diff --git a/build.gradle.kts b/build.gradle.kts
index 6c4cff979..b78dad738 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -113,10 +113,12 @@ subprojects {
 
   tasks.withType<KotlinCompile> {
     kotlinOptions {
+      apiVersion = "1.4"
       jvmTarget = JavaVersion.VERSION_1_8.toString()
+
       freeCompilerArgs = listOf(
         "-Xjvm-default=compatibility",
-        "-Xopt-in=kotlin.RequiresOptIn"
+        "-opt-in=kotlin.RequiresOptIn"
       )
     }
   }
diff --git a/mockwebserver/src/main/kotlin/mockwebserver3/MockWebServer.kt b/mockwebserver/src/main/kotlin/mockwebserver3/MockWebServer.kt
index 5add3a87a..a485b85a4 100644
--- a/mockwebserver/src/main/kotlin/mockwebserver3/MockWebServer.kt
+++ b/mockwebserver/src/main/kotlin/mockwebserver3/MockWebServer.kt
@@ -75,6 +75,7 @@ import okhttp3.internal.http2.Header
 import okhttp3.internal.http2.Http2Connection
 import okhttp3.internal.http2.Http2Stream
 import okhttp3.internal.immutableListOf
+import okhttp3.internal.lowercase
 import okhttp3.internal.platform.Platform
 import okhttp3.internal.threadFactory
 import okhttp3.internal.toImmutableList
diff --git a/okhttp-coroutines/src/jvmTest/kotlin/okhttp3/SuspendCallTest.kt b/okhttp-coroutines/src/jvmTest/kotlin/okhttp3/SuspendCallTest.kt
index 999984588..230d1ea47 100644
--- a/okhttp-coroutines/src/jvmTest/kotlin/okhttp3/SuspendCallTest.kt
+++ b/okhttp-coroutines/src/jvmTest/kotlin/okhttp3/SuspendCallTest.kt
@@ -15,7 +15,7 @@
  *
  */
 
-@file:OptIn(ExperimentalCoroutinesApi::class)
+@file:OptIn(ExperimentalCoroutinesApi::class, ExperimentalTime::class)
 
 package okhttp3
 
@@ -39,6 +39,7 @@ import org.junit.jupiter.api.fail
 import java.io.IOException
 import java.util.concurrent.TimeUnit
 import kotlin.time.Duration.Companion.seconds
+import kotlin.time.ExperimentalTime
 
 @ExtendWith(MockWebServerExtension::class)
 class SuspendCallTest(
diff --git a/okhttp-sse/src/main/kotlin/okhttp3/sse/internal/ServerSentEventReader.kt b/okhttp-sse/src/main/kotlin/okhttp3/sse/internal/ServerSentEventReader.kt
index 53d60aea0..1e3f9f362 100644
--- a/okhttp-sse/src/main/kotlin/okhttp3/sse/internal/ServerSentEventReader.kt
+++ b/okhttp-sse/src/main/kotlin/okhttp3/sse/internal/ServerSentEventReader.kt
@@ -16,6 +16,7 @@
 package okhttp3.sse.internal
 
 import java.io.IOException
+import okhttp3.internal.code
 import okhttp3.internal.toLongOrDefault
 import okio.Buffer
 import okio.BufferedSource
diff --git a/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerReader.kt b/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerReader.kt
index b51a19dca..eb63fdb27 100644
--- a/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerReader.kt
+++ b/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerReader.kt
@@ -17,6 +17,7 @@ package okhttp3.tls.internal.der
 
 import java.math.BigInteger
 import java.net.ProtocolException
+import okhttp3.internal.code
 import okio.Buffer
 import okio.BufferedSource
 import okio.ByteString
diff --git a/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerWriter.kt b/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerWriter.kt
index 7db879bfa..62b959612 100644
--- a/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerWriter.kt
+++ b/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/DerWriter.kt
@@ -16,6 +16,7 @@
 package okhttp3.tls.internal.der
 
 import java.math.BigInteger
+import okhttp3.internal.code
 import okio.Buffer
 import okio.BufferedSink
 import okio.ByteString
diff --git a/okhttp/src/commonMain/kotlin/okhttp3/CacheControl.kt b/okhttp/src/commonMain/kotlin/okhttp3/CacheControl.kt
index b6b7fc6e4..434c8658d 100644
--- a/okhttp/src/commonMain/kotlin/okhttp3/CacheControl.kt
+++ b/okhttp/src/commonMain/kotlin/okhttp3/CacheControl.kt
@@ -13,9 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+@file:OptIn(ExperimentalTime::class)
+
 package okhttp3
 
