Why doesn't this “undefined extern variable” result in a linker error in C++17?

Question

I have compiled and ran the following program in a C++17 compiler (Coliru). In the program, I declared an extern variable, but did not defi

Answer 1

This has alrady been answered, but if you are interested, cppreference.com has exactly this example for constexpr if:

Constexpr If

The statement that begins with if constexpr is known as the constexpr if statement.

In a constexpr if statement, the value of condition must be a contextually converted constant expression of type bool. If the value is true, then statement-false is discarded (if present), otherwise, statement-true is discarded.
[...]
The discarded statement can odr-use a variable that is not defined:

extern int x; // no definition of x required
int f() {
if constexpr (true)
    return 0;
else if (x)
    return x;
else
    return -x;
}