Comprehensions without a generator do not desugar

[jstolarek]

Say I have a table containing agencies:

data Agency = Agency
    { a_id       :: Integer
    , a_name     :: Text
    , a_based_in :: Text
    , a_phone    :: Text
    } deriving (Show)

agencies :: Q [Agency]
agencies = table "agencies"
                 ( "a_id" :|
                 [ "a_name"
                 , "a_based_in"
                 , "a_phone"
                 ])
                 (TableHints (pure $ Key (pure "a_id") ) NonEmpty)

And I write a query that looks like this:

q2 :: Q [Text]
q2 = [ z
     | a <- agencies
     , z <- [ a_nameQ a | a_nameQ a == "EdinTours" ]
     ]

This is a well-formed query and it executes correctly. However, trying to desugar it via showDesugaredOptQ optResugar q2 leads to:

DSH: Impossible happened at ("src/Database/DSH/Translate/CL2NKL.hs",288,37)
CallStack (from HasCallStack):
  error, called at src/Database/DSH/Translate/CL2NKL.hs:288:37 in DSH-0.13.0.1-EuPUolq064bCwy8rK7h5w4:Database.DSH.Translate.CL2NKL

The problem lies in the definition of desuagrQuals that expects that every comprehension will eventually contain a BindQ qualifier - clearly not the case for a comprehension containing only a GuardQ.

I know that q2 might look a bit contrived but I've actually been hit by this issue in my work on provenance, where queries get transformed to a form that does not contain a generator, just guards.