For decades we have been using assert(input) and assert(output), so it's not exactly new. Forcing sanitary checks by the language runtime is an option, hence requiring contracts to spell out the range of valid input/output which is checked in debug mode. Otherwise, if it's optional, it will only be used by a subset of engineers. If its mandatory, it can cripple performance (unless we have unsafe scopes). Opt-out is always better than opt-in.