 import kotlin.time.DurationUnit
+import kotlin.time.ExperimentalTime
 
 /**
  * A Cache-Control header with cache directives from a server or client. These directives set policy
diff --git a/okhttp/src/commonMain/kotlin/okhttp3/internal/-CacheControlCommon.kt b/okhttp/src/commonMain/kotlin/okhttp3/internal/-CacheControlCommon.kt
index 456b578d1..312a00b0c 100644
--- a/okhttp/src/commonMain/kotlin/okhttp3/internal/-CacheControlCommon.kt
+++ b/okhttp/src/commonMain/kotlin/okhttp3/internal/-CacheControlCommon.kt
@@ -13,9 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+@file:OptIn(ExperimentalTime::class)
+
 package okhttp3.internal
 
 import kotlin.time.DurationUnit
+import kotlin.time.ExperimentalTime
 import kotlin.time.toDuration
 import okhttp3.CacheControl
 import okhttp3.Headers
diff --git a/okhttp/src/commonMain/kotlin/okhttp3/internal/-UtilCommon.kt b/okhttp/src/commonMain/kotlin/okhttp3/internal/-UtilCommon.kt
index 0a3bf3588..aab0547e6 100644
--- a/okhttp/src/commonMain/kotlin/okhttp3/internal/-UtilCommon.kt
+++ b/okhttp/src/commonMain/kotlin/okhttp3/internal/-UtilCommon.kt
@@ -394,3 +394,17 @@ internal fun <T> interleave(a: Iterable<T>, b: Iterable<T>): List<T> {
     }
   }
 }
+
+@Suppress("DEPRECATION")
+inline val Char.code
+  get() = this.toInt()
+
+expect inline fun String.lowercase(): String
+
+expect inline fun String.uppercase(): String
+
+inline fun <E> buildList(builderAction: MutableList<E>.() -> Unit): List<E> {
+  return mutableListOf<E>().apply {
+    builderAction()
+  }
+}
diff --git a/okhttp/src/commonTest/kotlin/okhttp3/CacheControlTest.kt b/okhttp/src/commonTest/kotlin/okhttp3/CacheControlTest.kt
index 162f5b9ac..d4b9143b8 100644
--- a/okhttp/src/commonTest/kotlin/okhttp3/CacheControlTest.kt
+++ b/okhttp/src/commonTest/kotlin/okhttp3/CacheControlTest.kt
@@ -13,6 +13,9 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+@file:OptIn(ExperimentalTime::class)
+
 package okhttp3
 
 import assertk.assertThat
@@ -21,6 +24,7 @@ import assertk.assertions.isFalse
 import assertk.assertions.isTrue
 import kotlin.test.Test
 import kotlin.time.DurationUnit
+import kotlin.time.ExperimentalTime
 
 class CacheControlTest {
   @Test
diff --git a/okhttp/src/commonTest/kotlin/okhttp3/MediaTypeTest.kt b/okhttp/src/commonTest/kotlin/okhttp3/MediaTypeTest.kt
index 77e20e04b..9931205c7 100644
--- a/okhttp/src/commonTest/kotlin/okhttp3/MediaTypeTest.kt
+++ b/okhttp/src/commonTest/kotlin/okhttp3/MediaTypeTest.kt
@@ -20,6 +20,7 @@ import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNull
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
+import okhttp3.internal.uppercase
 
 /**
  * Test MediaType API and parsing.
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/Cache.kt b/okhttp/src/jvmMain/kotlin/okhttp3/Cache.kt
index 78bc29aa3..7bf0f2cb1 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/Cache.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/Cache.kt
@@ -31,6 +31,7 @@ import okhttp3.internal.cache.CacheRequest
 import okhttp3.internal.cache.CacheStrategy
 import okhttp3.internal.cache.DiskLruCache
 import okhttp3.internal.closeQuietly
+import okhttp3.internal.code
 import okhttp3.internal.concurrent.TaskRunner
 import okhttp3.internal.http.HttpMethod
 import okhttp3.internal.http.StatusLine
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/CacheControl.kt b/okhttp/src/jvmMain/kotlin/okhttp3/CacheControl.kt
index bbd45b5e5..ad0ed0f76 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/CacheControl.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/CacheControl.kt
@@ -13,10 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+@file:OptIn(ExperimentalTime::class)
+
 package okhttp3
 
