ghc: panic! (the 'impossible' happened)

I got:

(GHC version 8.0.2 for x86_64-unknown-linux):

initTc: unsolved constraints

WC {wc_insol = [W] spec_a86Q :: t_a86P[tau:1] (CHoleCan: spec)}

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

This happens after adding of this call in test file (called SimpleTaggingSpec.hs):

$(shrinkEnumTag AnyJT)

Implementation of this shrinkEnumTag looks:

shrinkEnumTag :: forall a. (Data a, IsTag a) => a -> Q [Dec]
shrinkEnumTag enm = do
  let dt = dataTypeOf enm
      ty = mkName $ dataTypeName dt
      tickAdd = (++"'")
      nameWith f = mkName . f . show
      derivs' = ConT . mkName <$> ["Eq", "Ord", "Enum"]
      mkShow tn tn' =
        [d|instance Show $(conT tn') where
             show a = show $ (toEnum $ fromEnum a :: $(conT tn))|]
  AlgRep algCs <- pure $ dataTypeRep dt
  let algCs' = [mkName $ show c | c <- algCs, fromConstr c /= (anyTag::a)]
      newCs = [NormalC (nameWith tickAdd n) [] | n <- algCs']
  TyConI tyCon <- reify ty
  decs <- case tyCon of
    DataD _cxt _nm tyVars _mbKind _cs _derivs -> do
      let ty' = nameWith tickAdd ty
      when (null newCs) $ fail "Can't find normal constructors"
      showInst <- mkShow ty ty'
      return $ [DataD [] ty' tyVars Nothing newCs derivs'] ++ showInst
    _ -> fail "expected Enum-like data type"
  return decs

If I add this call in not-test module, all is good, compilation is fine, application works. I use GHC 8.0.2, this happened with LTS-8.13 and LTS-9.18. Project uses stack tool. OS is Linux OpenSUSE Tumbleweed. I updated stack to the last version (1.6.1) but this did not help.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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ghc: panic! (the 'impossible' happened)