[project @ 2002-05-10 12:43:02 by simonpj]

If a type signature mentions a type variable that doesn't appear in
the type, GHC was dying horribly. Example (from happy -s):

	type T a = () -> ()

	f :: T a
	f () = ()

This commit fixes the problem.

diff --git a/ghc/compiler/typecheck/TcBinds.lhs b/ghc/compiler/typecheck/TcBinds.lhs
index 597a29d..045cdcc 100644 (file)
--- a/ghc/compiler/typecheck/TcBinds.lhs
+++ b/ghc/compiler/typecheck/TcBinds.lhs
@@ -37,7 +37,7 @@ import TcSimplify     ( bindInstsOfLocalFuns )
 import TcMType         ( newTyVar, newTyVarTy, newHoleTyVarTy,
                          zonkTcTyVarToTyVar, readHoleResult
                        )
-import TcType          ( mkTyVarTy, mkForAllTys, mkFunTys, tyVarsOfType, 
+import TcType          ( TcTyVar, mkTyVarTy, mkForAllTys, mkFunTys, tyVarsOfType, 
                          mkPredTy, mkForAllTy, isUnLiftedType, 
                          unliftedTypeKind, liftedTypeKind, openTypeKind, eqKind
                        )
@@ -431,16 +431,16 @@ generalise binder_names mbind tau_tvs lie_req sigs =
     tcSimplifyRestricted doc tau_tvs lie_req           `thenTc` \ (qtvs, lie_free, binds) ->
 
        -- Check that signature type variables are OK
-    checkSigsTyVars sigs                               `thenTc_`
+    checkSigsTyVars qtvs sigs                          `thenTc` \ final_qtvs ->
 
-    returnTc (qtvs, lie_free, binds, [])
+    returnTc (final_qtvs, lie_free, binds, [])
 
   else if null sigs then       -- UNRESTRICTED CASE, NO TYPE SIGS
     tcSimplifyInfer doc tau_tvs lie_req
 
   else                                 -- UNRESTRICTED CASE, WITH TYPE SIGS
        -- CHECKING CASE: Unrestricted group, there are type signatures
-       -- Check signature contexts are empty 
+       -- Check signature contexts are identical
     checkSigsCtxts sigs                        `thenTc` \ (sig_avails, sig_dicts) ->
     
        -- Check that the needed dicts can be
@@ -448,9 +448,9 @@ generalise binder_names mbind tau_tvs lie_req sigs =
     tcSimplifyInferCheck doc tau_tvs sig_avails lie_req        `thenTc` \ (forall_tvs, lie_free, dict_binds) ->
        
        -- Check that signature type variables are OK
-    checkSigsTyVars sigs                                       `thenTc_`
+    checkSigsTyVars forall_tvs sigs                    `thenTc` \ final_qtvs ->
 
-    returnTc (forall_tvs, lie_free, dict_binds, sig_dicts)
+    returnTc (final_qtvs, lie_free, dict_binds, sig_dicts)
 
   where
     tysig_names = map (idName . tcSigPolyId) sigs
@@ -489,7 +489,23 @@ checkSigsCtxts sigs@(TySigInfo id1 sig_tvs theta1 _ _ _ src_loc : other_sigs)
         checkTc (equalLength theta theta1) sigContextsErr      `thenTc_`
         unifyTauTyLists sig1_dict_tys (map mkPredTy theta)
 
-checkSigsTyVars sigs = mapTc_ check_one sigs
+checkSigsTyVars :: [TcTyVar] -> [TcSigInfo] -> TcM [TcTyVar]
+checkSigsTyVars qtvs sigs 
+  = mapTc check_one sigs       `thenTc` \ sig_tvs_s ->
+    let
+       -- Sigh.  Make sure that all the tyvars in the type sigs
+       -- appear in the returned ty var list, which is what we are
+       -- going to generalise over.  Reason: we occasionally get
+       -- silly types like
+       --      type T a = () -> ()
+       --      f :: T a
+       --      f () = ()
+       -- Here, 'a' won't appear in qtvs, so we have to add it
+
+       sig_tvs = foldr (unionVarSet . mkVarSet) emptyVarSet sig_tvs_s
+       all_tvs = mkVarSet qtvs `unionVarSet` sig_tvs
+    in
+    returnTc (varSetElems all_tvs)
   where
     check_one (TySigInfo id sig_tyvars sig_theta sig_tau _ _ src_loc)
       = tcAddSrcLoc src_loc                                            $