These are the problematic ones I've found:

> :t foldl
foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b
> :print foldl
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	isUnliftedType
  t1_a1H7[rt] :: TYPE t_a1H6[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1952:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
> :t fmap
fmap :: Functor f => (a -> b) -> f a -> f b
> :print fmap
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	isUnliftedType
  t1_a1Hu[rt] :: TYPE t_a1Ht[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1952:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

> :t return
return :: Monad m => a -> m a
> :print return
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	isUnliftedType
  t1_a1Wt[rt] :: TYPE t_a1Ws[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1952:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

> :t pure
pure :: Applicative f => a -> f a
> :print pure
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	isUnliftedType
  t1_a1WP[rt] :: TYPE t_a1WO[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1952:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

These that don't have constraints are fine:

> :print id
id = (_t1::a -> a)
> :print map
map = (_t2::(a1 -> b) -> [a1] -> [b])
> :print const
const = (_t3::a2 -> b1 -> a2)

This value and function created in the session are fine:

> let x :: Monad m => m (); x = return ()
Prelude| 
> :p x
x = (_t5::Monad m1 => m1 ())
> let f :: Monad m => Int -> m Int; f n = return (n + 1)
Prelude| 
> :p f
f = (_t6::Monad m2 => Int -> m2 Int)

mempty and mappend imported from Data.Monoid are fine:

> :m + Data.Monoid
> :p mempty
mempty = (_t7::Monoid a4 => a4)
> :p mappend
mappend = (_t8::Monoid a4 => a4 -> a4 -> a4)

foldl' and foldr imported from Data.List are not fine:

> :m + Data.List
> :p foldl'
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	isUnliftedType
  t1_a6Qy[rt] :: TYPE t_a6Qx[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1952:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

> :p Data.List.foldr
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	isUnliftedType
  t2_a6RG[rt] :: TYPE t1_a6RF[rt]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1952:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

but all and init from that same module are fine:

> :p Data.List.all
all = (_t13::Foldable t1 => (a22 -> Bool) -> t1 a22 -> Bool)
> :p Data.List.init
init = (_t14::[a29] -> [a29])

So, in a given module, among functions with class constraints, some have the error and others don't. I haven't found a value that's not a function with the error, and I haven't found a function with no class constraints exhibit the error.