Weird behavior trying to convert case classes to heterogeneous lists recursively with Shapeless

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This now works more or less as written using recent shapeless-2.1.0-SNAPSHOT builds, and a close relative of the sample in this question has been added there as an example.

The problem with the original is that each expansion of a Generic introduces a new HList type into the implicit resolution of the DeepHLister type class instances and, in principle, could produce an HList type that is related to but more complex than some type seen previously during the same resolution. This condition trips the divergence checker and aborts the resolution process.

The exact details of why this happens for D but not for C is lurking in the details of the implementation of Scala’s typechecker but, to a rough approximation, the differentiator is that during the resolution for C we see the B (larger) before the A (smaller) so the divergence checker is happy that our types are converging; conversely during the resolution for D we see the A (smaller) before the B (larger) so the divergence checker (conservatively) bails.

The fix for this in shapeless 2.1.0 is the recently enhanced Lazy type constructor and associated implicit macro infrastructure. This allows much more user control over divergence and supports the use of implicit resolution to construct the recursive implicit values which are crucial to the ability to automatically derive type class instances for recursive types. Many examples of this can be found in the shapeless code base, in particular the reworked type class derivation infrastructure and Scrap Your Boilerplate implementation, which no longer require dedicated macro support, but are implemented entirely in terms of the Generic and Lazy primitives. Various applications of these mechanisms can be found in the shapeless examples sub-project.

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