[kccqzy]

Don't even need to go that far. In C++ it is common to delete the default constructor anyways. So that achieves the opt-in.

[dieortin]

Having to delete the default constructor means it’s opt-out, not opt-in

[kccqzy]

I don't think you really know what a real-life C++ code base looks like. Deleting the default constructor often happens implicitly with no syntax needed. This often happens when you define a custom constructor or when its member doesn't have a default constructor.