Address comments.

Author: dongjoon-hyun

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

@@ -17,7 +17,6 @@
 
 package org.apache.spark.sql.catalyst.analysis
 
-import java.util.Locale
 import javax.annotation.Nullable
 
 import scala.annotation.tailrec
@@ -105,11 +104,10 @@ object TypeCoercion {
       Some(StructType(fields1.zip(fields2).map { case (f1, f2) =>
         // Since `t1.sameType(t2)` is true, two StructTypes have the same DataType
         // except `name` (in case of `spark.sql.caseSensitive=false`) and `nullable`.
-        // - Different names: use a lower case name because findTightestCommonType is commutative.
+        // - Different names: use f1.name
         // - Different nullabilities: `nullable` is true iff one of them is nullable.
-        val name = if (f1.name == f2.name) f1.name else f1.name.toLowerCase(Locale.ROOT)
         val dataType = findTightestCommonType(f1.dataType, f2.dataType).get
-        StructField(name, dataType, nullable = f1.nullable || f2.nullable)
+        StructField(f1.name, dataType, nullable = f1.nullable || f2.nullable)
       }))
 
     case _ => None

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

@@ -131,14 +131,17 @@ class TypeCoercionSuite extends AnalysisTest {
       widenFunc: (DataType, DataType) => Option[DataType],
       t1: DataType,
       t2: DataType,
-      expected: Option[DataType]): Unit = {
+      expected: Option[DataType],
+      isSymmetric: Boolean = true): Unit = {
     var found = widenFunc(t1, t2)
     assert(found == expected,
       s"Expected $expected as wider common type for $t1 and $t2, found $found")
     // Test both directions to make sure the widening is symmetric.
-    found = widenFunc(t2, t1)
-    assert(found == expected,
-      s"Expected $expected as wider common type for $t2 and $t1, found $found")
+    if (isSymmetric) {
+      found = widenFunc(t2, t1)
+      assert(found == expected,
+        s"Expected $expected as wider common type for $t2 and $t1, found $found")
+    }
   }
 
   test("implicit type cast - ByteType") {
@@ -419,10 +422,12 @@ class TypeCoercionSuite extends AnalysisTest {
         None)
     }
     withSQLConf(SQLConf.CASE_SENSITIVE.key -> "false") {
-      widenTest(
+      checkWidenType(
+        TypeCoercion.findTightestCommonType,
         StructType(Seq(StructField("a", IntegerType), StructField("B", IntegerType))),
         StructType(Seq(StructField("A", IntegerType), StructField("b", IntegerType))),
-        Some(StructType(Seq(StructField("a", IntegerType), StructField("b", IntegerType)))))
+        Some(StructType(Seq(StructField("a", IntegerType), StructField("B", IntegerType)))),
+        isSymmetric = false)
     }
   }