I had a similar initial reaction when I first saw the pitch, but I realized the logical basis for this feature is pretty simple and sound. We can probably polish the pitch to add some more details. Also, the implementation of rethrows conformances, together with 'reaync' functions, was based on a refactoring of this part of the type checker to deal with effects abstractly, and directly express effect polymorphism. The intended goal was to make it easier to add more kinds of composable effects in the future.
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