`__is_standard_layout` is wrong when a base class is the type of first non-static data member

[a62a2cf6-873b-4f58-ae25-048414c98fc7]

Bugzilla Link	48397
Version	trunk
OS	Windows NT
CC	@zygoloid

Extended Description

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 setM(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.)

[ec04fc15-fa35-46f2-80e1-5d271f2ef708]

Confirmed. We check for repeated base classes, but we don't check for a base class duplicating the first field of another base class.