Do not be so eager about loading family-instance modules

when doing the overlap check.  We only need to load the
ones for modules whose family instances we need to compare!

This means that programs that don't use type families are
not penalised, which is important.

compiler/typecheck/FamInst.lhs

@@ -7,6 +7,7 @@ module FamInst (
 
 import HscTypes
 import FamInstEnv
+import LoadIface
 import TcMType
 import TcRnMonad
 import TyCon
@@ -82,20 +83,17 @@ checkFamInstConsistency famInstMods directlyImpMods
        ; (eps, hpt) <- getEpsAndHpt
 
        ; let { -- Fetch the iface of a given module.  Must succeed as
- 	       -- all imported modules must already have been loaded.
+ 	       -- all directly imported modules must already have been loaded.
 	       modIface mod = 
 	         case lookupIfaceByModule dflags hpt (eps_PIT eps) mod of
                    Nothing    -> panic "FamInst.checkFamInstConsistency"
                    Just iface -> iface
 
              ; hmiModule     = mi_module . hm_iface
-	     ; hmiFamInstEnv = mkFamInstEnv . md_fam_insts . hm_details
-	     ; mkFamInstEnv  = extendFamInstEnvList emptyFamInstEnv
-             ; hptModInsts   = [ (hmiModule hmi, hmiFamInstEnv hmi) 
-			       | hmi <- eltsUFM hpt]
-             ; modInstsEnv   = eps_mod_fam_inst_env eps	-- external modules
-			       `extendModuleEnvList`	-- plus
-			       hptModInsts		-- home package modules
+	     ; hmiFamInstEnv = extendFamInstEnvList emptyFamInstEnv 
+                               . md_fam_insts . hm_details
+             ; hpt_fam_insts = mkModuleEnv [ (hmiModule hmi, hmiFamInstEnv hmi) 
+			                   | hmi <- eltsUFM hpt]
 	     ; groups        = map (dep_finsts . mi_deps . modIface) 
 				   directlyImpMods
 	     ; okPairs       = listToSet $ concatMap allPairs groups
@@ -106,22 +104,27 @@ checkFamInstConsistency famInstMods directlyImpMods
 	         -- the difference gives us the pairs we need to check now
 	     }
 
-       ; mapM_ (check modInstsEnv) toCheckPairs
+       ; mapM_ (check hpt_fam_insts) toCheckPairs
        }
   where
     allPairs []     = []
     allPairs (m:ms) = map (ModulePair m) ms ++ allPairs ms
 
-    -- The modules are guaranteed to be in the environment, as they are either
-    -- already loaded in the EPS or they are in the HPT.
-    --
-    check modInstsEnv (ModulePair m1 m2)
-      = let { instEnv1 = (expectJust "checkFamInstConsistency") . lookupModuleEnv modInstsEnv $ m1
-	    ; instEnv2 = (expectJust "checkFamInstConsistency") . lookupModuleEnv modInstsEnv $ m2
-	    ; insts1   = famInstEnvElts instEnv1
-	    }
-        in
-	mapM_ (checkForConflicts (emptyFamInstEnv, instEnv2)) insts1
+    check hpt_fam_insts (ModulePair m1 m2)
+      = do { env1 <- getFamInsts hpt_fam_insts m1
+           ; env2 <- getFamInsts hpt_fam_insts m2
+           ; mapM_ (checkForConflicts (emptyFamInstEnv, env2))   
+                   (famInstEnvElts env1) }
+
+getFamInsts :: ModuleEnv FamInstEnv -> Module -> TcM FamInstEnv
+getFamInsts hpt_fam_insts mod
+  | Just env <- lookupModuleEnv hpt_fam_insts mod = return env
+  | otherwise = do { _ <- initIfaceTcRn (loadSysInterface doc mod)
+                   ; eps <- getEps
+                   ; return (expectJust "checkFamInstConsistency" $
+                             lookupModuleEnv (eps_mod_fam_inst_env eps) mod) }
+  where
+    doc = ppr mod <+> ptext (sLit "is a family-instance module")
 \end{code}
 
 %************************************************************************