[SPARK-10080] [SQL] Fix binary incompatibility for $ column interpolation

Turns out that inner classes of inner objects are referenced directly, and thus moving it will break binary compatibility.

Author: Michael Armbrust <michael@databricks.com>

Closes #8281 from marmbrus/binaryCompat.

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -334,6 +334,17 @@ class SQLContext(@transient val sparkContext: SparkContext)
   @Experimental
   object implicits extends SQLImplicits with Serializable {
     protected override def _sqlContext: SQLContext = self
+
+    /**
+     * Converts $"col name" into an [[Column]].
+     * @since 1.3.0
+     */
+    // This must live here to preserve binary compatibility with Spark < 1.5.
+    implicit class StringToColumn(val sc: StringContext) {
+      def $(args: Any*): ColumnName = {
+        new ColumnName(sc.s(args: _*))
+      }
+    }
   }
   // scalastyle:on
 

sql/core/src/main/scala/org/apache/spark/sql/SQLImplicits.scala

@@ -33,16 +33,6 @@ import org.apache.spark.unsafe.types.UTF8String
 private[sql] abstract class SQLImplicits {
   protected def _sqlContext: SQLContext
 
-  /**
-   * Converts $"col name" into an [[Column]].
-   * @since 1.3.0
-   */
-  implicit class StringToColumn(val sc: StringContext) {
-    def $(args: Any*): ColumnName = {
-      new ColumnName(sc.s(args: _*))
-    }
-  }
-
   /**
    * An implicit conversion that turns a Scala `Symbol` into a [[Column]].
    * @since 1.3.0

sql/core/src/test/scala/org/apache/spark/sql/test/SharedSQLContext.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.sql.test
 
-import org.apache.spark.sql.SQLContext
+import org.apache.spark.sql.{ColumnName, SQLContext}
 
 
 /**
@@ -65,4 +65,14 @@ private[sql] trait SharedSQLContext extends SQLTestUtils {
     }
   }
 
+  /**
+   * Converts $"col name" into an [[Column]].
+   * @since 1.3.0
+   */
+  // This must be duplicated here to preserve binary compatibility with Spark < 1.5.
+  implicit class StringToColumn(val sc: StringContext) {
+    def $(args: Any*): ColumnName = {
+      new ColumnName(sc.s(args: _*))
+    }
+  }
 }