FIX: TypeFamilies: should_compile/Simple12

[ghc-hetmet.git] / compiler / typecheck / TcTyFuns.lhs

diff --git a/compiler/typecheck/TcTyFuns.lhs b/compiler/typecheck/TcTyFuns.lhs
index 64c9830..126a029 100644 (file)
--- a/compiler/typecheck/TcTyFuns.lhs
+++ b/compiler/typecheck/TcTyFuns.lhs
@@ -953,10 +953,14 @@ unifyMetaRule insts
         -- updatable meta variable meets non-variable type
         -- => occurs check, monotype check, and kinds match check, then update
        uMeta swapped tv (DoneTv (MetaTv _ ref)) ty cotv
-         = do { ty' <- checkTauTvUpdate tv ty       -- occurs + monotype check
-              ; checkUpdateMeta swapped tv ref ty'  -- update meta var
-              ; writeMetaTyVar cotv ty'             -- update the co var
-              ; return ([], True)
+         = do { mb_ty' <- checkTauTvUpdate tv ty    -- occurs + monotype check
+               ; case mb_ty' of
+                   Nothing  -> return ([inst], False)  -- tv occurs in faminst
+                   Just ty' ->
+                     do { checkUpdateMeta swapped tv ref ty'  -- update meta var
+                       ; writeMetaTyVar cotv ty'             -- update co var
+                       ; return ([], True)
+                        }
               }
 
         uMeta _ _ _ _ _ = panic "uMeta"