Article 1 of the Indian Constitution states that the territory of India shall consist of: the territories of the States, the Union Territories and any territory that may be acquired.
It also includes Jammu and Kashmir and says that Indian territory also consists of the Indian state of Jammu and Kashmir which existed immediately prior to the commencement of the constitution i.e. 26th January, 1950.
But at this time the North West portion of the state was already occupied by Pakistan. So according to the Article 1 of the Indian constitution, India cannot claim the Pakistan occupied Kashmir as its own territory.
Here the Constitution of Jammu & Kashmir comes into the picture. The constituent assembly of Jammu & Kashmir in the year 1956 gave its own constitution. And it unambiguously states that the state of Jammu and Kashmir which existed on 15th August 1947 is an integral part of India.
On 5th August 2019, the current government abrogated Article 370 of the Indian Constitution which used to give legitimacy to the Constitution of Jammu and Kashmir.
So now the Constitution of J&K is discarded which made the entire territory of J&K an integral part of India including PoK and Aksai chin.
Pay attention to the fact that Article 1 of the Indian constitution states that only that part of J&K belongs to India which existed at the time of the commencement of the constitution. And that does not include PoK.
As of today, since the Constitution of J&K has been discarded and rendered ineffective, there is no territorial definition of entire J&K anywhere.
This has basically given China an opening to interfere. If tomorrow both India and China come on the negotiating table and India would obviously claim the entire state of J&K as its own, China would ask for proof. And since the Constitution of J&K which solely gave the territorial definition has been thrown into the dustbin, we have no proof.
Remember the Home Minister said in Parliament that he would give his life for Aksai Chin. May be it is time.
Also remember the smug and disgusting Celebration of the 370 Abrogation all over India.