Undefined reference for unoptimised constexpr used as default parameter - c++

Question

I do not understand why the below code compiles with GCC optimised, but fails to link with "undefined reference to `base::A_VAL'" when unoptimised. Am I doing something dodgy? Is this something that's a compiler bug (never is)? This is with g++ 5.4.0 on Ubuntu.
base.h:
class base {
public:
static constexpr unsigned int A_VAL{0x69U};
};
derived.h:
#include "base.h"
#include <iostream>
using namespace std;
class derived : public base
{
public:
int some_func(void) {
cout << "Some func" << endl;
return 0;
}
};
concrete.h:
#include "derived.h"
#include <utility>
class concrete : public derived
{
public:
concrete(int a, std::pair<unsigned int, unsigned int> data = {A_VAL, A_VAL}) {
some_func();
std::cout << "First: " << data.first << " Second: " << data.second << endl;
}
};
test.cpp:
#include "concrete.h"
int main (int argc, char *argv[])
{
concrete c{1};
c.some_func();
}
g++ -O2 -std=c++14 -o test test.cpp
Fine.
g++ -O0 -std=c++14 -o test test.cpp
/tmp/ccm9NjMC.o: In function `main':
test.cpp:(.text+0x23): undefined reference to `base::A_VAL'
test.cpp:(.text+0x28): undefined reference to `base::A_VAL'
collect2: error: ld returned 1 exit status

Answer 1

When optimizing GCC is probably able to determine (after inlining an constant folding) that the body of concrete's constructor can be replaced pretty much by
some_func();
std::cout << "First: " << A_VAL << " Second: " << A_VAL << endl;
Since operator<< for the standard stream class takes integers by value, and A_VAL is a constant expression, the call above doesn't require there to be any storage for A_VAL. Its value is just plugged in. As such, GCC doesn't need there to be an out of class definition for A_VAL, as is normally required for static class members.
When not optimizing, GCC quite likely initializes the pair object. std::pair's constructor takes objects by reference, and a reference requires an object to bind to. So the the definition of A_VAL becomes required and so the linker complains.
You need to define the object somewhere (pre C++17)
// At namespace scope
constexpr unsigned base::A_VAL;
Or switch to compiling as C++17. Then A_VAL (like all constexpr static members data) will be implicitly an inline variable, and the compiler will resolve its definition by itself.

Answer 2

I'm not sure howconstexpr affecting this, but you just declared static class variable, but not defined it. I.e. usually you need to have constexpr unsigned int base::A_VAL{0x69U}; somewhere in .cpp file.