[SPARK-9200][SQL] Don't implicitly cast non-atomic types to string type.

Author: Reynold Xin <rxin@databricks.com>

Closes #7636 from rxin/complex-string-implicit-cast and squashes the following commits:

3e67327 [Reynold Xin] [SPARK-9200][SQL] Don't implicitly cast non-atomic types to string type.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercion.scala

@@ -720,7 +720,8 @@ object HiveTypeCoercion {
         case (StringType, DateType) => Cast(e, DateType)
         case (StringType, TimestampType) => Cast(e, TimestampType)
         case (StringType, BinaryType) => Cast(e, BinaryType)
-        case (any, StringType) if any != StringType => Cast(e, StringType)
+        // Cast any atomic type to string.
+        case (any: AtomicType, StringType) if any != StringType => Cast(e, StringType)
 
         // When we reach here, input type is not acceptable for any types in this type collection,
         // try to find the first one we can implicitly cast.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercionSuite.scala

@@ -115,6 +115,14 @@ class HiveTypeCoercionSuite extends PlanTest {
     shouldNotCast(IntegerType, ArrayType)
     shouldNotCast(IntegerType, MapType)
     shouldNotCast(IntegerType, StructType)
+
+    shouldNotCast(IntervalType, StringType)
+
+    // Don't implicitly cast complex types to string.
+    shouldNotCast(ArrayType(StringType), StringType)
+    shouldNotCast(MapType(StringType, StringType), StringType)
+    shouldNotCast(new StructType().add("a1", StringType), StringType)
+    shouldNotCast(MapType(StringType, StringType), StringType)
   }
 
   test("tightest common bound for types") {