Infer correct types when creating `MiddlewareHandler` dynamically

[yusukebe]

What is the feature you are proposing?

If you create a middleware from a function like a factory method, it can't infer types correctly in the handler:

const app = new Hono<{
  Variables: {
    foo: string
  }
}>()

type CreateMiddleware = <E extends Env, P extends string>(
  handler: (c: Context<E, P>) => any
) => MiddlewareHandler<E, P>

const createMiddleware: CreateMiddleware = () => {
  return async () => {}
}

app.get('/abc/:id', async (c) => {
  const foo = c.get('foo') // foo is string
  const id = c.req.param('id') // id is inferred correctly.
  type T = typeof id // string
})

app.get(
  '/abc/:id',
  createMiddleware(async (c) => {
    const foo = c.get('foo') // foo is never
    const id = c.req.param('id') // id is not inferred correctly.
    type T = typeof id // string | undefined
  })
)

This is a TypeScript limitation, but we have to find a way to resolve it.

Related to #3198