For constructors that can be called with a single argument, and can be called implicitly. For example:

struct A { auto A(int); };

would be equivalent to simply:

struct A { A(int); };

This would allows compilers to support an option to require that constructors callable with one argument (other that copy/move constructors) be preceded by either "explicit" or "auto", otherwise a diagnostic is output. This would be helpful to me and presumably others, who on occasion forget to make a conversion constructor explicit when it should be.