fix: make string coercion consistent to `toString` (#17774)

Closes #17758

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.changeset/thirty-groups-rhyme.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make string coercion consistent to `toString`

packages/svelte/src/internal/client/render.js

@@ -45,12 +45,12 @@ export function set_should_intro(value) {
  */
 export function set_text(text, value) {
 	// For objects, we apply string coercion (which might make things like $state array references in the template reactive) before diffing
-	var str = value == null ? '' : typeof value === 'object' ? value + '' : value;
+	var str = value == null ? '' : typeof value === 'object' ? `${value}` : value;
 	// @ts-expect-error
 	if (str !== (text.__t ??= text.nodeValue)) {
 		// @ts-expect-error
 		text.__t = str;
-		text.nodeValue = str + '';
+		text.nodeValue = `${str}`;
 	}
 }
 

packages/svelte/tests/runtime-runes/samples/set-text-stable-coercion/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+export default test({
+	html: 'toString <p>toString</p> <p>[toString]</p> toString',
+	async test() {}
+});

packages/svelte/tests/runtime-runes/samples/set-text-stable-coercion/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	class Widget {
+		toString() { return "toString" }
+		valueOf() { return "valueOf" }
+	}
+
+	const value = new Widget();
+</script>
+
+<!-- case 1 -->
+{value}
+
+<!-- case 2 -->
+<p>{value}</p>
+
+<!-- case 3 -->
+<p>[{value}]</p>
+
+<!-- case 4 -->
+{#if 1}
+	{value}
+{/if}