What do the words "undefined" mean in bullet point §5.19/2.3 in N4140?

Question

From N4140 §5.19/2.3 (emphasis mine)

— an invocation of an undefined constexpr function or an undefined constexpr constructor;

From §7.1.5/2 constexpr functions and constructors are implicitly inlined, that is, if a constexpr function is not defined in a TU the code will just not compile.

Answer 1

This bullet was added by defect report 699 and it requires that a constexpr function or constructor must be defined before use. The defect report added the following example to 7.1.5 to demonstrate the rule:

constexpr int square(int x);       //OK, declaration
constexpr struct pixel {           // error: pixel is a type
    int x;
    int y;
    constexpr pixel(int);            // OK, declaration
};
constexpr pixel::pixel(int a)
    : x(square(a)), y(square(a)) { } //OK, definition
constexpr pixel small(2);          // error: square not defined, so small(2)
                                     // not constant (5.19 [expr.const]), so constexpr not satisfied
constexpr int square(int x) {      // OK, definition
    return x * x;
}
constexpr pixel large(4);          // OK, square defined

Note, the wording from this report changed with defect report 1365 to the wording currently in the draft standard.