<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/60803>60803</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang 16-rc2 regression: confusion around single-element array class variant member in constexpr
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            bug,
            c++20,
            clang:frontend,
            regression,
            rejects-valid
      </td>
    </tr>

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

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

<pre>
    Caught this testing MSVCSTL with 16-rc2. One of the libc++ tests instantiates `std:basic_string` with a 16-byte `struct` type, which results in an SSO buffer large enough to hold a single character. I suspect this is a vanishingly rare use case outside of test suites, so this is not a release blocker, but a "would-be-nice" for LLVM 16.

Compiling this well-formed TU:
```c++
struct CharT { int x; };

struct basic_string {
  constexpr basic_string() { u_.buf_[0] = CharT{}; }

  union U {
    constexpr U() : buf_() {}

    CharT buf_[1];
  } u_;
};

constexpr bool test() {
  basic_string s;
  return true;
}

static_assert(test());
```
with `clang++ -std=c++20` diagnoses:
```
repro.cpp:18:15: error: static assertion expression is not an integral constant expression
static_assert(test());
 ^~~~~~
repro.cpp:4:41: note: member call on object outside its lifetime is not allowed in a constant expression
  constexpr basic_string() { u_.buf_[0] = CharT{}; }
 ^
repro.cpp:14:16: note: in call to 'basic_string()'
  basic_string s;
               ^
repro.cpp:18:15: note: in call to 'test()'
static_assert(test());
              ^
1 error generated.
```
This is particularly strange given that `U`'s default constructor activates `buf_`, and that the assignment `u_.buf_[0] = CharT{}` should activate `buf_` even were it not already the active member of `u_`. Indeed, changing `U`'s default constructor so as to not activate `buf_` - or removing the constructor entirely - results in successful compilation.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysVl1v4joT_jXmZkQUnJLABRcF3kor7Wov2u5t5SSTxO8xNvLYdLk5v_1okgCpylG10kEhX555nvl2FJFuLeJGLLdiuZ-pGDrnNztFeN4pH9DPSlefNzsV2y5A6DRBQAratvDj-dfu-eU7vOvQwSKf-0om8NMiuAZCh2B0WQm5FXLbqxBoS0HZoFVAApGnFGqRPZaKdPVGwWvbijwd4BQDlueAg5yPVeC1cD6ikDt473TVgUeKpscFZeH5-SeUsWnQg1G-RUDrYttBcNA5U4MC0rY1CFWnvKoC-gS-AUU6YjV6pgkUnJTV1LHoGbzyCJEQKkUILgbS9eAfUgCKOiCxPeSuANYFUODRIKuUxlV_oWeZMvKCkPLdRVPPS5xbXaGQEhrn4fv3Xz9gkSci3Yv0cTjv3OGoDYe6B39HY-aN8wes4eVVZKOUyNPhGIM9vB1iBrtO-RcQxRa0DfBbZFsQxV5k2ynPKDtNBKsMiwCVsxTw99F_kBByJeS6h45vSRmbN7HcpmK5B5HtB17G6Ml6zgkhQLTaWXid0kyJXi_o2SP00FeyT0gw-jhasBDLm3vAxBDfbv5-8n3inHOmz-uEbET5EBmawHsM0VsIPuIHjg_BVUFXb4oIPUPfGPi4Kl2SODz2PcApNYoj3ffQvG-X_ZhlycJQa9VaR0ifq2F49Hj0LqmOR5E9LlZ8WnJM0Xvn-WawDgbrOCUcCiTi20sxW64dbL0yQ4aUDROxP_ESxPJ_f_Pvs3EP_F-wSdYF5OsBDyV6qJQx4Cy48v_cp5ce1IHA6AaDPuDVUmPcO9b9PPh3U__bgmaX7oSa3VnkU3e0HVwJDoQsPhMLWXxZbh9-94lvOb5PPE1M8Ue5u0--GGoJWrToVcA6uVuHL-N0PCofdBWN8uYMFLyyLUKrT2ghdCpwzb-ykiwIamxUNGFIF08o50FVQZ8u-0efIxbegbL1AMAbz7CpHdD2eF8kM0-BOp7IV-wJNCBb9o6eC26sMY-qPg88rICXQnXNwCbyNIFvtkas2bKqU7btJ-oXrpEDRZylnuaOLXNwHjwe3GnYEvCDOtqgPZozzKf7IsWqQqImcu_ybqK4y5NZvcnqdbZWM9ws8iJfZmv5UMy6zbper1eYVmmzRLWq86wpVSHLoqlXUi6L1UxvZCqzVC7ydC3ThyIp0nxd1mtZqaxIi6wQDykelDaJMadD4nw700QRN3m6SrOZUSUa6r82pCxjK6QUciekvI2165t-9mWPjXc2oK2vCx7bS0PfXvFwoPlJGd0LLvczv2EL5mVsSTykRlOgm01BB4ObHVOMny4wgc0eObRN7Meg8i7aevx4mKPBvrCU9-oMlVFEcFJe86QZC4Eb7jJhZtGbTRfCsZ_Q8knIp1aHLpZJ5Q5CPrFB42V-9I69EPKpjxgJ-dQH7Z8AAAD__-cJ-zA">