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

Because the compiler produces compiler errors, the linker would yield linker errors ...

No, seriously:

if constexpr (true)

is always true, so the compiler ignores the rest of the if-clause because it is never reached. So i is never used actually.