fixed Not_found exception raised in certain coercion cases (#6574)

* fixed Not_found exception raised in certain coercion cases

* changelog

* match for specific exception

* add back changelog entry

Author: zth

CHANGELOG.md

@@ -13,6 +13,8 @@
 # 11.0.1 (Unreleased)
 
 #### :bug: Bug Fix
+
+- Fixed issue with coercions sometimes raising a `Not_found` instead of giving a proper error message. https://github.com/rescript-lang/rescript-compiler/pull/6574
 - Fix issue with recursive modules and uncurried. https://github.com/rescript-lang/rescript-compiler/pull/6575
 
 # 11.0.0

jscomp/ml/ctype.ml

@@ -3904,6 +3904,11 @@ let subtypes = TypePairs.create 17
 let subtype_error env trace =
   raise (Subtype (expand_trace env (List.rev trace), []))
 
+let extract_concrete_typedecl_opt env t = 
+  match extract_concrete_typedecl env t with 
+  | v -> Some v 
+  | exception Not_found -> None
+
 let rec subtype_rec env trace t1 t2 cstrs =
   let t1 = repr t1 in
   let t2 = repr t2 in
@@ -3960,23 +3965,23 @@ let rec subtype_rec env trace t1 t2 cstrs =
     | (Tconstr(p1, [], _), Tconstr(p2, [], _)) when Path.same p1 Predef.path_int && Path.same p2 Predef.path_float ->
         cstrs 
     | (Tconstr(path, [], _), Tconstr(_, [], _)) when Variant_coercion.can_coerce_primitive path && 
-        extract_concrete_typedecl env t2 |> Variant_coercion.can_try_coerce_variant_to_primitive |> Option.is_some
+        extract_concrete_typedecl_opt env t2 |> Variant_coercion.can_try_coerce_variant_to_primitive_opt |> Option.is_some
         ->
       (* type coercion for primitives (int/float/string) to elgible unboxed variants:
          - must be unboxed
          - must have a constructor case with a supported and matching primitive payload *)
-      (match Variant_coercion.can_try_coerce_variant_to_primitive (extract_concrete_typedecl env t2) with
+      (match Variant_coercion.can_try_coerce_variant_to_primitive_opt (extract_concrete_typedecl_opt env t2) with
       | Some (constructors, true) -> 
         if Variant_coercion.variant_has_catch_all_case constructors (fun p -> Path.same p path) then
           cstrs
         else 
           (trace, t1, t2, !univar_pairs)::cstrs
       | _ -> (trace, t1, t2, !univar_pairs)::cstrs)
     | (Tconstr(_, [], _), Tconstr(path, [], _)) when Variant_coercion.can_coerce_primitive path && 
-        extract_concrete_typedecl env t1 |> Variant_coercion.can_try_coerce_variant_to_primitive |> Option.is_some
+        extract_concrete_typedecl_opt env t1 |> Variant_coercion.can_try_coerce_variant_to_primitive_opt |> Option.is_some
         ->
       (* type coercion for variants to primitives *)
-      (match Variant_coercion.can_try_coerce_variant_to_primitive (extract_concrete_typedecl env t1) with
+      (match Variant_coercion.can_try_coerce_variant_to_primitive_opt (extract_concrete_typedecl_opt env t1) with
       | Some (constructors, unboxed) -> 
         if constructors |> Variant_coercion.variant_has_same_runtime_representation_as_target ~targetPath:path ~unboxed then
           cstrs

jscomp/ml/variant_coercion.ml

@@ -60,6 +60,11 @@ let can_try_coerce_variant_to_primitive
     Some (constructors, type_attributes |> Ast_untagged_variants.has_untagged)
   | _ -> None
 
+let can_try_coerce_variant_to_primitive_opt p = 
+  match p with 
+  | None -> None 
+  | Some p -> can_try_coerce_variant_to_primitive p
+
 let variant_representation_matches (c1_attrs : Parsetree.attributes)
     (c2_attrs : Parsetree.attributes) =
   match