A is accused of the murder of B. At his trial, A sets up that he had received grave and sudden…

Section 2(1)(g) treats as a fact in issue any fact from which the existence or extent of a liability necessarily follows, and the illustrations to that clause list, for a trial of A for the murder of B, that A caused B's death, that A intended it, that A had received grave and sudden provocation from B, and that A was of unsound mind. So provocation is "A fact that may be in issue at A's trial". It is a fact, not a question of law, and it bears on the liability itself, not merely on what follows a conviction.