Comment on what an "enumeration" type is

compiler/iface/BuildTyCl.lhs

@@ -112,15 +112,9 @@ mkDataTyConRhs :: [DataCon] -> AlgTyConRhs
 mkDataTyConRhs cons
   = DataTyCon {
         data_cons = cons,
-        is_enum = -- We define datatypes with no constructors to not be
-                  -- enumerations; this fixes trac #2578,  Otherwise we
-                  -- end up generating an empty table for
-                  --   <mod>_<type>_closure_tbl
-                  -- which is used by tagToEnum# to map Int# to constructors
-                  -- in an enumeration. The empty table apparently upset
-                  -- the linker.
-                  not (null cons) &&
+        is_enum = not (null cons) &&
                   all isNullarySrcDataCon cons
+		  -- See Note [Enumeration types] in TyCon
     }
 
 mkNewTyConRhs :: Name -> TyCon -> DataCon -> TcRnIf m n AlgTyConRhs

compiler/types/TyCon.lhs

@@ -440,7 +440,7 @@ data AlgTyConRhs
 			  --	  (see the tag assignment in DataCon.mkDataCon)
 
 	is_enum :: Bool   -- ^ Cached value: is this an enumeration type? 
-                          --   (See 'isEnumerationTyCon')
+                          --   See Note [Enumeration types]
     }
 
   -- | Information about those 'TyCon's derived from a @newtype@ declaration
@@ -580,6 +580,16 @@ data CoTyConDesc
   | CoUnsafe 
 \end{code}
 
+Note [Enumeration types]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We define datatypes with no constructors to not be
+enumerations; this fixes trac #2578,  Otherwise we
+end up generating an empty table for
+  <mod>_<type>_closure_tbl
+which is used by tagToEnum# to map Int# to constructors
+in an enumeration. The empty table apparently upset
+the linker.
+
 Note [Newtype coercions]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 The NewTyCon field nt_co is a a TyCon (a coercion constructor in fact)
@@ -983,6 +993,7 @@ isGadtSyntaxTyCon _                                    = False
 
 -- | Is this an algebraic 'TyCon' which is just an enumeration of values?
 isEnumerationTyCon :: TyCon -> Bool
+-- See Note [Enumeration types] in TyCon
 isEnumerationTyCon (AlgTyCon {algTcRhs = DataTyCon { is_enum = res }}) = res
 isEnumerationTyCon (TupleTyCon {tyConArity = arity}) = arity == 0
 isEnumerationTyCon _                                                   = False