Comments, plus adjust debug print of TcTyThing(ATyVar)

compiler/hsSyn/HsDecls.hs

@@ -1330,7 +1330,7 @@ type patterns, i.e. fv(pat_tys).  Note in particular
   '_' gets its own unique.  In this context wildcards behave just like
   an ordinary type variable, only anonymous.
 
-* The hsib_vars *including* type variables that are already in scope
+* The hsib_vars *includes* type variables that are already in scope
 
    Eg   class C s t where
           type F t p :: *

compiler/hsSyn/HsTypes.hs

@@ -280,12 +280,9 @@ isEmptyLHsQTvs _                = False
 
 ------------------------------------------------
 --            HsImplicitBndrs
--- Used to quantify the binders of a type in cases
--- when a HsForAll isn't appropriate:
+-- Used to quantify the implicit binders of a type
+--    * Implicit binders of a type signature (LHsSigType/LHsSigWcType)
 --    * Patterns in a type/data family instance (HsTyPats)
---    * Type of a rule binder (RuleBndr)
---    * Pattern type signatures (SigPatIn)
--- In the last of these, wildcards can happen, so we must accommodate them
 
 -- | Haskell Implicit Binders
 data HsImplicitBndrs pass thing   -- See Note [HsType binders]

compiler/typecheck/TcRnTypes.hs

@@ -1094,6 +1094,7 @@ instance Outputable TcTyThing where     -- Debugging only
                                  <> ppr (varType (tct_id elt)) <> comma
                                  <+> ppr (tct_info elt))
    ppr (ATyVar n tv)    = text "Type variable" <+> quotes (ppr n) <+> equals <+> ppr tv
+                            <+> dcolon <+> ppr (varType tv)
    ppr (ATcTyCon tc)    = text "ATcTyCon" <+> ppr tc <+> dcolon <+> ppr (tyConKind tc)
    ppr (APromotionErr err) = text "APromotionErr" <+> ppr err