Tighten up what it means to be an "enumeration data constructor"

See Note [Enumeration types] in TyCon, and comments in Trac #4528

2 files changed, 17 insertions, 3 deletions

diff --git a/compiler/iface/BuildTyCl.lhs b/compiler/iface/BuildTyCl.lhs
index 4319d1fc18..e71eefe339 100644
--- a/compiler/iface/BuildTyCl.lhs
+++ b/compiler/iface/BuildTyCl.lhs
@@ -112,10 +112,14 @@ mkDataTyConRhs :: [DataCon] -> AlgTyConRhs
 mkDataTyConRhs cons
   = DataTyCon {
         data_cons = cons,
-        is_enum = not (null cons) &&
-                  all isNullarySrcDataCon cons
+        is_enum = not (null cons) && all is_enum_con cons
 		  -- See Note [Enumeration types] in TyCon
     }
+  where
+    is_enum_con con
+       | (_tvs, theta, arg_tys, _res) <- dataConSig con
+       = null theta && null arg_tys
+
 
 mkNewTyConRhs :: Name -> TyCon -> DataCon -> TcRnIf m n AlgTyConRhs
 -- ^ Monadic because it makes a Name for the coercion TyCon
diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index 235097369f..2958107367 100644
--- a/compiler/types/TyCon.lhs
+++ b/compiler/types/TyCon.lhs
@@ -582,7 +582,7 @@ data CoTyConDesc
 
 Note [Enumeration types]
 ~~~~~~~~~~~~~~~~~~~~~~~~
-We define datatypes with no constructors to not be
+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
@@ -590,6 +590,16 @@ which is used by tagToEnum# to map Int# to constructors
 in an enumeration. The empty table apparently upset
 the linker.
 
+Moreover, all the data constructor must be enumerations, meaning
+they have type  (forall abc. T a b c).  GADTs are not enumerations.
+For example consider
+    data T a where
+      T1 :: T Int
+      T2 :: T Bool
+      T3 :: T a
+What would [T1 ..] be?  [T1,T3] :: T Int? Easiest thing is to exclude them.
+See Trac #4528.
+
 Note [Newtype coercions]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 The NewTyCon field nt_co is a a TyCon (a coercion constructor in fact)