Create new extension function rethrow along with tests for it

kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Rethrow.kt

@@ -0,0 +1,11 @@
+package com.github.michaelbull.result
+
+/**
+ * If the [Result] is [Err] and contains a [Throwable], throws the [error][Err.error]
+ * if it is an instance of [E].
+ *
+ * @throws E if [error][Err.error] is an instance of [E]
+ */
+public inline fun <V: Any?, reified E : Throwable> Result<V, Throwable?>.rethrow()
+    : Result<V, Throwable?> =
+    mapError { e -> if (e is E) throw e else e }

kotlin-result/src/commonTest/kotlin/com/github/michaelbull/result/RethrowTest.kt

@@ -0,0 +1,43 @@
+package com.github.michaelbull.result
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+
+class RethrowTest {
+
+    @Test
+    fun throwsInstancesOfChosenThrowable() {
+        val result: Result<Any, Throwable> = Err(RuntimeException())
+
+        assertFailsWith<RuntimeException> {
+            result.rethrow<Any, Exception>()
+        }
+    }
+
+    @Test
+    fun nothingThrownIfWrappedExceptionIsNotInstance() {
+        val result: Result<Any, Exception> = Err(NullPointerException())
+
+        result.rethrow<Any, IllegalArgumentException>()
+    }
+
+    @Test
+    fun nothingThrownIfErrorIsNull() {
+        val result: Result<Any, Throwable?> = Err(null)
+
+        result.rethrow<Any, Throwable>()
+    }
+
+    @Test
+    fun doesntAlterOkValue() {
+        val result: Result<String, Throwable> = Ok("ok")
+
+        assertEquals(
+            expected = "ok",
+            actual = result.rethrow<String, Exception>().unwrap()
+        )
+    }
+
+}
+