[GHC] #15247: Use empty types for TTG extension constructors

[GHC]

#15247: Use empty types for TTG extension constructors
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           Reporter:  adamgundry     |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:  TTG            |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I happened to be looking at `AmbiguousFieldOcc` and noticed that it has
 gained an `XAmbiguousFieldOcc` extension constructor for Trees That Grow,
 but lots of the destructor functions in `HsTypes` panic if they see this
 constructor (which is understandable, because they aren't expecting
 extensions).

 In general, perhaps it should be the case that for concrete phase
 descriptors (e.g. `GhcRn`) where extension constructors are not expected,
 the extension constructor argument type should be empty? It would then be
 clear that they should not be expected (barring abuse of laziness) and
 pattern matching on them could eliminate the empty type.

 That is, instead of the existing
 {{{#!hs
 data NoExt = NoExt

 type instance XXAmbiguousFieldOcc (GhcPass _) = NoExt
 }}}
 we would define
 {{{#!hs
 data NoExtConstructor -- empty data type

 noExtConstructor :: NoExtConstructor -> a
 noExtConstructor x = case x of {}

 type instance XXAmbiguousFieldOcc (GhcPass _) = NoExtConstructor
 }}}
 and similarly for other `XX...` type families.

 Alan, Shayan, is there any reason this couldn't work?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15247>
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