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So far, the only way to overcome this situation and produce a correct result was the "infinitesimal gap" that is explained in this page: Tips and Tricks: Liquid in a glass.
This method involves the liquid body being completely enclosed in the glass body, which doesn't have inner walls where the liquid exists. This way there is only one single wall, instead of two, to define the limit area between the glass and the liquid medium. The method also involves a third material - glass exit material - over the liquid upper surface to provide the correct order for the rays exiting the glass medium in certain directions.
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In render time, the overlapping liquid volume gets automatically removed, leaving one single face and the materials properly assigned with respect to each ray direction (right)
What if we add some ice cubes?
If we want to throw some ice cubes into the liquid, we will end up with the a similar situation: inside the ice cubes' volume there exists at first two different materials coexisting (liquid and ice), which is not what would happen in the real world, where only ice fills that space. In this case we need to assign a higher priority (lower value) to the ice and a lower priority (higher value) to the liquid.
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