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The code is:

data K = Foo
       | Bar

data D1 :: K -> * where
    F1 :: D1 Foo
    B1 :: D1 Bar

class C (a :: K -> *) where
    data D2 a :: K -> *
    foo :: a k -> D2 a k -> Bool

instance C D1 where
    data D2 D1 k where
              F2 :: D2 D1 Foo
              B2 :: D2 D1 Bar
    foo F1 F2 = True
    foo B1 B2 = True

With -Wall it gives the warning:

Test.hs:23:5: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for `foo':
        Patterns not matched:
            F1 B2
            B1 F2

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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