Java version `Inconsistent constant pool data` crash.

[supersaiyansubtlety]

A user of one of my mods gets this issue when on java 15, but not java 8 (I develop on 8):

java.lang.IncompatibleClassChangeError: Inconsistent constant pool data in classfile for class net/minecraft/class_1915. Method 'net.minecraft.class_1703 md81f247$lambda$sendOffers$0$0(int, net.minecraft.class_1661, net.minecraft.class_1657)' at index 66 is CONSTANT_MethodRef and should be CONSTANT_InterfaceMethodRef

The full crash log is commented in the issue on my repo.

Here's the mixin the crash log refers to.

[Chocohead]

This is SpongePowered#451