You can already do this as long as you set the compatibility level to JAVA_8 or higher.

Can you please give a example of how to do this?
I tried:
Interface:

package test;

public interface TestInterface {

	public double getValue();

	public default boolean greaterThan(double b) {
		double a = getValue();
		boolean c = a > b;
		return c;
	}

}

Mixin Class:

package mixin.test;

import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.gen.Invoker;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;

import test.TestInterface;

@Mixin(TestInterface.class)
public interface MixinTestInterface {

	@Invoker("getValue")
	public abstract double getValue();

	@Inject(at = @At(value = "HEAD"), method = "greaterThan(D)Z", cancellable = true)
	public default void greaterThan(double b, CallbackInfoReturnable<Boolean> cir) {
		double a = getValue();
		boolean c = a < b;
		cir.setReturnValue(c);
	}

}

And when using the interface in code i get a exception and this message:
Mixin apply failed klori-client.mixins.json:test.MixinTestInterface -> test.TestInterface: org.spongepowered.asm.mixin.transformer.throwables.InvalidInterfaceMixinException klori-client.mixins.json:test.MixinTestInterface->@Inject::greaterThan(DLorg/spongepowered/asm/mixin/injection/callback/CallbackInfoReturnable;)V is not supported on interface mixin method handler$zgn000$greaterThan [ -> PREINJECT Applicator Phase -> klori-client.mixins.json:test.MixinTestInterface -> Prepare Injections]

I am using compatibility level: JAVA_16

How i used the interface:
Implementation Class:

package test;

public class TestImplementation implements TestInterface {

	public final double value;

	public TestImplementation(double value) {
		this.value = value;
	}

	@Override
	public double getValue() {
		return value;
	}

}

test code:

	TestInterface ti = new TestImplementation(100);
	System.out.println("greaterThan: " + ti.greaterThan(10));

Uh oh!

[Suggestion] Allow interface mixins for default methods #421

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