[GHC] #15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness annotation

[GHC]

#15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness
annotation
-------------------------------------+-------------------------------------
        Reporter:  jkoppel           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  Research
       Component:  Compiler (Type    |  needed
  checker)                           |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 My strong instinct here is to go back to the paper.  In rule [UConVar] and
 friends we split up 'x' into a union of constructors {K x1, K2 x2 x3, K3}.
 Each of those split constructors comes with an extended `Delta`, which
 describes the constraints in scope.  If these constraints prove
 contradicatory we can drop the entire branch.

 So for a strict constructor, say
 {{{
 data T a where
   MkT :: a -> !a -> T a
 }}}
 we want to add a constraint to `Delta` that says that the type of that
 second argument is non-void; that is, it contains at least one element
 that terminates.  So maybe we need a constraint `NonVoid( tau )`.

 In the OP, `NonVoid( Bar A )` would be a contradiction.

 In the implementation we have a simple "term oracle" that works alongside
 the type oracle that is implemented by the constraint solver.  Maybe we
 could add the new constraint to the term oracle?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15305#comment:9>
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