[GHC] #9450: GHC instantiates Data instances before checking hs-boot files

[GHC]

#9450: GHC instantiates Data instances before checking hs-boot files
-------------------------------------+-------------------------------------
       Reporter:  alanz              |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:  boot,deriving      |        Operating System:
   Architecture:  x86_64 (amd64)     |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Compile-
     Blocked By:                     |  time crash
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 When compiling a file with deriving instances in it and making use of boot
 files, it appears that the derivation is attempted before the boot files
 are checked for consistency.

 This can lead to GHC panics, as when compiling GHC after this commit
 https://github.com/alanz/ghc/commit/c73182ca7345f0debba47d0b17a907bcac27c41f

 In the attached files, if you try to load HsLit.lhs into ghci it will
 complain about the missing Data instance, not about the boot file
 inconsistency. If you remove the Data from the deriving clause of
 HsOverLit, it complains about the boot file inconsistency.

 I have not managed to reproduce the original panic in this stripped down
 environment, but if it is needed to more fully understand the problem I
 can try again to do that.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9450>
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