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=235097369f22fe8e600fed4377757b3240a75390;hp=e7f8507d81a5aa3749c6d81eacfb6f78f5b771b3;hb=13adf25f23091c8ef1a14cfb11ed9379ffbaed90;hpb=afe4534704e8e0c25e2f90c6c0a2e397ecef24db

diff --git a/compiler/types/TyCon.lhs b/compiler/types/TyCon.lhs
index e7f8507..2350973 100644
--- a/compiler/types/TyCon.lhs
+++ b/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