fix(typescript): no loader required under Bun

lib/utils/loaderCheck.js

@@ -6,10 +6,16 @@
  * Check if a TypeScript loader is available for test files
  * Note: This checks if loaders are in the require array, not if packages are installed
  * Package installation is checked when actually requiring modules
+ * Always true under Bun, which transpiles TypeScript itself
  * @param {string[]} requiredModules - Array of required modules from config
  * @returns {boolean}
  */
 export function checkTypeScriptLoader(requiredModules = []) {
+  // Bun transpiles TypeScript natively, so no loader is needed.
+  // Node is not treated the same way: its native type stripping rejects enums
+  // and does not resolve extensionless relative imports.
+  if (process.versions.bun) return true
+
   // Check if a loader is configured in the require array
   return (
     requiredModules.includes('tsx/esm') ||

test/unit/utils/loaderCheck_test.js

@@ -0,0 +1,56 @@
+import { expect } from 'chai'
+import { checkTypeScriptLoader, validateTypeScriptSetup } from '../../../lib/utils/loaderCheck.js'
+
+describe('TypeScript loader check', () => {
+  const hadBun = 'bun' in process.versions
+  const originalBun = process.versions.bun
+
+  afterEach(() => {
+    if (hadBun) {
+      process.versions.bun = originalBun
+    } else {
+      delete process.versions.bun
+    }
+  })
+
+  describe('on Node', () => {
+    beforeEach(() => {
+      delete process.versions.bun
+    })
+
+    it('detects a configured loader', () => {
+      for (const loader of ['tsx/esm', 'tsx/cjs', 'tsx', 'ts-node/esm', 'ts-node/register', 'ts-node']) {
+        expect(checkTypeScriptLoader([loader]), loader).to.be.true
+      }
+    })
+
+    it('reports an error for TypeScript tests without a loader', () => {
+      expect(checkTypeScriptLoader([])).to.be.false
+
+      const validation = validateTypeScriptSetup(['basic_test.ts'], [])
+      expect(validation.hasError).to.be.true
+      expect(validation.message).to.include('TypeScript Test Files Detected')
+    })
+
+    it('passes when there are no TypeScript test files', () => {
+      expect(validateTypeScriptSetup(['basic_test.js'], []).hasError).to.be.false
+    })
+  })
+
+  describe('on Bun', () => {
+    beforeEach(() => {
+      process.versions.bun = '1.4.2'
+    })
+
+    // Bun transpiles TypeScript itself, so requiring tsx/ts-node is pointless (#5697)
+    it('needs no loader in the require array', () => {
+      expect(checkTypeScriptLoader([])).to.be.true
+      expect(validateTypeScriptSetup(['basic_test.ts'], []).hasError).to.be.false
+    })
+
+    it('still accepts a configured loader', () => {
+      expect(checkTypeScriptLoader(['tsx/esm'])).to.be.true
+      expect(validateTypeScriptSetup(['basic_test.ts'], ['tsx/esm']).hasError).to.be.false
+    })
+  })
+})