fix: prevent erroneous state_referenced_locally warnings on prop fallbacks (#17329)

* failing test

* fix: prevent erroneous `state_referenced_locally` warnings on prop fallbacks

Author: Rich-Harris

.changeset/rude-points-attend.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent erroneous `state_referenced_locally` warnings on prop fallbacks

packages/svelte/src/compiler/phases/2-analyze/visitors/VariableDeclarator.js

@@ -146,5 +146,15 @@ export function VariableDeclarator(node, context) {
 		}
 	}
 
-	context.next();
+	if (node.init && get_rune(node.init, context.state.scope) === '$props') {
+		// prevent erroneous `state_referenced_locally` warnings on prop fallbacks
+		context.visit(node.id, {
+			...context.state,
+			function_depth: context.state.function_depth + 1
+		});
+
+		context.visit(node.init);
+	} else {
+		context.next();
+	}
 }

packages/svelte/tests/validator/samples/static-state-reference/input.svelte

@@ -9,7 +9,8 @@
 	console.log(doubled);
 
 	let {
-		prop
+		prop,
+		other_prop = prop
 	} = $props();
 	let prop_state = $state(prop);
 	let prop_derived = $derived(prop);

packages/svelte/tests/validator/samples/static-state-reference/warnings.json

@@ -39,36 +39,36 @@
 		"code": "state_referenced_locally",
 		"end": {
 			"column": 29,
-			"line": 14
+			"line": 15
 		},
 		"message": "This reference only captures the initial value of `prop`. Did you mean to reference it inside a closure instead?",
 		"start": {
 			"column": 25,
-			"line": 14
+			"line": 15
 		}
 	},
 	{
 		"code": "state_referenced_locally",
 		"end": {
 			"column": 17,
-			"line": 16
+			"line": 17
 		},
 		"message": "This reference only captures the initial value of `prop`. Did you mean to reference it inside a closure instead?",
 		"start": {
 			"column": 13,
-			"line": 16
+			"line": 17
 		}
 	},
 	{
 		"code": "state_referenced_locally",
 		"end": {
 			"column": 25,
-			"line": 17
+			"line": 18
 		},
 		"message": "This reference only captures the initial value of `prop_derived`. Did you mean to reference it inside a closure instead?",
 		"start": {
 			"column": 13,
-			"line": 17
+			"line": 18
 		}
 	}
 ]