Track type variable scope more carefully.

The main job of this commit is to track more accurately the scope
of tyvars introduced by user-written foralls. For example, it would
be to have something like this:

  forall a. Int -> (forall k (b :: k). Proxy '[a, b]) -> Bool

In that type, a's kind must be k, but k isn't in scope. We had a
terrible way of doing this before (not worth repeating or describing
here, but see the old tcImplicitTKBndrs and friends), but now
we have a principled approach: make an Implication when kind-checking
a forall. Doing so then hooks into the existing machinery for
preventing skolem-escape, performing floating, etc. This also means
that we bump the TcLevel whenever going into a forall.

The new behavior is done in TcHsType.scopeTyVars, but see also
TcHsType.tc{Im,Ex}plicitTKBndrs, which have undergone significant
rewriting. There are several Notes near there to guide you. Of
particular interest there is that Implication constraints can now
have skolems that are out of order; this situation is reported in
TcErrors.

A major consequence of this is a slightly tweaked process for type-
checking type declarations. The new Note [Use SigTvs in kind-checking
pass] in TcTyClsDecls lays it out.

The error message for dependent/should_fail/TypeSkolEscape has become
noticeably worse. However, this is because the code in TcErrors goes to
some length to preserve pre-8.0 error messages for kind errors. It's time
to rip off that plaster and get rid of much of the kind-error-specific
error messages. I tried this, and doing so led to a lovely error message
for TypeSkolEscape. So: I'm accepting the error message quality regression
for now, but will open up a new ticket to fix it, along with a larger
error-message improvement I've been pondering. This applies also to
dependent/should_fail/{BadTelescope2,T14066,T14066e}, polykinds/T11142.

Other minor changes:
 - isUnliftedTypeKind didn't look for tuples and sums. It does now.

 - check_type used check_arg_type on both sides of an AppTy. But the left
   side of an AppTy isn't an arg, and this was causing a bad error message.
   I've changed it to use check_type on the left-hand side.

 - Some refactoring around when we print (TYPE blah) in error messages.
   The changes decrease the times when we do so, to good effect.
   Of course, this is still all controlled by
   -fprint-explicit-runtime-reps

Fixes #14066 #14749

Test cases: dependent/should_compile/{T14066a,T14749},
            dependent/should_fail/T14066{,c,d,e,f,g,h}

2 files changed, 10 insertions, 5 deletions

diff --git a/compiler/hsSyn/HsDecls.hs b/compiler/hsSyn/HsDecls.hs
index a3fe5a722e..6f1d5be849 100644
--- a/compiler/hsSyn/HsDecls.hs
+++ b/compiler/hsSyn/HsDecls.hs
@@ -19,7 +19,7 @@
 module HsDecls (
   -- * Toplevel declarations
   HsDecl(..), LHsDecl, HsDataDefn(..), HsDeriving,
-  HsDerivingClause(..), LHsDerivingClause,
+  HsDerivingClause(..), LHsDerivingClause, NewOrData(..), newOrDataToFlavour,
 
   -- ** Class or type declarations
   TyClDecl(..), LTyClDecl,
@@ -35,7 +35,7 @@ module HsDecls (
   FamilyDecl(..), LFamilyDecl,
 
   -- ** Instance declarations
-  InstDecl(..), LInstDecl, NewOrData(..), FamilyInfo(..),
+  InstDecl(..), LInstDecl, FamilyInfo(..),
   TyFamInstDecl(..), LTyFamInstDecl, instDeclDataFamInsts,
   DataFamInstDecl(..), LDataFamInstDecl, pprDataFamInstFlavour, pprFamInstLHS,
   FamInstEqn, LFamInstEqn, FamEqn(..),
@@ -958,7 +958,7 @@ famDeclHasCusk _ (FamilyDecl { fdInfo      = ClosedTypeFamily _
                              , fdResultSig = L _ resultSig })
   = hsTvbAllKinded tyvars && hasReturnKindSignature resultSig
 famDeclHasCusk mb_class_cusk _ = mb_class_cusk `orElse` True
-        -- all un-associated open families have CUSKs!
+        -- all un-associated open families have CUSKs
 
 -- | Does this family declaration have user-supplied return kind signature?
 hasReturnKindSignature :: FamilyResultSig a -> Bool
@@ -1114,6 +1114,11 @@ data NewOrData
   | DataType                    -- ^ @data Blah ...@
   deriving( Eq, Data )                -- Needed because Demand derives Eq
 
+-- | Convert a 'NewOrData' to a 'TyConFlavour'
+newOrDataToFlavour :: NewOrData -> TyConFlavour
+newOrDataToFlavour NewType  = NewtypeFlavour
+newOrDataToFlavour DataType = DataTypeFlavour
+
 -- | Located data Constructor Declaration
 type LConDecl pass = Located (ConDecl pass)
       -- ^ May have 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnSemi' when
diff --git a/compiler/hsSyn/HsTypes.hs b/compiler/hsSyn/HsTypes.hs
index 6503670130..15c570f0ea 100644
--- a/compiler/hsSyn/HsTypes.hs
+++ b/compiler/hsSyn/HsTypes.hs
@@ -1036,13 +1036,13 @@ splitLHsSigmaTy ty
   = (tvs, ctxt, ty2)
 
 splitLHsForAllTy :: LHsType pass -> ([LHsTyVarBndr pass], LHsType pass)
+splitLHsForAllTy (L _ (HsParTy ty)) = splitLHsForAllTy ty
 splitLHsForAllTy (L _ (HsForAllTy { hst_bndrs = tvs, hst_body = body })) = (tvs, body)
-splitLHsForAllTy (L _ (HsParTy t)) = splitLHsForAllTy t
 splitLHsForAllTy body              = ([], body)
 
 splitLHsQualTy :: LHsType pass -> (LHsContext pass, LHsType pass)
+splitLHsQualTy (L _ (HsParTy ty)) = splitLHsQualTy ty
 splitLHsQualTy (L _ (HsQualTy { hst_ctxt = ctxt, hst_body = body })) = (ctxt,     body)
-splitLHsQualTy (L _ (HsParTy t)) = splitLHsQualTy t
 splitLHsQualTy body              = (noLoc [], body)
 
 splitLHsInstDeclTy :: LHsSigType GhcRn