support coercing string to elgible untagged variants

jscomp/build_tests/super_errors/expected/variant_coercion_string_to_variant_no_payload.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_coercion_string_to_variant_no_payload.res:6:10-15
+
+  4 │ let x = "one"
+  5 │ 
+  6 │ let y = (x :> x)
+  7 │ 
+
+  Type string is not a subtype of x

jscomp/build_tests/super_errors/fixtures/variant_coercion_string_to_variant_no_payload.res

@@ -0,0 +1,6 @@
+@unboxed
+type x = One | Two
+
+let x = "one"
+
+let y = (x :> x)

jscomp/ml/ctype.ml

@@ -3951,6 +3951,25 @@ let rec subtype_rec env trace t1 t2 cstrs =
         end
     | (Tconstr(p1, _, _), _) when generic_private_abbrev env p1 ->
         subtype_rec env trace (expand_abbrev_opt env t1) t2 cstrs
+    | (Tconstr(path, [], _), Tconstr(_, [], _)) when Path.same path Predef.path_string && 
+        extract_concrete_typedecl env t2 |> Variant_coercion.can_try_coerce_variant_to_primitive |> Option.is_some
+        ->
+      (* type coercion for strings to elgible unboxed variants:
+         - must be unboxed
+         - must coercable to string
+         - must have a constructor case with a string payload *)
+      (match Variant_coercion.can_try_coerce_variant_to_primitive (extract_concrete_typedecl env t2) with
+      | Some (constructors, true) -> 
+        if constructors |> Variant_coercion.can_coerce_variant ~path ~unboxed:true 
+          && constructors |> List.exists(fun (c: constructor_declaration) -> 
+            match c.cd_args with 
+            | Cstr_tuple [{desc=Tconstr (p, [], _)}] when Path.same p Predef.path_string -> true 
+            | _ -> false) 
+        then
+          cstrs
+        else 
+          (trace, t1, t2, !univar_pairs)::cstrs
+      | _ -> (trace, t1, t2, !univar_pairs)::cstrs)
     | (Tconstr(_, [], _), Tconstr(path, [], _)) when Variant_coercion.can_coerce_path path && 
         extract_concrete_typedecl env t1 |> Variant_coercion.can_try_coerce_variant_to_primitive |> Option.is_some
         ->

jscomp/test/VariantCoercion.res

@@ -44,3 +44,11 @@ module CoerceWithPayload = {
   let d: float = (c :> float)
   let dd: float = (cc :> float)
 }
+
+module CoerceFromStringToVariant = {
+  @unboxed type strings = String(string) | First | Second | Third
+  let a = "hello"
+  let aa = "First"
+  let b: strings = (a :> strings)
+  let bb: strings = (aa :> strings)
+}