Inherited synthetic bridge methods throw an exception even if not using them

[BluSpring]

I'm not actually sure if that's a correct title to explain exactly what's happening, but basically, simply doing this (Minecraft 1.21.1, Mojmap):

@Mixin(MushroomCow.class)
public abstract class MushroomCowMixin extends Cow {
    public MushroomCowMixin(EntityType<? extends Cow> entityType, Level level) {
        super(entityType, level);
    }
}

causes a crash where the synthetic bridge method created in MushroomCow from Cow is considered invalid by mixin.

Full stack trace:

java.lang.BootstrapMethodError: java.lang.RuntimeException: Mixin transformation of net.minecraft.world.entity.animal.MushroomCow failed
	at knot//net.minecraft.world.entity.EntityType.<clinit>(EntityType.java:256)
	at knot//net.minecraft.world.item.Items.<clinit>(Items.java:854)
	at knot//net.minecraft.world.level.block.LightBlock.getShape(LightBlock.java:64)
	at knot//net.minecraft.world.level.block.state.BlockBehaviour$BlockStateBase.getShape(BlockBehaviour.java:1058)
	at knot//net.minecraft.world.level.block.state.BlockBehaviour$BlockStateBase.getShape(BlockBehaviour.java:1054)
	at knot//net.minecraft.world.level.block.state.BlockBehaviour.getCollisionShape(BlockBehaviour.java:355)
	at knot//net.minecraft.world.level.block.state.BlockBehaviour$BlockStateBase$Cache.<init>(BlockBehaviour.java:1341)
	at knot//net.minecraft.world.level.block.state.BlockBehaviour$BlockStateBase.initCache(BlockBehaviour.java:869)
	at knot//net.minecraft.world.level.block.Blocks.<clinit>(Blocks.java:1259)
	at knot//net.minecraft.world.level.block.FireBlock.bootStrap(FireBlock.java:336)
	at knot//net.minecraft.server.Bootstrap.bootStrap(Bootstrap.java:52)
	at knot//net.minecraft.client.main.Main.main(Main.java:130)
	at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:514)
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:72)
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
	at net.fabricmc.devlaunchinjector.Main.main(Main.java:86)
Caused by: java.lang.RuntimeException: Mixin transformation of net.minecraft.world.entity.animal.MushroomCow failed
	at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:440)
	at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:336)
	at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:231)
	at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
	... 16 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:381)
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:237)
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
	at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:435)
	... 20 more
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [mushroomcowtests.mixins.json:MushroomCowMixin from mod mushroomcowtests] from phase [DEFAULT] in config [mushroomcowtests.mixins.json] FAILED during APPLY
	at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinError(MixinProcessor.java:686)
	at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinApplyError(MixinProcessor.java:637)
	at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:368)
	... 23 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.InvalidMixinException: Conflicting synthetic bridge target method descriptor in synthetic bridge method getBreedOffspring(Lnet/minecraft/server/level/ServerLevel;Lnet/minecraft/world/entity/AgeableMob;)Lnet/minecraft/world/entity/AgeableMob; Existing:(Lnet/minecraft/server/level/ServerLevel;Lnet/minecraft/world/entity/AgeableMob;)Lnet/minecraft/world/entity/animal/MushroomCow; Incoming:(Lnet/minecraft/server/level/ServerLevel;Lnet/minecraft/world/entity/AgeableMob;)Lnet/minecraft/world/entity/animal/Cow;
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attachUniqueMethod(MixinPreProcessorStandard.java:564)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attachMethods(MixinPreProcessorStandard.java:351)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:299)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.createContextFor(MixinPreProcessorStandard.java:277)
	at org.spongepowered.asm.mixin.transformer.MixinInfo.createContextFor(MixinInfo.java:1291)
	at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:203)
	at org.spongepowered.asm.mixin.transformer.TargetClassContext.apply(TargetClassContext.java:437)
	at org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:418)

[MildestToucan]

Workaround:

It seems to me in my tests that shadowing the MushroomCow getBreedOffspring(ServerLevel serverlevel, AgeableMob ageableMob) method in your mixin or not extending Cow gets around the issue. The issue otherwise shows up when mixing into the same target class (using Fabric Loader 1.18.4, latest at the time of speaking, which bundles Fabric Mixin 0.17.0+mixin.0.8.7)

Findings from replicating issue

The following log is produced from replicating the same mixin class as OP: debug.log

Using -Dmixin.dumpTargetOnFailure=true, the following methods are found in the decompiled target class file:

The one returning a Cow matches the signature of the one described as "Incoming" in the error logs.

As mentioned in the workaround section, shadowing MushroomCow's getBreedOffSpring seems to allow the mixin class to work without causing the same crash.