Allow annotated field with JsonKey.unknownEnumValue to be of type Map

[Wesleychung93]

At present, the #585 and PR #691 allows us to annotate unknownEnumValue fields of type enum or on Iterable, List, or Set instances of an enum type. Is is possible to support Map ?

Example:

@JsonKey(unknownEnumValue: YourEnum.yourUnkownValue)
final Map<YourEnum, num> minWithdrawalAmount;

This is currently not possible. Enclosed error shown in terminal when generate the code

Error with `@JsonKey` on the `minWithdrawalAmount` field. `unknownEnumValue` can only be set on fields of type enum or on Iterable, List, or Set instances of an enum type.
package:yourfile.dart:19:22
   ╷
19 │   Map<YourEnum, num> minWithdrawalAmount;
   │                      ^^^^^^^^^^^^^^^^^^^
   ╵

[Yetispapa]

Same issue here

[t-beckmann]

The unknownEnumValue could apply to the key or value of a Map.

For the value the check can be passed when adding a case near https://github.com/google/json_serializable.dart/blob/master/json_serializable/lib/src/json_key_utils.dart#L164 like so:

        late DartType targetEnumType;
        if (element.type.isEnum) {
          targetEnumType = element.type;
        } else if (coreIterableTypeChecker.isAssignableFromType(element.type)) {
          targetEnumType = coreIterableGenericType(element.type);
        } else if (coreMapTypeChecker.isAssignableFromType(element.type)) {
          // Map handled here, index 1 denotes value type
          targetEnumType = element.type.typeArgumentsOf(coreMapTypeChecker)![1];
        } else {
          throwUnsupported(
            element,
            '`$fieldName` can only be set on fields of type enum or on '
            'Iterable, List, Set or Map instances of an enum type.',
          );
        }

The code generated then already properly passes the given unknownEnumValue to $enumDecode as the unknownValue without further changes, I tried.

For the key one way would be implementing a JsonKey.unknownEnumKey in a similar fashion, and some change to https://github.com/google/json_serializable.dart/blob/master/json_serializable/lib/src/type_helpers/map_helper.dart#L115. This however is less useful as such default key(s) could result in duplicate entries so that it's presence merely indicates such unknown key was encountered.