[GHC] #4955: increase error message detail for module lookups failure due to hi references

[GHC]

#4955: increase error message detail for module lookups failure due to hi
references
---------------------------------+------------------------------------------
    Reporter:  duncan            |       Owner:              
        Type:  bug               |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.1            |    Keywords:              
    Testcase:                    |   Blockedby:              
          Os:  Unknown/Multiple  |    Blocking:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------

Comment(by duncan):

 Replying to [ticket:4955 duncan]:

 > Clearly there is a deeper problem here, that a registered package is
 out-of-sync with its registration information: the .hi refers to a package
 that is not mentioned in the package registration.
 >
 > One way this could happen is due to inplace registrations, since it is
 easy in that case to rebuild the inplace .hi files without also re-
 registering. It may be sensible to rethink how Cabal does inplace
 registrations to prevent it getting out of sync.

 The user reports that one other possibility is that a dependency was
 `cabal copy`ied but not re-registered.

 The suggestion is that `cabal copy` could refuse to overwrite existing .hi
 files. So `copy` can be used to copy into a fresh temp dir, and
 subsequently registered, but not used to overwrite an existing installed
 package. For that the copy + register need to be done together.

 Filed as http://hackage.haskell.org/trac/hackage/ticket/802

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4955#comment:1>
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