 import java.util.concurrent.TimeUnit
 import kotlin.time.DurationUnit
+import kotlin.time.ExperimentalTime
 import okhttp3.internal.commonBuild
 import okhttp3.internal.commonClampToInt
 import okhttp3.internal.commonForceCache
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/Challenge.kt b/okhttp/src/jvmMain/kotlin/okhttp3/Challenge.kt
index d2d9593af..73e736810 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/Challenge.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/Challenge.kt
@@ -23,6 +23,7 @@ import kotlin.text.Charsets.ISO_8859_1
 import okhttp3.internal.commonEquals
 import okhttp3.internal.commonHashCode
 import okhttp3.internal.commonToString
+import okhttp3.internal.lowercase
 
 actual class Challenge actual constructor(
   @get:JvmName("scheme") actual val scheme: String,
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/Cookie.kt b/okhttp/src/jvmMain/kotlin/okhttp3/Cookie.kt
index fbe8b5c8f..60dd32723 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/Cookie.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/Cookie.kt
@@ -23,10 +23,12 @@ import java.util.Locale
 import java.util.regex.Pattern
 import okhttp3.internal.UTC
 import okhttp3.internal.canParseAsIpAddress
+import okhttp3.internal.code
 import okhttp3.internal.delimiterOffset
 import okhttp3.internal.http.MAX_DATE
 import okhttp3.internal.http.toHttpDateString
 import okhttp3.internal.indexOfControlOrNonAscii
+import okhttp3.internal.lowercase
 import okhttp3.internal.publicsuffix.PublicSuffixDatabase
 import okhttp3.internal.toCanonicalHost
 import okhttp3.internal.trimSubstring
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/FormBody.kt b/okhttp/src/jvmMain/kotlin/okhttp3/FormBody.kt
index 747086a8d..107c22c4b 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/FormBody.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/FormBody.kt
@@ -21,6 +21,7 @@ import okhttp3.HttpUrl.Companion.FORM_ENCODE_SET
 import okhttp3.HttpUrl.Companion.canonicalize
 import okhttp3.HttpUrl.Companion.percentDecode
 import okhttp3.MediaType.Companion.toMediaType
+import okhttp3.internal.code
 import okhttp3.internal.toImmutableList
 import okio.Buffer
 import okio.BufferedSink
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/Headers.kt b/okhttp/src/jvmMain/kotlin/okhttp3/Headers.kt
index 9f7358d77..295cfaede 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/Headers.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/Headers.kt
@@ -46,6 +46,7 @@ import okhttp3.internal.headersCheckName
 import okhttp3.internal.http.toHttpDateOrNull
 import okhttp3.internal.http.toHttpDateString
 import okhttp3.internal.http2.Header
+import okhttp3.internal.lowercase
 import org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement
 
