refactor: migrate checks.ts utility to core and deduplicate (#18139)

packages/core/src/utils/checks.test.ts

@@ -0,0 +1,32 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { checkExhaustive, assumeExhaustive } from './checks.js';
+
+describe('checks', () => {
+  describe('checkExhaustive', () => {
+    it('should throw an error with default message', () => {
+      expect(() => {
+        checkExhaustive('unexpected' as never);
+      }).toThrow('unexpected value unexpected!');
+    });
+
+    it('should throw an error with custom message', () => {
+      expect(() => {
+        checkExhaustive('unexpected' as never, 'custom message');
+      }).toThrow('custom message');
+    });
+  });
+
+  describe('assumeExhaustive', () => {
+    it('should do nothing', () => {
+      expect(() => {
+        assumeExhaustive('unexpected' as never);
+      }).not.toThrow();
+    });
+  });
+});

packages/core/src/utils/checks.ts

@@ -0,0 +1,28 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* Fail to compile on unexpected values. */
+export function assumeExhaustive(_value: never): void {}
+
+/**
+ * Throws an exception on unexpected values.
+ *
+ * A common use case is switch statements:
+ * switch(enumValue) {
+ *   case Enum.A:
+ *   case Enum.B:
+ *     break;
+ *   default:
+ *     checkExhaustive(enumValue);
+ * }
+ */
+export function checkExhaustive(
+  value: never,
+  msg = `unexpected value ${value}!`,
+): never {
+  assumeExhaustive(value);
+  throw new Error(msg);
+}