Hitting invulnerable guardians causes thorns damage

The bug

When you hit an invulnerable guardian in survival, you will still get the thorns damage, even though you didn't even deal damage to it. Punching an invulnerable mob with armor enchanted with thorns won't do that.

How to reproduce

1. Summon an invulnerable guardian

   /summon guardian ~ ~ ~ {Invulnerable:1b,NoAI:1b}
   

2. Switch to Survival mode

   /gamemode survival
   

3. Punch it

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.entity.monster.EntityGuardian.attackEntityFrom(DamageSource, float) attacks the damaging entity regardless of if the parent method successfully dealt damage.