Makes BackStackScreen<*> practical

Adds a few crucial `out` declarations to various `Container` and `Wrapper` types, so that declaring `RenderingT` types like `BackStackScreen<*>` is practical. Also required moving `fun BackStackScreen.plus` out to be an extension function.

Without these changes, this does not compile:

```kotlin
    val foo = BackStackScreen(FooScreen(1))
    val bar = BackStackScreen(BarScreen(1))
    val both = foo + bar
```

Instead we were forced to do things like:

```kotlin
    val foo: BackStackScreen<Screen> = BackStackScreen(FooScreen(1))
    val bar: BackStackScreen<Screen> = BackStackScreen(BarScreen(1))
    val both = foo + bar
```

It gets really old really fast.

benchmarks/performance-poetry/complex-poetry/src/main/java/com/squareup/benchmarks/performance/complex/poetry/MaybeLoadingGatekeeperWorkflow.kt

@@ -17,7 +17,7 @@ typealias IsLoading = Boolean
 
 @OptIn(WorkflowUiExperimentalApi::class)
 class MaybeLoadingGatekeeperWorkflow<T : Any>(
-  private val childWithLoading: Workflow<T, Any, OverviewDetailScreen>,
+  private val childWithLoading: Workflow<T, Any, OverviewDetailScreen<*>>,
   private val childProps: T,
   private val isLoading: Flow<Boolean>
 ) : StatefulWorkflow<Unit, IsLoading, Unit, MayBeLoadingScreen>() {

benchmarks/performance-poetry/complex-poetry/src/main/java/com/squareup/benchmarks/performance/complex/poetry/PerformancePoemWorkflow.kt

@@ -32,7 +32,6 @@ import com.squareup.workflow1.WorkflowAction
 import com.squareup.workflow1.WorkflowAction.Companion.noAction
 import com.squareup.workflow1.action
 import com.squareup.workflow1.runningWorker
-import com.squareup.workflow1.ui.Screen
 import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow1.ui.container.BackStackScreen
 import com.squareup.workflow1.ui.container.toBackStackScreen
@@ -60,7 +59,7 @@ import kotlinx.coroutines.flow.flow
 class PerformancePoemWorkflow(
   private val simulatedPerfConfig: SimulatedPerfConfig = SimulatedPerfConfig.NO_SIMULATED_PERF,
   private val isLoading: MutableStateFlow<Boolean>,
-) : PoemWorkflow, StatefulWorkflow<Poem, State, ClosePoem, OverviewDetailScreen>() {
+) : PoemWorkflow, StatefulWorkflow<Poem, State, ClosePoem, OverviewDetailScreen<*>>() {
 
   sealed class State {
     val isLoading: Boolean = false
@@ -100,7 +99,7 @@ class PerformancePoemWorkflow(
     renderProps: Poem,
     renderState: State,
     context: RenderContext
-  ): OverviewDetailScreen {
+  ): OverviewDetailScreen<*> {
     if (simulatedPerfConfig.simultaneousActions > 0) {
       repeat(simulatedPerfConfig.simultaneousActions) { index ->
         context.runningWorker(
@@ -222,7 +221,7 @@ class PerformancePoemWorkflow(
           }
 
         val stackedStanzas = visibleStanza?.let {
-          (previousStanzas + visibleStanza).toBackStackScreen<Screen>()
+          (previousStanzas + visibleStanza).toBackStackScreen()
         }
 
         val stanzaListOverview =

benchmarks/performance-poetry/complex-poetry/src/main/java/com/squareup/benchmarks/performance/complex/poetry/PerformancePoemsBrowserWorkflow.kt

