Constexpr conditions for constructor

Question

On this site, it is specified that:

"A constexpr function must satisfy the following requirements:

[...]

there exists at least one set of argument values such that an invocation of the function could be an evaluated subexpression of a core constant expression (for constructors, use in a constant initializer is sufficient) (since C++14). No diagnostic is required for a violation of this bullet."

What is the meaning of the bolded statement?

Answer 1

Looking at the linked defect report

struct X {
    std::unique_ptr<int> p;
    constexpr X() { }
};

Before C++14, this would be ill-formed due to [dcl.constexpr]

For a constexpr constructor, if no argument values exist such that after function invocation substitution, every constructor call and full-expression in the mem-initializers would be a constant expression (including conversions), the program is ill-formed; no diagnostic required.

Which mandates that there exists some argument (in this case, only the empty set) that can create a constant expression for the invocation of X::X, as in

constexpr X x;  // must be valid before C++14

Since std::unique_ptr isn't a literal type, it has a non-trivial destructor, this is impossible. Yet the defect report proposed that constexpr constructors should still be well-formed in such cases due to this kind of use case

X x;  // not constexpr, but initialization should be constant

Hence the rewording

For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, or, for a constructor, a constant initializer for some object , the program is ill-formed, no diagnostic required.

Translated, it means: a constexpr constructor is well-formed as long as it is a constexpr function, and its member initializations are also constexpr functions, even if the type itself can never be constexpr.