[SPARK-38864][FOLLOWUP] Add tests unpivoting struct expressions

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

@@ -535,6 +535,98 @@ class DatasetUnpivotSuite extends QueryTest
         "val"),
       longStructDataRows)
   }
+
+  test("unpivot with struct expressions") {
+    checkAnswer(
+      wideDataDs.unpivot(
+        Array($"id"),
+        Array(
+          struct($"str1".as("str"), $"int1".cast(LongType).as("long")).as("str-int"),
+          struct($"str2".as("str"), $"long1".as("long")).as("str-long")
+        ),
+        "var",
+        "val"),
+      Seq(
+        Row(1, "str-int", Row("one", 1L)),
+        Row(1, "str-long", Row("One", 1L)),
+        Row(2, "str-int", Row("two", null)),
+        Row(2, "str-long", Row(null, 2L)),
+        Row(3, "str-int", Row(null, 3L)),
+        Row(3, "str-long", Row("three", null)),
+        Row(4, "str-int", Row(null, null)),
+        Row(4, "str-long", Row(null, null))
+      )
+    )
+
+    checkAnswer(
+      // struct field types and names must match
+      wideDataDs.unpivot(
+        Array($"id"),
+        Array(
+          struct($"str1".as("str"), $"int1".cast(LongType).as("long")).as("str-int"),
+          struct($"str2".as("str"), $"long1".as("long")).as("str-long")
+        ),
+        "var",
+        "val").select(
+        $"id", $"var", $"val.*"
+      ),
+      Seq(
+        Row(1, "str-int", "one", 1L),
+        Row(1, "str-long", "One", 1L),
+        Row(2, "str-int", "two", null),
+        Row(2, "str-long", null, 2L),
+        Row(3, "str-int", null, 3L),
+        Row(3, "str-long", "three", null),
+        Row(4, "str-int", null, null),
+        Row(4, "str-long", null, null)
+      )
+    )
+
+    // different struct field types and names fail
+    val e1 = intercept[AnalysisException] {
+      wideDataDs.unpivot(
+        Array($"id"),
+        Array(
+          struct($"str1", $"int1").as("str-int"),
+          struct($"str2", $"long1").as("str-long")
+        ),
+        "var",
+        "val"
+      )
+    }
+    checkError(
+      exception = e1,
+      errorClass = "UNPIVOT_VALUE_DATA_TYPE_MISMATCH",
+      parameters = Map(
+        "types" -> (
+          """"STRUCT<str1: STRING, int1: INT>" (`str-int`), """ +
+            """"STRUCT<str2: STRING, long1: BIGINT>" (`str-long`)"""
+          )
+      )
+    )
+
+    // different struct field names fail
+    val e2 = intercept[AnalysisException] {
+      wideDataDs.unpivot(
+        Array($"id"),
+        Array(
+          struct($"str1", $"int1".cast(LongType).as("int1")).as("str-int"),
+          struct($"str2", $"long1").as("str-long")
+        ),
+        "var",
+        "val")
+    }
+    checkError(
+      exception = e2,
+      errorClass = "UNPIVOT_VALUE_DATA_TYPE_MISMATCH",
+      parameters = Map(
+        "types" -> (
+          """"STRUCT<str1: STRING, int1: BIGINT>" (`str-int`), """ +
+            """"STRUCT<str2: STRING, long1: BIGINT>" (`str-long`)"""
+          )
+      )
+    )
+  }
 }
 
 case class WideData(id: Int, str1: String, str2: String, int1: Option[Int], long1: Option[Long])