[GHC] #11348: Local open type families instances ignored during type checking

[GHC]

#11348: Local open type families instances ignored during type checking
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        Reporter:  alexvieth         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by alexvieth):

 > Yes, exactly. That is much simpler than "meta-nodes", and had the great
 merit of being correct (see third bullet).

 Agreed. My graph approach was all wrong, so I've ditched it.

 I think it would be ideal if we didn't have to plumb the `FreeVars` past
 `rnSrcDecls` and onto `tcTyAndClassDecls`, so I've come up with a solution
 which does essentially what you've described but during dependency
 analysis.

 1. Compute the SCCs for `TyClDecl`s as we do now in HEAD.
 2. For all of the `InstDecl`s (I includes class and data family instances.
 Any harm in doing so?), associate it with its `FreeVars`, intersected with
 the set of `Name`s of `TyClDecl`s that we just analysed.
 3. Fold the list of SCCs, at each step extracting the set of `InstDecl`s
 for which its `FreeVars` is empty, and then eliminating all of the `Name`s
 found in that SCC. That set of `InstDecl`s, if non-empty, comes before the
 current component in the output list.

 Will upload the patch in a few seconds.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11348#comment:8>
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