شَرِبْتُ الْكَأْسَ.
I drank the contents of the cup.
In ordinary usage one drinks the liquid, not the vessel. “Al-kaʾs” transfers from container to contents, a relationship of containment rather than resemblance.
A full explanation names both relationship and cue: the relation is container–contents, and the act of drinking favours the contents over the physical vessel. Calling it an ordinary simile-based metaphor misses the mechanism.