Generally a failed gift in the body of a will, left absolute to one person, falls into the residuary clause of the Will (if any).
However, if X is a issue of the testator, the Wills Act in your country may have provisions speaking to testator children pre-deceasing testator leaving issue who survive the testator- in which even, may save the gift from lapsing (in that, it would pass to the issue directly or via probate depending on how the child of the testator died (intestate or testator).
Kedia Delahaye