@@ -12,6 +12,7 @@ import com.squareup.benchmarks.performance.complex.poetry.instrumentation.Tracea
 import com.squareup.benchmarks.performance.complex.poetry.instrumentation.asTraceableWorker
 import com.squareup.benchmarks.performance.complex.poetry.views.BlankScreen
 import com.squareup.sample.container.overviewdetail.OverviewDetailScreen
+import com.squareup.sample.container.overviewdetail.plus
 import com.squareup.sample.poetry.ConfigAndPoems
 import com.squareup.sample.poetry.PoemListScreen.Companion.NO_POEM_SELECTED
 import com.squareup.sample.poetry.PoemListWorkflow
@@ -51,7 +52,7 @@ class PerformancePoemsBrowserWorkflow(
   private val isLoading: MutableStateFlow<Boolean>,
 ) :
   PoemsBrowserWorkflow,
-  StatefulWorkflow<ConfigAndPoems, State, Unit, OverviewDetailScreen>() {
+  StatefulWorkflow<ConfigAndPoems, State, Unit, OverviewDetailScreen<*>>() {
 
   sealed class State {
     object Recurse : State()
@@ -88,7 +89,7 @@ class PerformancePoemsBrowserWorkflow(
     renderProps: ConfigAndPoems,
     renderState: State,
     context: RenderContext
-  ): OverviewDetailScreen {
+  ): OverviewDetailScreen<*> {
     when (renderState) {
       is Recurse -> {
         val recursiveChild = PerformancePoemsBrowserWorkflow(
@@ -188,13 +189,13 @@ class PerformancePoemsBrowserWorkflow(
                 }
               }
             }
-            var poems = OverviewDetailScreen(
+            var poems: OverviewDetailScreen<*> = OverviewDetailScreen(
               overviewRendering = BackStackScreen(
                 poemListRendering.copy(selection = renderState.payload)
               )
             )
             if (renderState.payload != NO_POEM_SELECTED) {
-              val poem: OverviewDetailScreen = context.renderChild(
+              val poem = context.renderChild(
                 poemWorkflow,
                 renderProps.second[renderState.payload]
               ) { clearSelection }
@@ -209,7 +210,7 @@ class PerformancePoemsBrowserWorkflow(
                 poemListRendering.copy(selection = renderState.poemIndex)
               )
             )
-            val poem: OverviewDetailScreen = context.renderChild(
+            val poem = context.renderChild(
               poemWorkflow,
               renderProps.second[renderState.poemIndex]
             ) { clearSelection }

benchmarks/performance-poetry/complex-poetry/src/main/java/com/squareup/benchmarks/performance/complex/poetry/views/MayBeLoadingScreen.kt

@@ -8,11 +8,11 @@ import com.squareup.workflow1.ui.container.FullScreenModal
 
 @OptIn(WorkflowUiExperimentalApi::class)
 typealias MayBeLoadingScreen =
-  BodyAndOverlaysScreen<ScrimScreen<OverviewDetailScreen>, FullScreenModal<LoaderSpinner>>
+  BodyAndOverlaysScreen<ScrimScreen<OverviewDetailScreen<*>>, FullScreenModal<LoaderSpinner>>
 
 @OptIn(WorkflowUiExperimentalApi::class)
 fun MayBeLoadingScreen(
-  baseScreen: OverviewDetailScreen,
+  baseScreen: OverviewDetailScreen<*>,
   loaders: List<LoaderSpinner> = emptyList()
 ): MayBeLoadingScreen {
   return BodyAndOverlaysScreen(

samples/containers/android/src/main/java/com/squareup/sample/container/overviewdetail/OverviewDetailContainer.kt

@@ -14,6 +14,7 @@ import com.squareup.workflow1.ui.ViewEnvironment
 import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow1.ui.WorkflowViewStub
 import com.squareup.workflow1.ui.container.BackStackScreen
+import com.squareup.workflow1.ui.container.plus
 
 /**
  * Displays [OverviewDetailScreen] renderings in either split pane or single pane
@@ -25,7 +26,7 @@ import com.squareup.workflow1.ui.container.BackStackScreen
  * with [OverviewDetailScreen.overviewRendering] as the base of the stack.
  */
 @OptIn(WorkflowUiExperimentalApi::class)
-class OverviewDetailContainer(view: View) : ScreenViewRunner<OverviewDetailScreen> {
+class OverviewDetailContainer(view: View) : ScreenViewRunner<OverviewDetailScreen<*>> {
 
   private val overviewStub: WorkflowViewStub? = view.findViewById(R.id.overview_stub)
   private val detailStub: WorkflowViewStub? = view.findViewById(R.id.detail_stub)
@@ -42,7 +43,7 @@ class OverviewDetailContainer(view: View) : ScreenViewRunner<OverviewDetailScree
   }
 