 @Suppress("NAME_SHADOWING")
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/HttpUrl.kt b/okhttp/src/jvmMain/kotlin/okhttp3/HttpUrl.kt
index 2b8723e82..5f851e637 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/HttpUrl.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/HttpUrl.kt
@@ -26,6 +26,7 @@ import kotlin.text.Charsets.UTF_8
 import okhttp3.HttpUrl.Companion.toHttpUrl
 import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
 import okhttp3.internal.canParseAsIpAddress
+import okhttp3.internal.code
 import okhttp3.internal.delimiterOffset
 import okhttp3.internal.indexOfFirstNonAsciiWhitespace
 import okhttp3.internal.indexOfLastNonAsciiWhitespace
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/MultipartBody.kt b/okhttp/src/jvmMain/kotlin/okhttp3/MultipartBody.kt
index 93d685f86..99a6d73a0 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/MultipartBody.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/MultipartBody.kt
@@ -18,6 +18,7 @@ package okhttp3
 import java.io.IOException
 import java.util.UUID
 import okhttp3.MediaType.Companion.toMediaType
+import okhttp3.internal.code
 import okhttp3.internal.toImmutableList
 import okio.Buffer
 import okio.BufferedSink
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/-UtilJvm.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/-UtilJvm.kt
index 5362de662..89e356e1c 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/-UtilJvm.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/-UtilJvm.kt
@@ -313,3 +313,10 @@ internal inline fun Any.assertThreadDoesntHoldLock() {
     throw AssertionError("Thread ${Thread.currentThread().name} MUST NOT hold lock on $this")
   }
 }
+actual inline fun String.lowercase() = this.toLowerCase(Locale.ROOT)
+
+actual inline fun String.uppercase() = this.toUpperCase(Locale.ROOT)
+
+fun String.lowercase(locale: Locale) = this.toLowerCase(locale)
+
+fun String.uppercase(locale: Locale) = this.toUpperCase(locale)
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/cache/DiskLruCache.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/cache/DiskLruCache.kt
index c227de098..605768a79 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/cache/DiskLruCache.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/cache/DiskLruCache.kt
@@ -22,6 +22,7 @@ import java.io.IOException
 import okhttp3.internal.assertThreadHoldsLock
 import okhttp3.internal.cache.DiskLruCache.Editor
 import okhttp3.internal.closeQuietly
+import okhttp3.internal.code
 import okhttp3.internal.concurrent.Task
 import okhttp3.internal.concurrent.TaskRunner
 import okhttp3.internal.deleteContents
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/http/HttpHeaders.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/http/HttpHeaders.kt
index f149430c6..71ba05870 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/http/HttpHeaders.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/http/HttpHeaders.kt
@@ -27,6 +27,7 @@ import okhttp3.CookieJar
 import okhttp3.Headers
 import okhttp3.HttpUrl
 import okhttp3.Response
+import okhttp3.internal.code
 import okhttp3.internal.headersContentLength
 import okhttp3.internal.platform.Platform
 import okhttp3.internal.skipAll
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Hpack.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Hpack.kt
index 6ec4e3ea5..9b219eaa8 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Hpack.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Hpack.kt
@@ -20,6 +20,7 @@ import java.util.Arrays
 import java.util.Collections
 import java.util.LinkedHashMap
 import okhttp3.internal.and
+import okhttp3.internal.code
 import okhttp3.internal.http2.Header.Companion.RESPONSE_STATUS
 import okhttp3.internal.http2.Header.Companion.TARGET_AUTHORITY
 import okhttp3.internal.http2.Header.Companion.TARGET_METHOD
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Http2ExchangeCodec.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Http2ExchangeCodec.kt
index bb463eb7a..63374e4c1 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Http2ExchangeCodec.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/http2/Http2ExchangeCodec.kt
@@ -42,6 +42,7 @@ import okhttp3.internal.http2.Header.Companion.TARGET_PATH_UTF8
 import okhttp3.internal.http2.Header.Companion.TARGET_SCHEME
 import okhttp3.internal.http2.Header.Companion.TARGET_SCHEME_UTF8
 import okhttp3.internal.immutableListOf
+import okhttp3.internal.lowercase
 import okio.Sink
 import okio.Source
 
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt
index 0ed111fe4..60b02fb89 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt
@@ -21,6 +21,7 @@ import java.net.IDN
 import java.util.concurrent.CountDownLatch
 import java.util.concurrent.atomic.AtomicBoolean
 import okhttp3.internal.and
+import okhttp3.internal.code
 import okhttp3.internal.platform.Platform
 import okio.GzipSource
 import okio.buffer
diff --git a/okhttp/src/jvmMain/kotlin/okhttp3/internal/tls/OkHostnameVerifier.kt b/okhttp/src/jvmMain/kotlin/okhttp3/internal/tls/OkHostnameVerifier.kt
index a43791bb2..6ef6e876a 100644
--- a/okhttp/src/jvmMain/kotlin/okhttp3/internal/tls/OkHostnameVerifier.kt
+++ b/okhttp/src/jvmMain/kotlin/okhttp3/internal/tls/OkHostnameVerifier.kt
@@ -25,6 +25,7 @@ import java.util.Locale
 import javax.net.ssl.HostnameVerifier
 import javax.net.ssl.SSLException
 import javax.net.ssl.SSLSession
+import okhttp3.internal.lowercase
 
