Require strict return type for params.stringify

[caillou]

Motivation

• I opened a bug report for something that was a user error (Link bug with splat and params.stringify #3650).
• As of now, this user-error is not caught by the type system.

Change Request

Say we have the following route: foo/$bar/$

As of now, I can do the following, and the type system does not complain:

params: {
  stringify: () => ({})
}

Ideally, the type system would require for me to return an object that contains _splat.