Blacklist error, undefined, unsafeCoerce, and unsafeCoerce# from valid hole fits

Motivation

Let's say I'm writing some code and I remember that there a function that converts Word8# to Word#, but I cannot remember what it is:

import GHC.Exts

index# :: ByteArray# -> Int# -> Word8#
index# a i = _ (indexWord8Array# a i)

GHC makes several recommendations:

    • Found hole: _ :: Word# -> Word8#
    ...
    • Relevant bindings include
      ...
      Valid hole fits include
        narrowWord8# :: Word# -> Word8#
          (imported from ‘GHC.Exts’ at src-imp/Word8.hs:10:1-15
           (and originally defined in ‘ghc-prim-0.5.3:GHC.Prim’))
        magicDict :: forall a. a
          with magicDict @(Word# -> Word8#)
          (imported from ‘GHC.Exts’ at src-imp/Word8.hs:10:1-15
           (and originally defined in ‘ghc-prim-0.5.3:GHC.Prim’))
        unsafeCoerce# :: forall a b. a -> b
          with unsafeCoerce# @'WordRep @'Word8Rep @Word# @Word8#
          (imported from ‘GHC.Exts’ at src-imp/Word8.hs:10:1-15
           (and originally defined in ‘ghc-prim-0.5.3:GHC.Prim’))

GHC telling me that unsafeCoerce# typechecks as a fit for a hole is not helpful. I've seen this happen with other functions too. Namely: error, undefined, errorWithoutStackTrace, unsafeCoerce. Also, in this example, magicDict shows up. I don't what that is, but it looks like something that shouldn't be recommended.

Proposal

Have a blacklist for valid hole fits. This wouldn't always shoot down everything worthless (it could only target known-key stuff), but it could clean up some of these unhelpful suggestions that GHC makes fairly often.