fix: Prevent `$state` `bigint`s incrementing and decrementing from throwing (#14485)

* Fix `$.update` and `$.update_pre` for bigints

* resolve conflicts

* fix some things

* fix thing i definitely didn't just break

* hopefully this will fix it

* fix formatting

* simplify

* style consistency

* simplify

* changeset

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/young-beds-beam.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly increment/decrement bigints

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

@@ -1068,23 +1068,32 @@ function get_parent_context(component_context) {
 }
 
 /**
- * @param {Value<number>} signal
+ * @template {number | bigint} T
+ * @param {Value<T>} signal
  * @param {1 | -1} [d]
- * @returns {number}
+ * @returns {T}
  */
 export function update(signal, d = 1) {
-	var value = +get(signal);
-	set(signal, value + d);
-	return value;
+	var value = get(signal);
+	var result = d === 1 ? value++ : value--;
+
+	set(signal, value);
+
+	// @ts-expect-error
+	return result;
 }
 
 /**
- * @param {Value<number>} signal
+ * @template {number | bigint} T
+ * @param {Value<T>} signal
  * @param {1 | -1} [d]
- * @returns {number}
+ * @returns {T}
  */
 export function update_pre(signal, d = 1) {
-	return set(signal, +get(signal) + d);
+	var value = get(signal);
+
+	// @ts-expect-error
+	return set(signal, d === 1 ? ++value : --value);
 }
 
 /**

packages/svelte/tests/signals/test.ts

@@ -765,4 +765,20 @@ describe('signals', () => {
 			assert.deepEqual(a.deriveds, null);
 		};
 	});
+
+	test('bigint states update correctly', () => {
+		return () => {
+			const count = state(0n);
+
+			assert.doesNotThrow(() => $.update(count));
+			assert.equal($.get(count), 1n);
+			assert.doesNotThrow(() => $.update(count, -1));
+			assert.equal($.get(count), 0n);
+
+			assert.doesNotThrow(() => $.update_pre(count));
+			assert.equal($.get(count), 1n);
+			assert.doesNotThrow(() => $.update_pre(count, -1));
+			assert.equal($.get(count), 0n);
+		};
+	});
 });