[Clang] Linker error at -O0 due to missing static const int definition, hidden at -O2

[shzaiz]

In Clang 20.1.0, a class with static const int members that are declared but not defined results in a linker error at -O0, but compiles successfully at -O2.

This seems to be due to constant propagation at higher optimization levels, which removes the need for symbol resolution, masking the fact that a required definition is missing.

This behavior differs from MSVC, which compiles successfully in both optimization levels.

Code:

#include <map>
#include <vector>
using namespace std;
const pair<int, int> START = make_pair(0, 0);
class GridWorld {
public:
    // change `const' to `constexpr', it will successfully execute
    // or change compiler option with -O2
    static const int NORTH = 123456, SOUTH = 1, EAST = 2, WEST = 3;
    typedef pair<int, int> State;
    GridWorld(int x = 0, int y = 0) {}
    State state() { return make_pair(x, y); }
    map<State, double> benefit_matrix;
    void learn_benefit() {
        GridWorld temp(0, 0);
        for (auto action_state : temp.available_actions())
            ;
    }
private:
    int x, y;
    vector<pair<int, State>> available_actions() {
        vector<pair<int, State>> actions;
        if (y != 0)
            actions.push_back(make_pair(NORTH, make_pair(x, y - 1)));
        if (y != 4)
            actions.push_back(make_pair(SOUTH, make_pair(x, y + 1)));
        if (x != 4)
            actions.push_back(make_pair(EAST, make_pair(x + 1, y)));
        if (x != 0)
            actions.push_back(make_pair(WEST, make_pair(x - 1, y)));
        return actions;
    }
};

int main() {
    GridWorld env = GridWorld(START.first, START.second);
    env.learn_benefit();
    return 0;
} 

Reproduction Link: https://godbolt.org/z/dTdxWzMjP

[frederick-vs-ja]

std::make_pair take arguments by reference (since C++11), so the static data members are ord-used and need to be defined. If you change const to constexpr, the static data members become inline variables and thus get defined (since C++17). Using unary operator+ is a possible workaround, which avoids odr-using.

In the original example, the variables are odr-used but not defined, and thus the program is ill-formed, no diagnostic required. Compilers can either accept or reject it.

MSVC seems to make these initialized const static data members effectively inline, which is a conforming extension, IIUC.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Guangwei Zhang (AUGPath) (shzaiz)

In `Clang 20.1.0`, a class with static const int members that are declared but not defined results in a linker error at `-O0`, but compiles successfully at `-O2`.

This seems to be due to constant propagation at higher optimization levels, which removes the need for symbol resolution, masking the fact that a required definition is missing.

This behavior differs from MSVC, which compiles successfully in both optimization levels.

Code:

#include <map>
#include <vector>
using namespace std;
const pair<int, int> START = make_pair(0, 0);
class GridWorld {
public:
    // change `const' to `constexpr', it will successfully execute
    // or change compiler option with -O2
    static const int NORTH = 123456, SOUTH = 1, EAST = 2, WEST = 3;
    typedef pair<int, int> State;
    GridWorld(int x = 0, int y = 0) {}
    State state() { return make_pair(x, y); }
    map<State, double> benefit_matrix;
    void learn_benefit() {
        GridWorld temp(0, 0);
        for (auto action_state : temp.available_actions())
            ;
    }
private:
    int x, y;
    vector<pair<int, State>> available_actions() {
        vector<pair<int, State>> actions;
        if (y != 0)
            actions.push_back(make_pair(NORTH, make_pair(x, y - 1)));
        if (y != 4)
            actions.push_back(make_pair(SOUTH, make_pair(x, y + 1)));
        if (x != 4)
            actions.push_back(make_pair(EAST, make_pair(x + 1, y)));
        if (x != 0)
            actions.push_back(make_pair(WEST, make_pair(x - 1, y)));
        return actions;
    }
};

int main() {
    GridWorld env = GridWorld(START.first, START.second);
    env.learn_benefit();
    return 0;
} 

Reproduction Link: https://godbolt.org/z/dTdxWzMjP