[llvm-bugs] [Bug 48397] New: __is_standard_layout is wrong when a base class is the type of first non-static data member

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48397

            Bug ID: 48397
           Summary: __is_standard_layout is wrong when a base class is the
                    type of first non-static data member
           Product: clang
           Version: trunk
          Hardware: PC
               URL: https://wandbox.org/permlink/YHg7sBk5XEY5pz1B
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++14
          Assignee: unassignedclangbugs at nondot.org
          Reporter: k_satoda at f2.dion.ne.jp
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The static_assert in the following code fails on clang HEAD 12.0.0
(https://github.com/llvm/llvm-project.git
1c19900f9417bd0b832c1cb70863b2439a18647f)
but should succeed as D is not a standard-layout class.

  struct B1 {};
  struct B2 { B1 b1; };
  struct D : B1, B2 {};
  static_assert(!__is_standard_layout(D), "D is not a standard-layout class");

Tested at: https://wandbox.org/permlink/ha8cStNKHEKl2BaJ

The standard says: https://timsong-cpp.github.io/cppwp/n4659/class#7
> A class S is a standard-layout class if it: 
>   - ...
>   - has no element of the set M(S) of types (defined below) as a base class.
>     M(X) is defined as follows:
>       - ...
>       - If X is a non-union class type whose first non-static data member
>         has type X0 (where said member may be an anonymous union), the
>         set M(X) consists of X0 and the elements of M(X0).

The set M(D) consists of just B1 because D is a non-union class type
whose first non-static data member has type B1. Then, because D has B1
as a base class, D is not a standard-layout class.

(If D is a standard-layout class, it must be layout compatible with B2.
But sizeof(D) is 2 while sizeof(B2) is 1, which means they can't be
layout compatible.)

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