Untagged variants: consider regexp as an object type.

Fixes #6140

Author: cristianoc

CHANGELOG.md

@@ -16,6 +16,7 @@
 
 - Introduced a new  `%ffi` extension (*experimental* - not for production use!) that provides a more robust mechanism for JavaScript function interoperation by considering function arity in type constraints. This enhancement improves safety when dealing with JavaScript functions by enforcing type constraints based on the arity of the function. https://github.com/rescript-lang/rescript-compiler/pull/6251
 - Extended untagged variants with function types. https://github.com/rescript-lang/rescript-compiler/pull/6279
+- Untagged variants: consider regexp as an object type. https://github.com/rescript-lang/rescript-compiler/pull/6296
 
 #### :boom: Breaking Change
 

jscomp/ml/ast_untagged_variants.ml

@@ -117,7 +117,7 @@ let type_is_builtin_object (t : Types.type_expr) =
   match t.desc with
   | Tconstr (path, _, _) ->
     let name = Path.name path in
-    name = "Js.Dict.t" || name = "Js_dict.t"
+    name = "Js.Dict.t" || name = "Js_dict.t" || name = "Js.Re.t" || name = "RescriptCore.Re.t"
   | _ -> false
 
 let get_block_type ~env (cstr : Types.constructor_declaration) :