Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Data.Linq/SqlClient/Query/SqlGatherConsumedAliases.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace System.Data.Linq.SqlClient {
+    /// <summary>
+    /// Walk a tree and return the set of unique aliases it consumes.
+    /// </summary>
+    class SqlGatherConsumedAliases {
+        internal static HashSet<SqlAlias> Gather(SqlNode node) {
+            Gatherer g = new Gatherer();
+            g.Visit(node);
+            return g.Consumed;
+        }
+
+        private class Gatherer : SqlVisitor {
+            internal HashSet<SqlAlias> Consumed = new HashSet<SqlAlias>();
+
+            internal void VisitAliasConsumed(SqlAlias a) {
+                Consumed.Add(a);
+            }
+            internal override SqlExpression VisitColumn(SqlColumn col) {
+                VisitAliasConsumed(col.Alias);
+                VisitExpression(col.Expression);
+                return col;
+            }
+            internal override SqlExpression VisitColumnRef(SqlColumnRef cref) {
+                VisitAliasConsumed(cref.Column.Alias);
+                return cref;
+            }
+        }
+    }
+}