[SPARK-31025][SQL] Support foldable CSV strings by schema_of_csv

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/csvExpressions.scala

@@ -165,10 +165,14 @@ case class SchemaOfCsv(
   @transient
   private lazy val csv = child.eval().asInstanceOf[UTF8String]
 
-  override def checkInputDataTypes(): TypeCheckResult = child match {
-    case Literal(s, StringType) if s != null => super.checkInputDataTypes()
-    case _ => TypeCheckResult.TypeCheckFailure(
-      s"The input csv should be a string literal and not null; however, got ${child.sql}.")
+  override def checkInputDataTypes(): TypeCheckResult = {
+    if (child.foldable && csv != null) {
+      super.checkInputDataTypes()
+    } else {
+      TypeCheckResult.TypeCheckFailure(
+        "The input csv should be a foldable string expression and not null; " +
+        s"however, got ${child.sql}.")
+    }
   }
 
   override def eval(v: InternalRow): Any = {

sql/core/src/test/resources/sql-tests/results/csv-functions.sql.out

@@ -91,7 +91,7 @@ select schema_of_csv(null)
 struct<>
 -- !query output
 org.apache.spark.sql.AnalysisException
-cannot resolve 'schema_of_csv(NULL)' due to data type mismatch: The input csv should be a string literal and not null; however, got NULL.; line 1 pos 7
+cannot resolve 'schema_of_csv(NULL)' due to data type mismatch: The input csv should be a foldable string expression and not null; however, got NULL.; line 1 pos 7
 
 
 -- !query
@@ -108,7 +108,7 @@ SELECT schema_of_csv(csvField) FROM csvTable
 struct<>
 -- !query output
 org.apache.spark.sql.AnalysisException
-cannot resolve 'schema_of_csv(csvtable.`csvField`)' due to data type mismatch: The input csv should be a string literal and not null; however, got csvtable.`csvField`.; line 1 pos 7
+cannot resolve 'schema_of_csv(csvtable.`csvField`)' due to data type mismatch: The input csv should be a foldable string expression and not null; however, got csvtable.`csvField`.; line 1 pos 7
 
 
 -- !query

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

@@ -200,4 +200,11 @@ class CsvFunctionsSuite extends QueryTest with SharedSparkSession {
       assert(readback(0).getAs[Row](0).getAs[Date](0).getTime >= 0)
     }
   }
+
+  test("schema_of_csv - infers the schema of foldable CSV string") {
+    val input = concat_ws(",", lit(0.1), lit(1))
+    checkAnswer(
+      spark.range(1).select(schema_of_csv(input)),
+      Seq(Row("struct<_c0:double,_c1:int>")))
+  }
 }