[GHC] #13506: Spurious extra error message due to functional dependencies

[GHC]

#13506: Spurious extra error message due to functional dependencies
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        Reporter:  gelisam           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 This error message has had an interesting history. In GHC 7.4.2 and
 earlier, you get the error message that you desire:

 {{{
 GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:16:12:
     Couldn't match type `Char' with `()'
     When using functional dependencies to combine
       FunDep [a] a,
         arising from the dependency `lista -> a'
         in the instance declaration at Bug.hs:5:10
       FunDep [Char] (),
         arising from a use of `singleton' at Bug.hs:16:12-20
     In the expression: singleton ()
     In an equation for `illTyped': illTyped = singleton ()
 Failed, modules loaded: none.
 }}}

 In GHC 7.6.3, it changed to the current behavior:

 {{{
 GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:16:12:
     Couldn't match type `Char' with `()'
     When using functional dependencies to combine
       FunDep [a] a,
         arising from the dependency `lista -> a'
         in the instance declaration at Bug.hs:5:10
       FunDep [Char] (),
         arising from a use of `singleton' at Bug.hs:16:12-20
     In the expression: singleton ()
     In an equation for `illTyped': illTyped = singleton ()

 Bug.hs:24:13:
     Couldn't match type `()' with `Char'
     When using functional dependencies to combine
       FunDep [Char] (),
         arising from a use of `singleton' at Bug.hs:16:12-20
       FunDep [Char] Char,
         arising from a use of `singleton' at Bug.hs:24:13-21
     In the expression: singleton 'a'
     In an equation for `wellTyped': wellTyped = singleton 'a'
 }}}

 But in GHC 7.8.4, it was briefly fixed again!

 {{{
 GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:16:12:
     No instance for (FunDep [Char] ())
       arising from a use of ‘singleton’
     In the expression: singleton ()
     In an equation for ‘illTyped’: illTyped = singleton ()
 }}}

 But in GHC 7.10.3 and later, we get the current error message again.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13506#comment:1>
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