[GHC] #11793: Confusing type error from constrained class method

[GHC]

#11793: Confusing type error from constrained class method
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           Reporter:  gridaphobe     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc2
           Keywords:  error-message  |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 import GHC.Stack

 class Foo a where
   foo :: HasCallStack => a -> String

 instance Foo () where
   foo () = prettyCallStack callStack

 main = putStrLn (foo ())
 }}}

 {{{
 ~/S/ghc> ./inplace/bin/ghc-stage2 --make Foo.hs
 [1 of 1] Compiling Main             ( Foo.hs, Foo.o )

 Foo.hs:4:3: error:
     • Constraint ‘HasCallStack’ in the type of ‘foo’
         constrains only the class type variables
       Use ConstrainedClassMethods to allow it
     • When checking the class method:
         foo :: forall a. (Foo a, HasCallStack) => a -> String
       In the class declaration for ‘Foo’
 }}}

 The error says that `HasCallStack` constrains the class type variable `a`,
 which is incorrect. Is the error message supposed to be that
 `HasCallStack` '''should''' constrain the class type variable and nothing
 else?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11793>
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