Related: What exact dimensions does a physical cone AoE template need to have?
The rules on areas of effect for spells state (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, the description of the fireball spell states, in part:
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)?