fix special case in canonical comparison

cpmpy/transformations/linearize.py

@@ -299,24 +299,30 @@ def canonical_comparison(lst_of_expr):
                 assert not is_num(lhs), "lhs cannot be an integer at this point!"
 
                 # bring all const to rhs
-                if lhs.name == "sum":
-                    new_args = []
-                    for i, arg in enumerate(lhs.args):
-                        if is_num(arg):
-                            rhs -= arg
-                        else:
-                            new_args.append(arg)
-                    lhs = Operator("sum", new_args)
-
-                elif lhs.name == "wsum":
-                    new_weights, new_args = [], []
-                    for i, (w, arg) in enumerate(zip(*lhs.args)):
-                        if is_num(arg):
-                            rhs -= w * arg
-                        else:
-                            new_weights.append(w)
-                            new_args.append(arg)
-                    lhs = Operator("wsum", [new_weights, new_args])
+                if isinstance(lhs, Operator):
+                    if lhs.name == "sum":
+                        new_args = []
+                        for i, arg in enumerate(lhs.args):
+                            if is_num(arg):
+                                rhs -= arg
+                            else:
+                                new_args.append(arg)
+                        lhs = Operator("sum", new_args)
+
+                    elif lhs.name == "wsum":
+                        new_weights, new_args = [], []
+                        for i, (w, arg) in enumerate(zip(*lhs.args)):
+                            if is_num(arg):
+                                rhs -= w * arg
+                            else:
+                                new_weights.append(w)
+                                new_args.append(arg)
+                        lhs = Operator("wsum", [new_weights, new_args])
+                    else:
+                        raise ValueError(f"lhs should be sum or wsum, but got {lhs}")
+                else:
+                    assert isinstance(lhs, _NumVarImpl)
+                    lhs = Operator("sum", [lhs])
 
             newlist.append(eval_comparison(cpm_expr.name, lhs, rhs))
         else:   # rest of expressions