Related: What exact dimensions does a physical cone AoE template need to have?
From PHB, p.204;
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover
However, from the text of the fireball spell,
The fire spreads around corners.
In the case of someone hiding around a corner, I would personally rule that they have total cover. Does this mean that the flame from a fireball will ignore total cover? Or does it only apply to one specific condition under which a creature can have total cover from another (for instance, if the creature was inside of a welded steel box)?