src/Pure/sign.ML

       val Sign {naming = _, syn = syn1, tsig = tsig1, consts = consts1} = sign1;
       val Sign {naming = _, syn = syn2, tsig = tsig2, consts = consts2} = sign2;
 
       val naming = Name_Space.default_naming;
       val syn = Syntax.merge_syntaxes syn1 syn2;
-      val tsig = Type.merge_tsigs pp (tsig1, tsig2);
+      val tsig = Type.merge_tsig pp (tsig1, tsig2);
       val consts = Consts.merge (consts1, consts2);
     in make_sign (naming, syn, tsig, consts) end;
 );
 
 fun rep_sg thy = Data.get thy |> (fn Sign args => args);