 /**
  * A HostnameVerifier consistent with [RFC 2818][rfc_2818].
diff --git a/okhttp/src/jvmTest/java/okhttp3/CallTest.kt b/okhttp/src/jvmTest/java/okhttp3/CallTest.kt
index ed9be3ebd..571abc80b 100644
--- a/okhttp/src/jvmTest/java/okhttp3/CallTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/CallTest.kt
@@ -67,6 +67,7 @@ import okhttp3.internal.DoubleInetAddressDns
 import okhttp3.internal.RecordingOkAuthenticator
 import okhttp3.internal.addHeaderLenient
 import okhttp3.internal.closeQuietly
+import okhttp3.internal.code
 import okhttp3.internal.http.RecordingProxySelector
 import okhttp3.internal.userAgent
 import okhttp3.okio.LoggingFilesystem
diff --git a/okhttp/src/jvmTest/java/okhttp3/ResponseBodyJvmTest.kt b/okhttp/src/jvmTest/java/okhttp3/ResponseBodyJvmTest.kt
index f1e6187fd..285bbc42b 100644
--- a/okhttp/src/jvmTest/java/okhttp3/ResponseBodyJvmTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/ResponseBodyJvmTest.kt
@@ -32,6 +32,7 @@ import java.lang.StringBuilder
 import java.nio.charset.StandardCharsets
 import java.util.concurrent.atomic.AtomicBoolean
 import okhttp3.internal.and
+import okhttp3.internal.code
 import okio.ByteString.Companion.decodeHex
 import org.assertj.core.api.Assertions.assertThat
 
diff --git a/okhttp/src/jvmTest/java/okhttp3/URLConnectionTest.kt b/okhttp/src/jvmTest/java/okhttp3/URLConnectionTest.kt
index 24f6b92ba..d4a0dbdc7 100644
--- a/okhttp/src/jvmTest/java/okhttp3/URLConnectionTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/URLConnectionTest.kt
@@ -63,8 +63,10 @@ import okhttp3.TestUtil.assertSuppressed
 import okhttp3.internal.RecordingAuthenticator
 import okhttp3.internal.RecordingOkAuthenticator
 import okhttp3.internal.addHeaderLenient
+import okhttp3.internal.code
 import okhttp3.internal.http.HTTP_PERM_REDIRECT
 import okhttp3.internal.http.HTTP_TEMP_REDIRECT
+import okhttp3.internal.lowercase
 import okhttp3.internal.platform.Platform.Companion.get
 import okhttp3.internal.userAgent
 import okhttp3.testing.Flaky
diff --git a/okhttp/src/jvmTest/java/okhttp3/UrlComponentEncodingTester.kt b/okhttp/src/jvmTest/java/okhttp3/UrlComponentEncodingTester.kt
index c82691793..d9a712663 100644
--- a/okhttp/src/jvmTest/java/okhttp3/UrlComponentEncodingTester.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/UrlComponentEncodingTester.kt
@@ -17,7 +17,9 @@ package okhttp3
 
 import okhttp3.HttpUrl.Companion.toHttpUrl
 import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
+import okhttp3.internal.code
 import okhttp3.internal.format
+import okhttp3.internal.lowercase
 import okio.Buffer
 import okio.ByteString.Companion.encodeUtf8
 import org.junit.jupiter.api.Assertions.fail
diff --git a/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt b/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt
index 458d3f91d..da5a0aa77 100644
--- a/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt
@@ -15,6 +15,7 @@
  */
 package okhttp3
 
+import okhttp3.internal.code
 import okhttp3.internal.format
 import okio.Buffer
 import okio.BufferedSource
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/cache/DiskLruCacheTest.kt b/okhttp/src/jvmTest/java/okhttp3/internal/cache/DiskLruCacheTest.kt
index a96d741f7..4e151af95 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/cache/DiskLruCacheTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/cache/DiskLruCacheTest.kt
@@ -41,6 +41,7 @@ import java.io.FileNotFoundException
 import java.io.IOException
 import java.util.ArrayDeque
 import java.util.NoSuchElementException
+import okhttp3.internal.code
 
