Forbid empty strings in month and day-of-week names and AM/PM markers

core/common/src/format/DateTimeFormatBuilder.kt

@@ -129,6 +129,8 @@ public sealed interface DateTimeFormatBuilder {
          *
          * [am] is used for the AM marker (0-11 hours), [pm] is used for the PM marker (12-23 hours).
          *
+         * Empty strings can not be used as markers.
+         *
          * @see [amPmHour]
          */
         public fun amPmMarker(am: String, pm: String)

core/common/src/format/LocalDateFormat.kt

@@ -12,6 +12,8 @@ import kotlinx.datetime.internal.format.parser.Copyable
 
 /**
  * A description of how month names are formatted.
+ *
+ * An empty string can not be a month name.
  */
 public class MonthNames(
     /**
@@ -21,6 +23,7 @@ public class MonthNames(
 ) {
     init {
         require(names.size == 12) { "Month names must contain exactly 12 elements" }
+        names.forEach { require(it.isNotEmpty()) { "A month name can not be empty" } }
     }
 
     /**
@@ -63,6 +66,8 @@ internal fun MonthNames.toKotlinCode(): String = when (this.names) {
 
 /**
  * A description of how day of week names are formatted.
+ *
+ * An empty string can not be a day-of-week name.
  */
 public class DayOfWeekNames(
     /**
@@ -72,6 +77,7 @@ public class DayOfWeekNames(
 ) {
     init {
         require(names.size == 7) { "Day of week names must contain exactly 7 elements" }
+        names.forEach { require(it.isNotEmpty()) { "A month name can not be empty" } }
     }
 
     /**

core/common/src/internal/format/parser/ParserOperation.kt

@@ -157,6 +157,7 @@ internal class StringSetParserOperation<Output>(
 
     init {
         for (string in strings) {
+            require(string.isNotEmpty()) { "Found an empty string in $whatThisExpects" }
             var node = trie
             for (char in string) {
                 val searchResult = node.children.binarySearchBy(char.toString()) { it.first }