It would be a disaster of Biblical proportions between two nuclear-armed combatants with enormous manpower.
The conflict could escalate into something that makes WW2 look like a picnic.
What would China have to gain from attacking India?
Don’t worry. That’s just drama. Indian use this scheme to get US’s help and attention. It helps confirm that China is a threat and India is reliable ally. 20 years ago China played as US’s ally against USSR. Now India just replays China’s former role, but this time the common enemy is China.