X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypes%2FTyCon.lhs;fp=compiler%2Ftypes%2FTyCon.lhs;h=2958107367318bd9062e98e23fe00704122aab54;hp=235097369f22fe8e600fed4377757b3240a75390;hb=bee517d218a9546db88ee3ad4e15fb2010562e4a;hpb=35a77384d096e9cec8488f09ddaee533ad61051a

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index 2350973..2958107 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)