument type mismatch errors

CHANGELOG.md

@@ -24,6 +24,8 @@
 
 #### :nail_care: Polish
 
+- Improve default argument type mismatch errors. https://github.com/rescript-lang/rescript/pull/8389
+
 #### :house: Internal
 
 # 13.0.0-alpha.4

compiler/ml/typecore.ml

@@ -2452,6 +2452,7 @@ 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
@@ -2540,13 +2541,15 @@ 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/dict_show_no_coercion.res.expected

@@ -9,4 +9,4 @@
   3 │ 
 
   This has type: float
-  But it's expected to have type: JSON.t (defined as JSON.t)
+  But it's expected to have type: JSON.t (defined as JSON.t)

tests/build_tests/super_errors/expected/for_await_requires_async_iterable.res.expected

@@ -9,4 +9,4 @@
   7 │   }
 
   This has type: Iterable.t<int> (defined as iterable<int>)
-  But it's expected to have type: asyncIterable<'a>
+  But it's expected to have type: asyncIterable<'a>

tests/build_tests/super_errors/expected/for_of_non_array.res.expected

@@ -8,4 +8,4 @@
   4 │ }
 
   This has type: int
-  But it's expected to have type: iterable<'a>
+  But it's expected to have type: iterable<'a>

tests/build_tests/super_errors/expected/for_of_string_requires_iterable.res.expected

@@ -9,4 +9,4 @@
   6 │ }
 
   This has type: string
-  But it's expected to have type: iterable<'a>
+  But it's expected to have type: iterable<'a>

tests/build_tests/super_errors/expected/for_of_type_mismatch.res.expected

@@ -11,4 +11,4 @@
   This has type: int
   But it's expected to have type: string
 
-  You can convert int to string with Int.toString.
+  You can convert int to string with Int.toString.

tests/build_tests/super_errors/expected/jsx_invalid_prop_ast0_conversion.res.expected

@@ -7,6 +7,4 @@
   18 │ let _ = <Component bar="hello" />
   19 │ 
 
-  The prop bar does not belong to the JSX component <Component /> 
-
-
+  The prop bar does not belong to the JSX component <Component />

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) => ()