 class FileSystemParamProvider: SimpleProvider() {
   override fun arguments() = listOf(
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/http2/Http2ConnectionTest.kt b/okhttp/src/jvmTest/java/okhttp3/internal/http2/Http2ConnectionTest.kt
index 8e9b22364..a4f26f974 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/http2/Http2ConnectionTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/http2/Http2ConnectionTest.kt
@@ -26,6 +26,7 @@ import okhttp3.TestUtil.headerEntries
 import okhttp3.TestUtil.repeat
 import okhttp3.internal.EMPTY_BYTE_ARRAY
 import okhttp3.internal.EMPTY_HEADERS
+import okhttp3.internal.code
 import okhttp3.internal.concurrent.TaskFaker
 import okhttp3.internal.concurrent.TaskRunner
 import okhttp3.internal.notifyAll
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/http2/HttpOverHttp2Test.kt b/okhttp/src/jvmTest/java/okhttp3/internal/http2/HttpOverHttp2Test.kt
index faa0c08e2..763a99111 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/http2/HttpOverHttp2Test.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/http2/HttpOverHttp2Test.kt
@@ -65,6 +65,7 @@ import okhttp3.TestUtil.repeat
 import okhttp3.internal.DoubleInetAddressDns
 import okhttp3.internal.EMPTY_REQUEST
 import okhttp3.internal.RecordingOkAuthenticator
+import okhttp3.internal.code
 import okhttp3.internal.connection.RealConnection
 import okhttp3.internal.discard
 import okhttp3.testing.Flaky
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/idn/Punycode.kt b/okhttp/src/jvmTest/java/okhttp3/internal/idn/Punycode.kt
index 5d8718828..c64b618ab 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/idn/Punycode.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/idn/Punycode.kt
@@ -15,6 +15,7 @@
  */
 package okhttp3.internal.idn
 
+import okhttp3.internal.code
 import okio.Buffer
 
 /**
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/idn/StringprepTablesReaderTest.kt b/okhttp/src/jvmTest/java/okhttp3/internal/idn/StringprepTablesReaderTest.kt
index 460787216..41a912cf1 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/idn/StringprepTablesReaderTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/idn/StringprepTablesReaderTest.kt
@@ -17,6 +17,7 @@ package okhttp3.internal.idn
 
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import okhttp3.internal.code
 import okio.Buffer
 import okio.FileSystem
 import okio.Path.Companion.toPath
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/idn/stringprepTablesReader.kt b/okhttp/src/jvmTest/java/okhttp3/internal/idn/stringprepTablesReader.kt
index 17a092c3b..b22aebb10 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/idn/stringprepTablesReader.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/idn/stringprepTablesReader.kt
@@ -15,6 +15,7 @@
  */
 package okhttp3.internal.idn
 
+import okhttp3.internal.code
 import okio.Buffer
 import okio.BufferedSource
 import okio.ByteString.Companion.encodeUtf8
diff --git a/okhttp/src/jvmTest/java/okhttp3/internal/ws/WebSocketWriterTest.kt b/okhttp/src/jvmTest/java/okhttp3/internal/ws/WebSocketWriterTest.kt
index 6b9a9b34b..94562f0d5 100644
--- a/okhttp/src/jvmTest/java/okhttp3/internal/ws/WebSocketWriterTest.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/internal/ws/WebSocketWriterTest.kt
@@ -17,6 +17,7 @@ package okhttp3.internal.ws
 
 import java.util.Random
 import okhttp3.TestUtil.repeat
+import okhttp3.internal.code
 import okhttp3.internal.format
 import okhttp3.internal.ws.WebSocketProtocol.OPCODE_BINARY
 import okhttp3.internal.ws.WebSocketProtocol.OPCODE_TEXT
diff --git a/okhttp/src/nonJvmMain/kotlin/okhttp3/internal/-UtilNonJvm.kt b/okhttp/src/nonJvmMain/kotlin/okhttp3/internal/-UtilNonJvm.kt
index 282d4edcf..72de79033 100644
--- a/okhttp/src/nonJvmMain/kotlin/okhttp3/internal/-UtilNonJvm.kt
+++ b/okhttp/src/nonJvmMain/kotlin/okhttp3/internal/-UtilNonJvm.kt
@@ -16,4 +16,11 @@
 
 package okhttp3.internal
 
+import kotlin.text.lowercase as kotlinLowerCase
+import kotlin.text.uppercase as kotlinUpperCase
+
 actual typealias HttpUrlRepresentation = String
+
+actual inline fun String.lowercase() = this.kotlinLowerCase()
+
+actual inline fun String.uppercase() = this.kotlinUpperCase()