<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/111185>111185</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            c++23: constexpr std::unique_ptr::~unique_ptr()/std::default_delete<T> instantiation too strict
        </td>
    </tr>

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

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

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

<pre>
    ```cpp
#include <memory>

class Foo {
  inline static std::unique_ptr<Foo> instance_;
};
```

See: https://godbolt.org/z/MqMxs9nzr

This fails to compile due to `unique_ptr<T>::~unique_ptr()` (and thus `default_delete<T>::operator()`) being instantiated at the point of the variable declaration (when the class isn't fully defined yet). Since c++23, `unique_ptr` and `default_delete::operator()` are constexpr, but the declaration of the unique_ptr should not attempt to fully instantiate the destructor or std::default_delete<T>::operator() yet. Or at least the above usage pattern should be allowed, as this is a valid pattern for singletons (that aren't Meyers style). Should this fall back to non-constexpr unique_ptr/default_delete?

This fails using both libc++ and libstdc++. gcc and msvc both compile it fine.

```
In file included from test.cpp:1:
In file included from /builds/clang-19.1.0/bin/../include/c++/v1/memory:944:
In file included from /builds/clang-19.1.0/bin/../include/c++/v1/__memory/inout_ptr.h:16:
In file included from /builds/clang-19.1.0/bin/../include/c++/v1/__memory/shared_ptr.h:32:
/builds/clang-19.1.0/bin/../include/c++/v1/__memory/unique_ptr.h:78:19: error: invalid application of 'sizeof' to an incomplete type 'Foo'
   78 | static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete type");
      | ^~~~~~~~~~~
/builds/clang-19.1.0/bin/../include/c++/v1/__memory/unique_ptr.h:292:7: note: in instantiation of member function 'std::default_delete<Foo>::operator()' requested here
  292 | __ptr_.second()(__tmp);
      | ^
/builds/clang-19.1.0/bin/../include/c++/v1/__memory/unique_ptr.h:261:71: note: in instantiation of member function 'std::unique_ptr<Foo>::reset' requested here
  261 |   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 ~unique_ptr() { reset(); }
      | ^
simple.cpp:4:38: note: in instantiation of member function 'std::unique_ptr<Foo>::~unique_ptr' requested here
    4 |   inline static std::unique_ptr<Foo> instance;
      | ^
simple.cpp:3:7: note: definition of 'Foo' is not complete until the closing '}'
    3 | class Foo {
```

</pre>
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