Compaction fails when using compact index keys as container keys

[niklasl]

In the JSON-LD 1.1 Compaction algorithm, step 12.8.9.6.1 currently reads "Reinitialize container key by IRI compacting index key". This does not always work, since the index key may already be compacted at this point.

In fact, compaction test 0112 ("Compact property index using Compact IRI index") fails here if implemented verbatim, since IRI Compaction step 9 detects an IRI confused with prefix error on the index key ex:name as defined in the context.

To fix this, a condition can be added to step 12.8.9.6.1, to only compact the index key if it is not already compacted; along the lines of: "Reinitialize container key to the value of index key. If the container key expands to itself, reinitialize it by IRI compacting index key." (Note that this is similar to the check in step 12.8.9.8.4.)

(For context, note also that step 20.2 of the Create Term Definition algorithm says "Initialize index to the value associated with the @index entry", which means its exact value, not its expansion. It even elaborates on that with "If the result of IRI expanding that value is not an IRI, an invalid term definition has been detected and processing is aborted.". Compaction tests 0113 and 0114 also rely on this exact behavior.)

[niklasl]

Implementations passing compaction test 0112 appear to have implemented these details differently. I discussed this with @gkellogg last year (in relation to compaction test 0114) and he came to similar conclusions. Alas, we didn't get to the bottom of it back then.