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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?

Trac metadata

Trac field	Value
Version	8.0.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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