unsafe coerce without extensions

The following program implements an unsafe coerce and works on versions of GHC at least as far back as 7.0.3 but is fixed in HEAD. I'm reporting this because I don't know if it was fixed on purpose and might merit at least a fix to the 7.6.X branch if not the 7.4.X branch (due to its use in the platform)

newtype Id a = Id { runId :: a }

-- Providing this type triggers a type-error
-- cast :: a -> b
cast v = runId z
  where
  z :: Id v
  z = aux
    where
    aux = Id v

Example usage:

*Main> map cast [97..107 :: Int] :: String
"abcdefghijk"

*Main> cast (Just True) :: String
"\-1152921504589708267Segmentation fault: 11

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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