[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
            Resolution:              |         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:              |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 This is awkward.  GHC currently typechecks the source file, and then
 compares with the hs-boot file for consistency.  And indeed, we ''must''
 do that, because until we have done at least some typechecking we don't
 have any type from the source file to compare with those in the hs-boot
 file.

 I suppose that the difficulty is that the very act of typechecking the
 source file leads to an unexpected situation and a crash.  I have not
 worked out the details.

 I'm not sure how to solve this.  One route (which seems like the right
 one) might be to make the check ''whenever extending the type
 environment''; that is, as soon as we have got to the point of adding a
 data type T, or function f, to the environment, at that moment check
 compatibility.

 Probably not too hard, but a bit fiddly, and I'm definitely not going to
 have time to do it soon.  And yet it's bad that GHC actually crashes.  It
 should never do that.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9450#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler