[GHC] #10294: Missing instances if compiling with -fplugin

13 Apr 2015

      #10294: Missing instances if compiling with -fplugin
-------------------------------------+-------------------------------------
              Reporter:  jscholl     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
  (Type checker)                     |  Operating System:  Linux
              Keywords:              |   Type of failure:  GHC rejects
          Architecture:  x86_64      |  valid program
  (amd64)                            |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 Using the example from "9.3.3.2. Using Annotations" of the GHC user manual
 does not work correctly with GHC 7.10. The following fails:

 $ ghc -c Bug1.hs -fplugin=SayAnnNames.Plugin

 {{{
 Bug1.hs:5:1:
     No instance for (Data SomeAnn) arising from an annotation
     In the annotation: {-# ANN foo SomeAnn #-}
 }}}

 It does not seem to matter why the type class is required:

 $ ghc -c Bug2.hs -fplugin=SayAnnNames.Plugin

 {{{
 Bug2.hs:10:7:
     No instance for (Data SomeAnn) arising from a use of ‘bar’
     In the expression: bar SomeAnn
     In an equation for ‘baz’: baz = bar SomeAnn
 }}}

 Compiling without -fplugin works fine, though. If SomeAnn is defined in a
 different module than SayAnnNames.Plugin and reexported from it, the
 instances are in scope, too.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10294
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