3 files changed, 41 insertions, 2 deletions

diff --git a/src/type_check.ml b/src/type_check.ml
index 4424fe8d..ac0d8af2 100644
--- a/src/type_check.ml
+++ b/src/type_check.ml
@@ -112,6 +112,7 @@ type env =
     union_ids : (typquant * typ) Bindings.t;
     registers : (effect * effect * typ) Bindings.t;
     variants : (typquant * type_union list) Bindings.t;
+    scattered_variant_envs : env Bindings.t;
     mappings : (typquant * typ * typ) Bindings.t;
     typ_vars : (Ast.l * kind_aux) KBindings.t;
     shadow_vars : int KBindings.t;
@@ -421,6 +422,7 @@ module Env : sig
   val add_scattered_variant : id -> typquant -> t -> t
   val add_variant_clause : id -> type_union -> t -> t
   val get_variant : id -> t -> typquant * type_union list
+  val get_scattered_variant_env : id -> t -> t
   val add_mapping : id -> typquant * typ * typ -> t -> t
   val add_union_id : id -> typquant * typ -> t -> t
   val get_union_id  : id -> t -> typquant * typ
@@ -499,6 +501,7 @@ end = struct
       union_ids = Bindings.empty;
       registers = Bindings.empty;
       variants = Bindings.empty;
+      scattered_variant_envs = Bindings.empty;
       mappings = Bindings.empty;
       typ_vars = KBindings.empty;
       shadow_vars = KBindings.empty;
@@ -1140,7 +1143,10 @@ end = struct
 
   let add_scattered_variant id typq env =
     typ_print (lazy (adding ^ "scattered variant " ^ string_of_id id));
-    { env with variants = Bindings.add id (typq, []) env.variants }
+    { env with
+      variants = Bindings.add id (typq, []) env.variants;
+      scattered_variant_envs = Bindings.add id env env.scattered_variant_envs
+    }
 
   let add_variant_clause id tu env =
     match Bindings.find_opt id env.variants with
@@ -1152,6 +1158,11 @@ end = struct
     | Some (typq, tus) -> typq, tus
     | None -> typ_error env (id_loc id) ("union " ^ string_of_id id ^ " not found")
 
+  let get_scattered_variant_env id env =
+    match Bindings.find_opt id env.scattered_variant_envs with
+    | Some env' -> env'
+    | None -> typ_error env (id_loc id) ("scattered union " ^ string_of_id id ^ " has not been declared")
+            
   let is_register id env =
     Bindings.mem id env.registers
 
@@ -5155,7 +5166,13 @@ and check_scattered : 'a. Env.t -> 'a scattered_def -> (tannot def) list * Env.t
      [DEF_scattered (SD_aux (SD_unioncl (id, tu), (l, None)))],
      let env = Env.add_variant_clause id tu env in
      let typq, _ = Env.get_variant id env in
-     check_type_union l env env id typq tu
+     let definition_env = Env.get_scattered_variant_env id env in
+     (try check_type_union l definition_env env id typq tu with
+      | Type_error (env, l', err) ->
+         let msg = "As this is a scattered union clause, this could \
+                    also be caused by using a type defined after the \
+                    'scattered union' declaration" in
+         raise (Type_error (env, l', Err_because (err, id_loc id, Err_other msg))))
   | SD_funcl (FCL_aux (FCL_Funcl (id, _), (l, _)) as funcl) ->
      let typq, typ = Env.get_val_spec id env in
      let funcl_env = add_typquant l typq env in
diff --git a/test/typecheck/fail/scattered_union_rec.expect b/test/typecheck/fail/scattered_union_rec.expect
new file mode 100644
index 00000000..cbc9f70a
--- /dev/null
+++ b/test/typecheck/fail/scattered_union_rec.expect
@@ -0,0 +1,16 @@
+Type error:
+[scattered_union_rec.sail]:6:24-25
+6 |union clause U = Ctor : E
+  |                        ^
+  | Undefined type E
+  | This error was caused by:
+  | [scattered_union_rec.sail]:6:0-25
+  | 6 |union clause U = Ctor : E
+  |   |^-----------------------^
+  |   | Types are not well-formed within this type definition. Note that recursive types are forbidden.
+  | This error was caused by:
+  | [scattered_union_rec.sail]:6:13-14
+  | 6 |union clause U = Ctor : E
+  |   |             ^
+  |   | As this is a scattered union clause, this could also be caused by using a type defined after the 'scattered union' declaration
+  | 
diff --git a/test/typecheck/fail/scattered_union_rec.sail b/test/typecheck/fail/scattered_union_rec.sail
new file mode 100644
index 00000000..9f005f4e
--- /dev/null
+++ b/test/typecheck/fail/scattered_union_rec.sail
@@ -0,0 +1,6 @@
+
+scattered union U
+
+enum E = A | B | C
+
+union clause U = Ctor : E