[GHC] #15796: Core Lint error with invalid newtype declaration

[GHC]

#15796: Core Lint error with invalid newtype declaration
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.6.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I'm pretty sure I know what's happening:

 * The newtype declaration is bogus. It kind-checks, but then fails the
 validity check.

 * If a declaration fails a validity check, we manufacture a "recovery
 tycon", which is a `TcTyCon`. See `Note [Recover from validity error]` in
 TcTyClsDecls.

 * When we create a type family instance, we (optionally) do a quick core-
 lint check on it, as there's no other time we ever do so.

 * Core lint falls over when it sees a `TcTyCon`, which should never make
 it past the type checker.

 I don't want to remove this last check. And I don't want to abolish
 recovery tycons, as they produce nice error messages. Maybe we tell core-
 lint (in a new parameter) that it's being called in the type checker?
 Maybe we skip this check if there are errors? Yes, that seems sensible.
 Patch incoming.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15796#comment:6>
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