[bug] Creating Custom Biome Modifiers: example code throws error

[Bittorn]

For the record, I am using NeoForge 21.1.82 for MC 1.21.1 for my project, but I have tested in the latest 21.4.88-beta and it throws the same error.

The relevant code:

RecordCodecBuilder.mapCodec(instance -> instance.group(
                        Biome.LIST_CODEC.fieldOf("biomes").forGetter(ExampleBiomeModifier::biomes),
                        Codec.INT.fieldOf("value").forGetter(ExampleBiomeModifier::value)
                ).apply(instance, ExampleBiomeModifier::new)

Throws this error:

      Required type                                                  Provided
t1: App<Mu<BiomeModifier>, T1>                     RecordCodecBuilder<ExampleBiomeModifier,HolderSet<Biome>>
t2: App<Mu<BiomeModifier>, T2>                     RecordCodecBuilder<ExampleBiomeModifier,Integer>
reason: no instance(s) of type variable(s) exist so that BiomeModifier conforms to ExampleBiomeModifier

I'm pretty new to Minecraft modding so this may be my issue, but I have copied the example code exactly and even tried modifying it to fix it myself and yet no luck. This might be a holdover from NeoForge 1.20 or maybe even older Forge implementation.

[ChampionAsh5357]

So, this is not an issue with the codec itself, but rather with the ambiguous generic being set. This can be fixed by holding an instance of Supplier<ExampleBiomeModifier> instead of the holder for the constant. The docs should probably be updated to use suppliers or the deferred holder objects by default, leaving holders only when absolutely necessary.