<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/64619>64619</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Missing diagnostic for non-standard layout types in `offsetof`
        </td>
    </tr>

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

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

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

<pre>
    Consider the following example (https://godbolt.org/z/o77v1nad9):
```cpp
struct X1 { int a; };
struct X2 { int b; };
struct Y : X1, X2 {};

static_assert(__builtin_offsetof(Y, b) == 4);
static_assert(offsetof(Y, b) == 4);

static_assert(!std::is_standard_layout_v<Y>, "");
```
Only GCC warns that non-standard layout type is passed to `offsetof`, which is [conditionally-supported](http://eel.is/c++draft/support.types.layout#1.sentence-2).
```
<source>:8:34: warning: 'offsetof' within non-standard-layout type 'Y' is conditionally-supported [-Winvalid-offsetof]
    8 | static_assert(__builtin_offsetof(Y, b) == 4);
      | ~~~~~~~~~~~~~~~~~~~^~~~~
In file included from /opt/compiler-explorer/gcc-trunk-20230811/include/c++/14.0.0/cstddef:50,
 from <source>:1:
<source>:9:24: warning: 'offsetof' within non-standard-layout type 'Y' is conditionally-supported [-Winvalid-offsetof]
    9 | static_assert(offsetof(Y, b) == 4);
 |                        ^
```
GCC issues it by default, whereas Clang, MSVC, and EDG doesn't even in pedantic modes (or `/W4`).

I'm not sure what the solution should be, but I feel like there is a problem to solve.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzMVU2P4ygT_jWVS8mWDfHXwYd8tEdzGL2HV9qZPkXYlBN2CFiA05M97G9f4aS7p3e7pTnsYRESCX6q4KmqpxDeq6MhaqHYQrFfiTmcrGsfjFRa61Vv5bXdWeOVJIfhRDhare2TMkekH-I8aUJg9SmEyQPfAOuAdUcre6tDat0RWPcHsM5W1SU3QjbAmgjL9pBtoMxuc5im244Pbh4CfssRqi0qE1AA3yJUe-DbtxD2Auk_gDwi8A1-y4Ht7vCfQc9QEdRwEN6TC8Dqw6GflQ7KHOw4egp2BFY_Rg89sAaB74Hvcb2w2L7v4pcN3zcHlvsgY4z4RvmDD8JI4eRBi6udw-ECfPcI_CF6BsaW-ZPL54je_v7P6Ct-2u3wSTjjMZxEQGNN8uwUb04xXCdC5XGKt5AYLEKZvfAos3jY00kNpwiCYjtYI1VQ1gitr4mfp8m6QBKK_b0UXiqBSKfKA-sGYFtgW-nEGIB1d5s0nuzT2zWA8Tz1ZAKZgZLIK32XFfCdt7MbKIaBb2rgG76OqY4slTnGn8Cq1zxU-KTCSZk33JOfuQOrHiNOefyAW6SdfFXmIrSSyYvvYn-7EyJijVDt8F-oKFxGdPbnPwcUD8uyID8bHJUmVGbQsySJo7NnjHqbYpQHe56UJpfQj0lbRy5qcxiS4GbzPWEZ41md58C6u_1rnoB1-TrN0ixu-SAljcA3RayE-x1vJ71NRf4q7bcfoubZfyFHzbs5-vXMROsPBhQP75Zr1J_yfiaPKmB_RUmjmHW4iYocCY87Lcwxbnz5_2-7uAoj8WH_CaUlb4BVAelCBpXBiaQwQQ14tpJ87L3W4aLR7ut6WV9kcysRYNUZjQ3oZ0f4FHtAbOPe6jmGEP3JzlpiTwvzOeBnHIk0avWdItItrUHg5Gyv6Ry7g7f6QulKtlw2vBEravOyyauiqfNsdWoH0Ze9GMdCjg0rhnocmzKXZbXu-6Kq1-VKtc-1l_O8LLK0rrOybIaR1eW64nkG64zOQulU68s5PiOrJYJtuS7zZqVFT9ovDxZjwxI6vpFKHI31QQ0-NsViv3JttE76-ehhnWnlg3_1F1TQ1H5R3seX7NUYR-s-7JE-JuBta1zNTrd_e_1UOM19OtgzsC4eeF-SydnfaYjCvNUDsG4h9FcAAAD__3DdTPE">