Support nullary variants.

Author: cristianoc

compiler/ml/ast_untagged_variants.ml

@@ -97,6 +97,13 @@ let block_type_can_be_undefined = function
     false
   | UnknownType -> true
 
+let tag_can_be_undefined tag =
+  match tag.tag_type with
+  | None -> false
+  | Some (String _ | Int _ | Float _ | BigInt _ | Bool _ | Null) -> false
+  | Some (Untagged block_type) -> block_type_can_be_undefined block_type
+  | Some Undefined -> true
+
 let has_untagged (attrs : Parsetree.attributes) =
   Ext_list.exists attrs (function {txt}, _ -> txt = untagged)
 

compiler/ml/parmatch.ml

@@ -551,6 +551,11 @@ let all_record_args lbls =
             in
             match cdecl with
             | None -> x
+            | Some cstr
+              when Ast_untagged_variants.is_nullary_variant cstr.cd_args ->
+              let _, tag = Ast_untagged_variants.get_cstr_loc_tag cstr in
+              if Ast_untagged_variants.tag_can_be_undefined tag then x
+              else (id, lbl, pat_construct)
             | Some cstr -> (
               match
                 Ast_untagged_variants.get_block_type ~env:pat.pat_env cstr

tests/tests/src/pattern_match_json.mjs

@@ -30,8 +30,8 @@ function decodeGroup(group) {
 }
 
 function decodeNull(x) {
-  let match = x.field;
-  if (match !== undefined && match === null) {
+  let tmp = x.field;
+  if (tmp === null) {
     return "yes it's null";
   } else {
     return "no";