<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/139227>139227</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] Linker error at -O0 due to missing static const int definition, hidden at -O2
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          shzaiz
      </td>
    </tr>
</table>

<pre>
    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:

```cpp
#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

</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJykVlFz4jYQ_jXiZQfGyIDxAw8cCW1nek3nkjaPjGytsS6y5Eoygfz6jmQ5hJBc2-mc54ik3f32-3bXFrNW7BXiisy_kPnNiHWu1mZl6xcmXkaF5qfVLwrIItlIpvZAk8l0kpBFQugGGJSSWQvPwtVgHXOihFIr60AoBw02BRoLrmYOmEHgWEpmkEPROVDaAcdKKORg0HbSWRAKGEihntAAGqMNMOehx3cDovcsddMKiRZsV5ZobdVJeXq1pGSRTEiyJsn6oRYWLGJjwWkoEHiH_q-QIlMOWqNbtmdOaOX9a7Gv0YBunWjES78t8YDSeujnWpQ1GGz0AT0pBIXIodIG7KkptPQ0tOy8m7dvmH0Sah8sK1a6qAMY_KsTXoXAXgQUYaER1gq1f5t6gTU7CG2Ai6rySlZGN57l1_s_N1GQPquPJREKCu3qjwhFmI3mSNJ1v_ARw1O2rV_SVKhSdhyBpJuGtSS9vdo-YOm06U86nz8o1qBtWYlgHSfpF5Ks-5ZomTAk3QjlfN7-J72F-4f1twcg6Q007Al3wYYuA7OE0Dz6hyb7yQj-qI3kQDK_23aFFGWfPQAAoVtCt1DWTO3RqxRgCc18yYclHltDaBYycPAspLyUDI9Ydg4vQ2ozRI069z2iVd_4vud6-6sR-O3u28PPgd6UprP5wuPe3_0x7Pnl7fq-F4D61eNtXKU9dR_VnVrkWH0ioGMOz7avIhG69AkcQ7AkOsBpWOZBxOwmuoUoIX0kdBmPwaDrjLqozNFHOvWVgbN_6I5NnwvdANddIdGnV6DCSrhdw5wRx5jnQQsOEplRu3h-Bo0BL7iAw6a9bovBzk8goUvWOQ2s9HXZBSZA0nVwnbADE5IVEnf9ue3x_HMOEwr-Grjn1hpxCPqGJguC9gK82g0DsLmsTizLrdfgE_SB7b-JEP0uaIvKsz4BodOhqP402k7azta7gpVPhC7fVjC0ZP9-el9WGPuWzOPzI7DZtXT_hBva_hNcQr9cI_eQx_8B6UfrCnEAi238Kdfje2H_A7Cf4mvg8Y9h47Rd1brvRP9_2CLJOnxcmVCXfXQeF1SHMOdvXwbhRTuphLF9d4WlxVIrfs4E1WHywVyec4k5JjGV7Abefjf67L5hazTvAg34VagnP4a1c631YxTeqHvNCy3dRJs9odsXQrf8gR8fX75-_50k6xFfpTxPczbC1TSbLbLlNMuyUb2aL2ecsiRf8GS6YHSRZ3Qxy3POccayihcjsaIJnSfzJPf_0mySZ2W6xPlySafzapnPySzBhgk5kfLQePiRsLbD1TTNKc1GkhUo7XATMitvNS66vSWzRArr7NnPCSfDnSlci8j8JlB9e28Z3yXDhSN-26-_D-crgC9KLThH1fvSUWfk6p1swtVdMSl1Q-jWZxJ_xq3R37F0hG4DHUvoNjI6rOjfAQAA___AjfmJ">