[GHC] #15362: Template Haskell ignores bad type family definitions

[GHC]

#15362: Template Haskell ignores bad type family definitions
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Template       |           Version:  8.4.3
  Haskell                            |
           Keywords:                 |  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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 This garbage is accepted:

 {{{#!hs
 data Nat = Zero | Succ Nat

 $( [d| type family a + b where
          Maybe Zero b = b
          Succ a + b = Succ (a + b) |] )
 }}}


 Note the `Maybe` photo-bombing in the first equation. The problem is that
 TH uses the symbol declared in the family head for all the equations,
 ignoring what's actually there. This is sad.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15362>
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