<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/123700>123700</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Should operands in ternary operator be allowed to have explicit copy constructor?
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          Rush10233
      </td>
    </tr>
</table>

<pre>
    I've got a very confusing output with the following test:

```c++
struct QString
{
    QString()=default;
 explicit QString( const QString & );
};

QString def(){
    return QString{};
};

void foo( bool b, QString s )
{
     auto res = b ? def() : s;
}
```

It turns out that GCC accepts this with `-std=c++17` or above; EDG and ICC always accept, and clang and MSVC always reject even with `-std=c++17` or above:  https://godbolt.org/z/zhza8Krfa


The diagnostic of clang and MSVC points out that if `s`  is taken, the conversion from `s` to `res`  will implicitly use the copy ctor of `QString`, which is not allowed:

```c++
<source>:13:29: error: no matching constructor for initialization of 'QString'
   13 |      auto res = b ? def() : s;
      | ^
<source>:4:14: note: explicit constructor is not a candidate
    4 |     explicit QString( const QString & );
      | ^
<source>:3:5: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
    3 |     QString()=default;
      | ^
1 error generated.
```

Although the diagnostic sounds reasonable here, I still wonder if the standard has been made some changes since C++17 according to the behavior of GCC.

</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJyUlc9v6zYMx_8a5UK0kC0nTg4-pElTFMMOW4fdZYu2tSlSJtHO0r9-oJ2mWR_wtgc4iCDzx0fUl7ROyXYesRLLJ7HcL_RAfYjVr0PqM5krtaiDuVSvIi9HhC4QaBgxXqAJvh2S9R2EgU4DwdlSD9QjtMG5cOY3hImE2go5PSs5P43In_iR20RxaAh-eaNofcc2JW8DwG0vX4t8I9TeYKsHR0JNBvj3ydnG0p0ZA6XbBoh8BZPn0xR2f13I7YeBwfYa_JYzIg3R32KWT59-dwHGYA20IXDOOgQHtch3t7xpynp_EtADBYiYQKg91CDU4TM5CLWF9JnkrkxztlcChkpcZaBeE7zsdqCbBk-UgHqb5sKLlXxIZITaX8ublWIlIUTQdRhRqCd43r-A9gZeOYA760u6xmF-ftE47btp9fPb7zebiH9gQ4Aj-v-ZagvQE50SX31-EPmhC6YOjh5D7ER-eOdf_67XP8VWX7Uht7_1CMbqzodEtoHQfsU5Bevprg62ZZLEqcEmIP0nej4JS7AJfsSYbPDQxnC8GVLgZcTZ62ydA3ucpeQuMCS8ep8u0FCITCFW8kMQK8nxz71tes7ouRlY6mi-J3KhdikMsUGhnoXaZkqoLQtzCxhjiLzwAY6amp4FNMmY2yJEaEME6y1Z7ey7Jj4OE-XlTfblLLJMgSh38ENym63ZTSyfv-UsmLWY6Wi61FvT3SN-1AEa7Y01mvAaurgR_WCz_gcXl295j3VL_C8uhhqtrt1kE_GvwXJNJOjYDUf0lPgu64Egg7NOcIphtAbNFeCznt8fRF9Ys_lOoUOPUROax287euuoD0M3z8o7xacweMPtplPwDA49RmTKV0jEUj0HbzCy8Nk1kfZGRwO9TlAjejhqg5DCEaHpte8wQbK-Qdh9tCn3e4hmGs1hClJjr0c7K_1lt2PchamU2aiNXmCVlaosCpWVxaKvpForI7N2iWrZyGWxalEVZYnrbF1vVptsYatc5kuZ5Zks5SorHtcrpaRS67Zu6mIjlSgkHrV1j86NR54GC5vSgFWWq1LKhdM1ujR9iPLc4xmmtyLP-bsUK3Z6qIcuiUI6myh9hiFLDqu3PgzOQDhh1FxK64Eweh0v8x7rosaPnuUS9HrEe12fLvciEuqwGKKrvgwzS_1QPzbhKPIDE1z_Hk4x8KQU-WHiTiI_XA82Vvk_AQAA__93s0Vk">