xo-expression: bugfix: ensure in-layer uniqueness of vars

include/xo/expression/Variable.hpp

@@ -25,6 +25,11 @@ namespace xo {
                 return new Variable(name, var_type);
             }
 
+            /** return copy of x:  same var, different object identity **/
+            static ref::rp<Variable> copy(ref::brw<Variable> x) {
+                return new Variable(x->name(), x->valuetype());
+            }
+
             /** downcast from Expression **/
             static ref::brw<Variable> from(ref::brw<Expression> x) {
                 return ref::brw<Variable>::from(x);

src/expression/Lambda.cpp

@@ -103,9 +103,11 @@ namespace xo {
 
                                 auto ix = var_map.find(var->name());
                                 if (ix == var_map.end()) {
-                                    /* add to var_map */
+                                    /* add to var_map,  copy to ensure Variable
+                                     * is unique to layer
+                                     */
 
-                                    var_map[var->name()] = var.get();
+                                    var_map[var->name()] = Variable::copy(var);
 
                                     return var.get();
                                 } else {