<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/64154>64154</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Incorrectly rejecting qualified names as a member designator of offsetof
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            c,
            c++,
            rejects-valid
      </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/jzTKMdMdn):
```cpp
#include <cstddef>

struct X2 {
    int field;
};

static_assert(offsetof(X2, X2::field) == 0);
static_assert(__builtin_offsetof(X2, X2::field) == 0);
```
Clang issues the following errors:
```
<source>:7:30: error: expected ')'
    7 | static_assert(offsetof(X2, X2::field) == 0);
      |                              ^
<source>:7:15: note: to match this '('
    7 | static_assert(offsetof(X2, X2::field) == 0);
      |               ^
/opt/compiler-explorer/clang-trunk-20230727/lib/clang/18/include/stddef.h:116:42: note: expanded from macro 'offsetof'
  116 | #define offsetof(t, d) __builtin_offsetof(t, d)
      |                                          ^
<source>:8:40: error: expected ')'
    8 | static_assert(__builtin_offsetof(X2, X2::field) == 0);
      |                                        ^
<source>:8:33: note: to match this '('
    8 | static_assert(__builtin_offsetof(X2, X2::field) == 0);
```
GCC, EDG, and MSVC accept the code.

The C++ Standard [says](http://eel.is/c++draft/support.types.layout#1.sentence-1) the following: _The macro offsetof(type, member-designator) has the same semantics as the corresponding macro in the C standard library header [<stddef.h>], but accepts a restricted set of type arguments in this document._

C23 (N3096) §7.21 p4: <...> _the value of which is the offset in bytes, to the subobject (designated by member-designator), from the beginning of any object of type type. The type and member designator shall be such that given_ `static type t;` _then the expression &(t. member-designator) evaluates to an address constant._

Since in C++ qualified names are allowed to the right of dot, `&(x2.X2::field)` is a well-formed expression ([CE](https://godbolt.org/z/8Yfz33e89)). I believe it's in the spirit of the wording to accept aforementioned example.
</pre>
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