<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62129>62129</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            clang rejects valid code with `if constexpr` in a `constexpr` function template
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:frontend
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          sfc-gh-sgiesecke
      </td>
    </tr>
</table>

<pre>
    The following code is accepted by gcc 10.2 and 12.2, but not by clang 16, both with `--std=c++17`:
```
#include <string>

struct A {
 static constexpr bool flag = false;
};

template <typename T>
constexpr auto bar() {
    if constexpr (T::flag) {
 std::string x;
    }
    return 42;
}
constexpr auto barVar = bar<A>();
```
clang errs with:
```
<source>:19:21: error: variable of non-literal type 'std::string' (aka 'basic_string<char>') cannot be defined in a constexpr function before C++2b
```

I think gcc is correct in accepting this, as the `if constexpr` shouldn't be evaluated in this case. 

(Even if `A::flag` were true, the error message would be misleading as this is a `constexpr` function template which isn't required to be `constexpr` for all `A`. In that case, there should only be an error on `constexpr auto barVar = bar<A>();` because `bar<A>` isn't `constexpr`.)
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJyMlEGT6igQxz8NXrpMEdBoDjk4Ola9-9RetwjpJOwguED0zbffauJzdHan9lVZkQTo_vW__6BiNINDbNj6ha0PCzWl0Ycm9no5jMs4GIyo33HR-u6jeRsRem-tvxo3gPYdgomgtMZzwg7aDxi0hpIXApTroBSFYGIP7ZTA-UTz2io3QFnlzz6NcDVpBFbx5TKmjsmDZuKFiZdywyrO5I7xA-M7Gs-_-VVI47SdOgQm9zEF4wYmX2-T-RlTmHSCHbDNy_wFYlLJaNDexYQ_zwFa7y30Vg3A5AF6ZSMyeVvNNofPcX4mPJ2tSjll-jijUyeEt3vaz7BqSh5aFZjYMlE_AACA6R_yM7F9oxLljiCe12YxaGquDn7eaSgM0d1fAqYpOFiJJ_hvoP5QIVdLfHK_I_yM-bn3Weq5XxhCzJ36tiNyH_0UNFJAuStrJneiZHJHW32gwUUFo1qL4Htw3i2tSRiUBdISmNh8qZiJDQmk3hVNtioa_eevTu_1SPivTGxINa1cdhdCh71x2IFxoB6E7ienk_EOWux9QNjPHhPtf9eSnz8gjca9Z0ObCNqHgDrlyNnu1JQ0mkhGVhHSiOTix_6yikMc_WQ7x8Qm8-FF2UmlmZB2g1YRC3hMzMT29YKOrMIqUvZukIrDFQNCChNSWsqZ5YUTxqgGhCtlo0QnEy2qjiAznIn5oFLEJ767Mnd3X0ejRzBxZg7492QCdkDuwX9v9wGUtTNoxQv4QWWplMu6IQa8qQDe2Q-KotwN27uniL9l0opDi1pNMdM8rKj4nfoLZsFEvega2dWyVgtsympbrqq65uVibMpec1m3a66kbDtd150s11qjbMVKVeV2YRrBheSrclXy9VaUhdiutF6vt7wVvNNiw1YcT8rYwtrLqfBhWJgYJ2wqUYp6YVWLNubbVYh8mqidwbuErmNC0J0bGtq6bKchshW3Jqb4GSyZZLGZz2HAv1CnCBdlTTffv7_uz6_Oy0fgf9u9mIJtxpTOkXwmjkwcB5PGqS20PzFxJIrb3_IcPGVn4pjri0wcc4n_BAAA___V3-o8">