   override fun showRendering(
-    rendering: OverviewDetailScreen,
+    rendering: OverviewDetailScreen<*>,
     environment: ViewEnvironment
   ) {
     if (singleStub == null) {
@@ -53,7 +54,7 @@ class OverviewDetailContainer(view: View) : ScreenViewRunner<OverviewDetailScree
   }
 
   private fun renderSplitView(
-    rendering: OverviewDetailScreen,
+    rendering: OverviewDetailScreen<*>,
     viewEnvironment: ViewEnvironment
   ) {
     if (rendering.detailRendering == null && rendering.selectDefault != null) {
@@ -81,7 +82,7 @@ class OverviewDetailContainer(view: View) : ScreenViewRunner<OverviewDetailScree
   }
 
   private fun renderSingleView(
-    rendering: OverviewDetailScreen,
+    rendering: OverviewDetailScreen<*>,
     viewEnvironment: ViewEnvironment,
     stub: WorkflowViewStub
   ) {
@@ -92,7 +93,7 @@ class OverviewDetailContainer(view: View) : ScreenViewRunner<OverviewDetailScree
     stub.show(combined, viewEnvironment + Single)
   }
 
-  companion object : ScreenViewFactory<OverviewDetailScreen> by ScreenViewFactory.fromLayout(
+  companion object : ScreenViewFactory<OverviewDetailScreen<*>> by ScreenViewFactory.fromLayout(
     layoutId = R.layout.overview_detail,
     constructor = ::OverviewDetailContainer
   )

samples/containers/android/src/main/java/com/squareup/sample/container/panel/PanelOverlayDialogFactory.kt

@@ -4,7 +4,6 @@ import android.content.Context
 import android.graphics.Rect
 import androidx.appcompat.app.AppCompatDialog
 import com.squareup.sample.container.R
-import com.squareup.workflow1.ui.Screen
 import com.squareup.workflow1.ui.ViewEnvironment
 import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow1.ui.container.OverlayDialogFactory
@@ -17,22 +16,22 @@ import kotlin.reflect.KClass
  * Android support for [PanelOverlay].
  */
 @OptIn(WorkflowUiExperimentalApi::class)
-internal object PanelOverlayDialogFactory : OverlayDialogFactory<PanelOverlay<Screen>> {
-  override val type: KClass<in PanelOverlay<Screen>> = PanelOverlay::class
+internal object PanelOverlayDialogFactory : OverlayDialogFactory<PanelOverlay<*>> {
+  override val type: KClass<in PanelOverlay<*>> = PanelOverlay::class
 
   override fun buildDialog(
-    initialRendering: PanelOverlay<Screen>,
+    initialRendering: PanelOverlay<*>,
     initialEnvironment: ViewEnvironment,
     context: Context
-  ): OverlayDialogHolder<PanelOverlay<Screen>> {
+  ): OverlayDialogHolder<PanelOverlay<*>> {
     val dialog = AppCompatDialog(context, R.style.PanelDialog)
 
     val realHolder = dialog.asDialogHolderWithContent(initialRendering, initialEnvironment)
 
     // We replace the default onUpdateBounds function with one that gives the
     // panel a square shape on tablets. See OverlayDialogFactory for more details
     // on the bounds mechanism.
-    return object : OverlayDialogHolder<PanelOverlay<Screen>> by realHolder {
+    return object : OverlayDialogHolder<PanelOverlay<*>> by realHolder {
       override val onUpdateBounds: ((Rect) -> Unit) = { bounds ->
         val refinedBounds: Rect = if (!dialog.context.isTablet) {
           // On a phone, fill the bounds entirely.

samples/containers/common/src/main/java/com/squareup/sample/container/overviewdetail/OverviewDetailScreen.kt

@@ -3,6 +3,7 @@ package com.squareup.sample.container.overviewdetail
 import com.squareup.workflow1.ui.Screen
 import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow1.ui.container.BackStackScreen
+import com.squareup.workflow1.ui.container.plus
 
