Why does the C++ linker allow undefined functions?

Question

This C++ code, perhaps surprisingly, prints out 1.

#include 

std::string x();

int main() {

    std::cout << \"x: \" <&l         


        

Answer 1

[basic.def.odr]/2:

A function whose name appears as a potentially-evaluated expression is odr-used if it is the unique lookup result or the selected member of a set of overloaded functions (3.4, 13.3, 13.4), unless it is a pure virtual function and its name is not explicitly qualified.

Hence, strictly speaking, the code odr-uses the function and therefore requires a definition.
But modern compilers will realize that the functions exact address is not actually relevant for the behavior of the program, and will thus elide the use and not require a definition.

Also note what [basic.def.odr]/3 specifies:

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program; no diagnostic required.

An implementation is not obliged to halt compilation and issue an error message (=diagnostic). It can do what it considers best. In other words, any action is allowed and we have UB.