case expression with mixed use of Num instances cause spurious overlap warning (#8016) · Issues · Glasgow Haskell Compiler / GHC

case expression with mixed use of Num instances cause spurious overlap warning

newtype Whomp = Whomp Int
  deriving (Eq)

instance Num Whomp where
  Whomp a + Whomp b = Whomp (a+b)
  Whomp a * Whomp b = Whomp (a*b)
  abs (Whomp a) = Whomp (abs a)
  signum (Whomp a) = Whomp (signum a)
  fromInteger = Whomp . fromInteger

foo :: Whomp -> String
foo cc = case cc of
    Whomp 0 -> "zero"
    10 -> "ten"

produces

Foo.hs:12:10: Warning:
    Pattern match(es) are overlapped
    In a case alternative: Whomp 0 -> ...

Trac metadata

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