On Saturday, June 8, 2024, Robert A.H. Leahy wrote:

Classes with unconstrained unary constructors are just broken, as you've discovered.

So then should we write a paper to deprecate unary unconstrained constructors in C++26?

I don't think there's anything wrong with unary unconstrained constructors. I see how they're inconvenient for 'std::elide', but doesn't make them inherently broken.