kotlin-test: make assertIs<T> returning value cast to T KT-45296

libraries/kotlin.test/common/src/main/kotlin/kotlin/test/Assertions.kt

@@ -87,9 +87,10 @@ fun <@OnlyInputTypes T> assertNotSame(illegal: T, actual: T, message: String? =
 @SinceKotlin("1.5")
 @InlineOnly
 @OptIn(ExperimentalStdlibApi::class)
-inline fun <reified T> assertIs(value: Any?, message: String? = null) {
+inline fun <reified T> assertIs(value: Any?, message: String? = null): T {
     contract { returns() implies (value is T) }
     assertIsOfType(value, typeOf<T>(), value is T, message)
+    return value as T
 }
 
 @PublishedApi

libraries/kotlin.test/common/src/test/kotlin/kotlin/test/tests/BasicAssertionsTest.kt

@@ -210,8 +210,10 @@ class BasicAssertionsTest {
     @Test
     fun testAssertIsOfType() {
         val s: Any = "test"
-        assertIs<String>(s)
+        val result = assertIs<String>(s)
         assertEquals(4, s.length)
+        assertEquals(s, result)
+        assertEquals(4, result.length)
     }
 
     @OptIn(ExperimentalStdlibApi::class)