<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/60620>60620</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            libc++-16 regression: unique_ptr no longer works for incomplete type 
        </td>
    </tr>

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

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

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

<pre>
    In 16.0.0-rc2, the following code

```
#include <memory>

struct ForwardDeclared;

struct MyStruct {
   std::unique_ptr<ForwardDeclared> member;
   MyStruct() {}
   ~MyStruct();
};
```

now produces the error

```
In file included from clang_regression.cpp:1:
In file included from external/clang_linux/include/c++/v1/memory:898:
In file included from external/clang_linux/include/c++/v1/__memory/shared_ptr.h:31:
external/clang_linux/include/c++/v1/__memory/unique_ptr.h:63:19: error: invalid application of 'sizeof' to an incomplete type 'ForwardDeclared'
    static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete type");
                  ^~~~~~~~~~~
external/clang_linux/include/c++/v1/__memory/unique_ptr.h:297:7: note: in instantiation of member function 'std::default_delete<ForwardDeclared>::operator()' requested here
      __ptr_.second()(__tmp);
      ^
external/clang_linux/include/c++/v1/__memory/unique_ptr.h:263:75: note: in instantiation of member function 'std::unique_ptr<ForwardDeclared>::reset' requested here
  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX23 ~unique_ptr() { reset(); }
                                                                          ^
clang_regression.cpp:7:4: note: in instantiation of member function 'std::unique_ptr<ForwardDeclared>::~unique_ptr' requested here
   MyStruct() {}
   ^
clang_regression.cpp:3:8: note: forward declaration of 'ForwardDeclared'
struct ForwardDeclared;
       ^
1 error generated.
```

To my understanding, this is in violation of the standard which states:
* https://eel.is/c++draft/unique.ptr#general-4.sentence-3: The template parameter T of unique_­ptr may be an incomplete type[.](https://eel.is/c++draft/unique.ptr#general-4.sentence-3)
* https://eel.is/c++draft/unique.ptr#dltr.general-2: The template parameter T of default_­delete may be an incomplete type[.](https://eel.is/c++draft/unique.ptr#dltr.general-2.sentence-1)

Afaict, the two mandates:
* https://eel.is/c++draft/unique.ptr#dltr.dflt-3: T is a complete type for `default_­delete::operator()(T* ptr) const`
* https://eel.is/c++draft/unique.ptr#single.dtor-note-1: for `~unique_ptr`: [Note 1: The use of default_­delete requires T to be a complete type[.](https://eel.is/c++draft/unique.ptr#single.dtor-1.sentence-1) — end note]

do not apply for this code snippet, as I am not calling `~unique_ptr`. The destructor `~MyStruct()` is only declared, but not implemented in the header
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy0V12P4rgS_TXmpUSUOJDAAw80HxLS3L6j2-hq9ikycSV417EzttM97MP89pWdNNDf0k5PlE5DbJerTp06ZZi1olaICzK9IdP1iHXuqM3iXtcobY2WqXp00Py02ClIsiiO4rEpKaErcEeESkupH4SqodQcSbwm8XJ4ZvFw919pKlQpO45A0lWDjTYnkm6uV1hnutLBVpsHZvgaS8kMcpLevDLpP6e7_gPJh2EAsI6TdEnSZafE9w6L1hmSrl7Y20CDzQHN2TLA2R6hM0LnwWq-Po_-fDp8cSlfXz4_izc8lX6A1mjelWgDXmiMNu_AtFNQCYkwYMWhMrqBUjJVFwZrg9YKraKybUm6THy07yzDHw6NYpLQbW9BCtX9IHQ7TPPvCb0J9_Y-IXT7mJflbD77dONFMZinW3v0qfAJio4kXaaXSH7J7CXvwWyWepDmJF0OuKdLEOqeScGBta0UJXNCK9AVEJpb8TfqitAcnAamfLi6aSU6BHdq0U95TiWanykC1jEnyoJZi8bz5NHcrNi3gVLphqRriH3lEEpLppR2wDFs8HI7Quk10-DFRaabn-fr87Gj85ykS_8HSjvsoQOhrGPKiTNufSVB1akyvPI4PlYhx4p10hV9jK9WYj9Rt2iY02aoLpqDwe8dWoccjmjwGoLCe1hEFkut-OOCWVG4pn2JF5lufgMygVb59Feg-UCg-kkGLbq30Si-7G5WX78Wu9svu9tN8f_d3e5m92W3_-M8svrv7d1-8-3r_4q73e1qU6y-faMp_Lza_Kx2MGw2yBtcq98nXedkvKFmnmuT3w7qk-jfJNr77eCjSDw_ZteRVL0zwIM316LzlqJ80AufQ5r0Agc1Kl9KyKN3mtJeQ3OCTnE0Hl0uVN23c2HB3wruhZZnL33bCvN8BA9HUR6D1qE9azahSzg614Y3dEvoFlFGwl6qihtWuXMpRQH8tHdWjieRReVQlTj2yMH-iOCwaSVzCC0zrEGHBvbemSF7ZEXJct06Aw07weFVAZ3eRGS6JnT2Wa7R-S-Fy6Uz0aNh-lGgj-LZRzq0id8T7FPHLhEnl4jDc1kx4UuiP_m5Bw2NZ8UnMCF4wCvpBgJ4GjJ42n4rbYBk8Wu4vN5FZnvvTdhgDqVW1l0dRv-Vm1aoWmLEnTZjX9rjZChu79i1sGSxHyDTm1vtEJLHXHcW30ytFyJh0MLeHz98kuHzMnztePI0wUA2lMxiMp8AKt4r1nR9nXau_dtwYDqFYINQ-OM-WCXaFgMlmIUdsCZMLZmU_ifBC1SiAAPHXt0G3J6drbPYZ18reRrk0qviCg6dC7aFx6TxEXCvVJ6JR2QczYgvUj5P52yEiyTLp3M6mWT56LiYVUmeTfJZntJZQqtDTmmSTmYJn6d5QpGOxILGNI1pPI-zdELzKEszXuUsKSdYTpJ0QiYxNkzISMr7JtKmHglrO1xkcUbjkWQHlDb8fKJU4QOEQX9-m65HZuHXjA9dbckklsI6e7HihJO4kOIw5G6cZHBpJp41F_RAaZBa1WjgQZu_bMjE8yPqqDNy8ZQftXDH7hCVuiF063ce_o1bo_9Ej_o2-OspFOL5JwAA___ccGAA">