[GHC] #13272: DeriveAnyClass regression involving a rigid type variable

[GHC]

#13272: DeriveAnyClass regression involving a rigid type variable
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  Generics       |  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:
-------------------------------------+-------------------------------------
 I was writing up an example to show off how `DeriveAnyClass` has improved
 since 639e702b6129f501c539b158b982ed8489e3d09c, and wouldn't you know it,
 the example doesn't actually compile anymore post-
 639e702b6129f501c539b158b982ed8489e3d09c. Oopsie.

 {{{#!hs
 module TypeName where

 import GHC.Generics

 class TypeName a where
   typeName         :: forall proxy.
                       proxy a -> String
   default typeName :: forall proxy d f.
                       (Generic a, Rep a ~ D1 d f, Datatype d)
                    => proxy a -> String
   typeName _ = gtypeName $ from (undefined :: a)

 gtypeName :: Datatype d => D1 d f p -> String
 gtypeName = datatypeName

 data T a = MkT a
   deriving (Generic, TypeName)
 }}}

 This compiles before that commit. After it, however, it fails with the
 error:

 {{{
 [1 of 1] Compiling TypeName         ( Bug.hs, interpreted )

 Bug.hs:23:22: error:
     • Couldn't match type ‘f’
                      with ‘C1
                              ('MetaCons "MkT" 'PrefixI 'False)
                              (S1
                                 ('MetaSel
                                    'Nothing 'NoSourceUnpackedness
 'NoSourceStrictness 'DecidedLazy)
                                 (Rec0 a))’
         arising from the 'deriving' clause of a data type declaration
       ‘f’ is a rigid type variable bound by
         the deriving clause for ‘TypeName (T a)’ at Bug.hs:14:38
     • When deriving the instance for (TypeName (T a))
    |
 23 |   deriving (Generic, TypeName)
    |                      ^^^^^^^^
 }}}

 I'm not sure why it's complaining only about `f` and not, say, `d`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13272>
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