Add context to the DayOfWeek constructor precondition (#371)

core/common/src/DayOfWeek.kt

@@ -27,6 +27,6 @@ public val DayOfWeek.isoDayNumber: Int get() = ordinal + 1
  * Returns the [DayOfWeek] instance for the given ISO-8601 week day number. Monday is 1, Sunday is 7.
  */
 public fun DayOfWeek(isoDayNumber: Int): DayOfWeek {
-    require(isoDayNumber in 1..7)
+    require(isoDayNumber in 1..7) { "Expected ISO day-of-week number in 1..7, got $isoDayNumber" }
     return DayOfWeek.entries[isoDayNumber - 1]
 }

core/common/test/DayOfWeekTest.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2019-2024 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.test
+
+import kotlinx.datetime.*
+import kotlin.test.*
+
+class DayOfWeekTest {
+
+    @Test
+    fun testDayOfWeek() {
+        for (i in 1..7) {
+            assertEquals(i, DayOfWeek(i).isoDayNumber)
+        }
+        assertFailsWith<IllegalArgumentException> { DayOfWeek(-1) }
+        assertFailsWith<IllegalArgumentException> { DayOfWeek(8) }
+        assertFailsWith<IllegalArgumentException> { DayOfWeek(Int.MIN_VALUE) }
+    }
+}