Add YearMonth.days: range of dates in a given month

Author: dkhalanskyjb

core/common/src/YearMonth.kt

@@ -115,7 +115,7 @@ public constructor(year: Int, month: Int) : Comparable<YearMonth> {
      *
      * @sample kotlinx.datetime.test.samples.YearMonthSamples.days
      */
-    // val days: LocalDateRange get() = firstDay..lastDay // no ranges yet
+    public val days: LocalDateRange
 
     /**
      * Constructs a [YearMonth] instance from the given year-month components.

core/common/test/YearMonthTest.kt

@@ -165,12 +165,18 @@ class YearMonthTest {
     }
 
     @Test
-    fun firstAndLastDay() {
+    fun daysRange() {
+        @OptIn(ExperimentalStdlibApi::class)
         fun test(year: Int, month: Int) {
             val yearMonth = YearMonth(year, month)
             assertEquals(LocalDate(year, month, 1), yearMonth.firstDay)
             assertEquals(LocalDate(year, month, yearMonth.numberOfDays), yearMonth.lastDay)
             assertEquals(yearMonth.plusMonth().firstDay, yearMonth.lastDay.plus(1, DateTimeUnit.DAY))
+            assertEquals(LocalDateRange(yearMonth.firstDay, yearMonth.lastDay), yearMonth.days)
+            assertContains(yearMonth.days as OpenEndRange<LocalDate>, yearMonth.firstDay)
+            assertContains(yearMonth.days as OpenEndRange<LocalDate>, yearMonth.lastDay)
+            assertFalse(yearMonth.days.contains(yearMonth.firstDay.minus(1, DateTimeUnit.DAY)))
+            assertFalse(yearMonth.days.contains(yearMonth.lastDay.plus(1, DateTimeUnit.DAY)))
         }
         for (month in 1..12) {
             for (year in 2000..2005) {

core/common/test/samples/YearMonthSamples.kt

@@ -73,6 +73,22 @@ class YearMonthSamples {
         }
     }
 
+    @Test
+    fun days() {
+        // Getting the range of days in a YearMonth
+        val yearMonth = YearMonth(2024, Month.APRIL)
+        // The range consists of all the days in the month:
+        check(yearMonth.days.size == 30)
+        check(yearMonth.days.first() == LocalDate(2024, Month.APRIL, 1))
+        check(yearMonth.days.last() == LocalDate(2024, Month.APRIL, 30))
+        check(yearMonth.days.contains(LocalDate(2024, Month.APRIL, 15)))
+        // The range allows iterating over the days:
+        for (day in yearMonth.days) {
+            check(day.month == Month.APRIL)
+            check(day.year == 2024)
+        }
+    }
+
     @Test
     fun compareToSample() {
         // Comparing YearMonth values

core/commonKotlin/src/YearMonth.kt

@@ -30,7 +30,7 @@ public actual constructor(public actual val year: Int, month: Int) : Comparable<
 
     public actual val numberOfDays: Int get() = monthNumber.monthLength(isLeapYear(year))
 
-    // val days: LocalDateRange get() = firstDay..lastDay // no ranges yet
+    public actual val days: LocalDateRange get() = firstDay..lastDay // no ranges yet
 
     public actual constructor(year: Int, month: Month): this(year, month.number)
 

core/jvm/src/YearMonthJvm.kt

@@ -26,8 +26,7 @@ public actual class YearMonth internal constructor(
     public actual val firstDay: LocalDate get() = LocalDate(value.atDay(1))
     public actual val lastDay: LocalDate get() = LocalDate(value.atEndOfMonth())
     public actual val numberOfDays: Int get() = value.lengthOfMonth()
-
-    // val days: LocalDateRange get() = firstDay..lastDay // no ranges yet
+    public actual val days: LocalDateRange get() = firstDay..lastDay // no ranges yet
 
     public actual constructor(year: Int, month: Int): this(try {
         jtYearMonth.of(year, month)