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

In your case the variable is used in discarded statements only. However, even if we ignore that fact, C++ language specification still explicitly states that no diagnostic is required for missing definitions

3.2 One-definition rule

4 Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (6.4.1); no diagnostic required.

The language specification understands that an optimizing compiler might be smart enough to eliminate all odr-uses of a variable. In that case it would be excessive and unnecessary to require the implementation to detect and report the potential ODR violations.