only create fallback content if no slot is provided - fixes #2354

Author: Rich-Harris

src/compile/render-dom/wrappers/Slot.ts

@@ -106,6 +106,7 @@ export default class SlotWrapper extends Wrapper {
 		let mount_before = block.builders.mount.toString();
 
 		block.builders.create.push_condition(`!${slot}`);
+		block.builders.claim.push_condition(`!${slot}`);
 		block.builders.hydrate.push_condition(`!${slot}`);
 		block.builders.mount.push_condition(`!${slot}`);
 		block.builders.update.push_condition(`!${slot}`);
@@ -118,6 +119,7 @@ export default class SlotWrapper extends Wrapper {
 		block.event_listeners = listeners;
 
 		block.builders.create.pop_condition();
+		block.builders.claim.pop_condition();
 		block.builders.hydrate.pop_condition();
 		block.builders.mount.pop_condition();
 		block.builders.update.pop_condition();

test/hydration/samples/text-fallback/Nested.svelte

@@ -0,0 +1,7 @@
+<div>
+	<slot name="foo">
+		foo fallback
+	</slot>
+
+	<slot></slot>
+</div>

test/hydration/samples/text-fallback/_after.html

@@ -0,0 +1,4 @@
+<div>
+	<div slot="foo">foo override</div>
+	default
+</div>

test/hydration/samples/text-fallback/_before.html

@@ -0,0 +1,4 @@
+<div>
+	<div slot="foo">foo override</div>
+	default
+</div>

test/hydration/samples/text-fallback/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Nested from './Nested.svelte'
+</script>
+
+<Nested>
+	<div slot="foo">
+		foo override
+	</div>
+
+	default
+</Nested>