RelTraits (e.g. RelCollationTrait and RelDistributionTrait) fail to convert in calcite core. Convention is handled specially at the root node (with VolcanoPlanner.ensureRootConverters()), but this assumes that convention conversions are not necessary underneath the root, which may not necessarily be true. In order for RelTrait conversions to work, there is a need to have converters (via AbstractConverters which use RelTraitDef.convert()).
In a previous commit (b312031), AbstractConverters were added too aggressively. For example, even if RelTraitDef.canConvert() returns false, AbstractConverters were superfluously added. As a result, many plans would take a lot longer to plan (as pointed by 3b55c35). But removing the AbstractConverters means that RelTrait conversions are not handled properly in calcite.