 /**
  * Rendering type for overview / detail containers, with [BackStackScreen] in both roles.
@@ -16,51 +17,33 @@ import com.squareup.workflow1.ui.container.BackStackScreen
  * or a null [selectDefault]. **[selectDefault] cannot be a no-op.**
  */
 @OptIn(WorkflowUiExperimentalApi::class)
-class OverviewDetailScreen private constructor(
-  val overviewRendering: BackStackScreen<Screen>,
-  val detailRendering: BackStackScreen<Screen>? = null,
+class OverviewDetailScreen<out T : Screen> private constructor(
+  val overviewRendering: BackStackScreen<T>,
+  val detailRendering: BackStackScreen<T>? = null,
   val selectDefault: (() -> Unit)? = null
 ) : Screen {
   constructor(
-    overviewRendering: BackStackScreen<Screen>,
-    detailRendering: BackStackScreen<Screen>
+    overviewRendering: BackStackScreen<T>,
+    detailRendering: BackStackScreen<T>
   ) : this(overviewRendering, detailRendering, null)
 
   /**
    * @param selectDefault optional function that a split view container may call to request
    * that a selection be made to fill a null [detailRendering].
    */
   constructor(
-    overviewRendering: BackStackScreen<Screen>,
+    overviewRendering: BackStackScreen<T>,
     selectDefault: (() -> Unit)? = null
   ) : this(overviewRendering, null, selectDefault)
 
-  operator fun component1(): BackStackScreen<Screen> = overviewRendering
-  operator fun component2(): BackStackScreen<Screen>? = detailRendering
-
-  /**
-   * Returns a new [OverviewDetailScreen] appending the [overviewRendering] and
-   * [detailRendering] of [other] to those of the receiver. If the new screen's
-   * [detailRendering] is `null`, it will have the [selectDefault] function of [other].
-   */
-  operator fun plus(other: OverviewDetailScreen): OverviewDetailScreen {
-    val newOverview = overviewRendering + other.overviewRendering
-    val newDetail = detailRendering
-      ?.let { it + other.detailRendering }
-      ?: other.detailRendering
-
-    return if (newDetail == null) {
-      OverviewDetailScreen(newOverview, other.selectDefault)
-    } else {
-      OverviewDetailScreen(newOverview, newDetail)
-    }
-  }
+  operator fun component1(): BackStackScreen<T> = overviewRendering
+  operator fun component2(): BackStackScreen<T>? = detailRendering
 
   override fun equals(other: Any?): Boolean {
     if (this === other) return true
     if (javaClass != other?.javaClass) return false
 
-    other as OverviewDetailScreen
+    other as OverviewDetailScreen<*>
 
     return overviewRendering == other.overviewRendering &&
       detailRendering == other.detailRendering &&
@@ -80,3 +63,26 @@ class OverviewDetailScreen private constructor(
       "selectDefault=$selectDefault)"
   }
 }
+
+/**
+ * Returns a new [OverviewDetailScreen] appending the
+ * [overviewRendering][OverviewDetailScreen.overviewRendering] and
+ * [detailRendering][OverviewDetailScreen.detailRendering] of [other] to those of the receiver.
+ * If the new screen's `detailRendering` is `null`, it will have the
+ * [selectDefault][OverviewDetailScreen.selectDefault] function of [other].
+ */
+@OptIn(WorkflowUiExperimentalApi::class)
+operator fun <T : Screen> OverviewDetailScreen<T>.plus(
+  other: OverviewDetailScreen<T>
+): OverviewDetailScreen<T> {
+  val newOverview = overviewRendering + other.overviewRendering
+  val newDetail = detailRendering
+    ?.let { it + other.detailRendering }
+    ?: other.detailRendering
+
+  return if (newDetail == null) {
+    OverviewDetailScreen(newOverview, other.selectDefault)
+  } else {
+    OverviewDetailScreen(newOverview, newDetail)
+  }
+}

samples/containers/common/src/main/java/com/squareup/sample/container/panel/PanelOverlay.kt

@@ -6,7 +6,7 @@ import com.squareup.workflow1.ui.container.ModalOverlay
 import com.squareup.workflow1.ui.container.ScreenOverlay
 
 @OptIn(WorkflowUiExperimentalApi::class)
-class PanelOverlay<C : Screen>(
+class PanelOverlay<out C : Screen>(
   override val content: C
 ) : ScreenOverlay<C>, ModalOverlay {
   override fun <D : Screen> map(transform: (C) -> D): PanelOverlay<D> =