CWG3169 [class.mem.general] Issues with the definition of layout-compatibility #869

Full name of submitter (unless configured in github; will be published with the issue): Jay Ghiron

Reference (section label): [class.mem.general]

Issue description:

This is split off from cplusplus/draft#8787. Some other issues were covered in #867.

Issue 1: Layout-compatible unions are not defined as being layout-compatible classes despite being classes and layout-compatible.

Issue 2: The wording "if their common initial sequence comprises all members and bit-fields of both classes" for layout-compatible classes includes all members, not just non-static data members when that clearly is not the intent.

Issue 3: Layout-compatible unions are not defined as considering the widths of bit-field members, but all implementations do consider the widths of bit-field members.

Issue 4: Layout-compatible unions are not defined as considering unnamed bit-fields since they are not members, but all implementations do consider the presence of unnamed bit-fields.

Issue 5: Layout-compatible unions are not defined as considering the use of no_unique_address, but all implementations do consider the use of no_unique_address if they support the attribute.

The solution to the second issue is provided in the pull request mentioned before.