Why is this constexpr static member function not seen as constexpr when called?

Question

Why is this constexpr static member function, identified by the //! Nah comment, not seen as constexpr when called?

Answer 1

[class.mem]/2

Within the class member-specification, the class is regarded as complete within function bodies, default arguments, exception-specifications, and default member initializers (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.

In the initializer of a static data member of a class, the class is incomplete. The initializer can only see the declarations of members which precede it, and any member functions it can see are considered declared but not defined. A call to a function that is declared but not defined can not be a constant expression.