According to 7.7 [expr.const] bullet 5.4, an expression is a constant expression unless (among other reasons) it would evaluate

• an invocation of an undefined constexpr function

This does not address the question of the point at which a constexpr function must be defined. The intent, in order to allow mutually-recursive constexpr functions, was that the function must be defined prior to the outermost evaluation that eventually results in the invocation, but this is not clearly stated.