test: add custom extensions

Author: hanspagel

packages/openapi-types/src/openapi.test-d.ts

@@ -1,4 +1,4 @@
-import { expectTypeOf } from 'vitest'
+import { assertType, expectTypeOf } from 'vitest'
 import { describe, it } from 'vitest'
 
 import { OpenAPI, OpenAPIV2, OpenAPIV3, OpenAPIV3_1 } from './index'
@@ -35,4 +35,13 @@ describe('OpenAPI', () => {
 
     expectTypeOf(specification).toMatchTypeOf<OpenAPIV3_1.Document>()
   })
+
+  it('types a custom extension', () => {
+    const specification: OpenAPI.Document<{
+      'x-custom'?: boolean
+    }> = {}
+
+    expectTypeOf(specification['random-attribute']).toEqualTypeOf<any>()
+    expectTypeOf(specification['x-custom']).toEqualTypeOf<boolean>()
+  })
 })