Subclause 7.7 [expr.const] paragraph 2 defines "constant-initialized" using the following rule:

A variable or temporary object o is constant-initialized if

• either it has an initializer or its default-initialization results in some initialization being performed, and
• ...

However, the rules for constexpr are slightly different, per 9.2.6 [dcl.constexpr] paragraph 6:

A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized. ...

The difference manifests for an example such as:

  struct S {};
  int main() {
    constexpr S s;       // OK
    constexpr S s2 = s;  // error: s is not constant-initialized
  }

Is the difference intentional?