Add test for struct as map key type

Author: JoshRosen

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetIOSuite.scala

@@ -231,6 +231,23 @@ class ParquetIOSuite extends QueryTest with ParquetTest with SharedSparkSession
     }
   }
 
+  testStandardAndLegacyModes("nested map with struct as key type") {
+    val data = (1 to 4).map { i =>
+      Tuple1(
+        Map(
+          (i, s"kA_$i") -> s"vA_$i",
+          (i, s"kB_$i") -> s"vB_$i"
+        )
+      )
+    }
+    withParquetDataFrame(data) { df =>
+      // Structs are converted to `Row`s
+      checkAnswer(df, data.map { case Tuple1(m) =>
+        Row(m.map { case (k, v) => Row(k.productIterator.toSeq: _*) -> v })
+      })
+    }
+  }
+
   testStandardAndLegacyModes("nested map with struct as value type") {
     val data = (1 to 4).map { i =>
       Tuple1(