fix: migrate default slots to children snippet (#13760)

Closes #13758

.changeset/five-dingos-push.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: migrate default slots to children snippet

packages/svelte/src/compiler/migrate/index.js

@@ -1152,6 +1152,10 @@ function migrate_slot_usage(node, path, state) {
 			is_text_attribute(attribute)
 		) {
 			snippet_name = attribute.value[0].data;
+			// the default slot in svelte 4 if what the children slot is for svelte 5
+			if (snippet_name === 'default') {
+				snippet_name = 'children';
+			}
 			if (!regex_is_valid_identifier.test(snippet_name)) {
 				has_migration_task = true;
 				state.str.appendLeft(

packages/svelte/tests/migrate/samples/slot-usages/input.svelte

@@ -77,6 +77,14 @@
     <div slot="bar">bar</div>
 </Component>
 
+<Component>
+	<span slot="default">should be children</span>
+</Component>
+
+<Component>
+	<span slot="default" let:with_prop>should be children {with_prop} too</span>
+</Component>
+
 <c-e>
     <div slot="named">unchanged</div>
 </c-e>

packages/svelte/tests/migrate/samples/slot-usages/output.svelte

@@ -121,6 +121,16 @@
     {/snippet}
 </Component>
 
+<Component>
+	<span >should be children</span>
+</Component>
+
+<Component>
+	<span  >{#snippet children({ with_prop })}
+                should be children {with_prop} too            {/snippet}
+        </span>
+</Component>
+
 <c-e>
     <div slot="named">unchanged</div>
 </c-e>