[Bug c++/91064] New: __is_standard_layout incorrect for a class with multiple bases

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91064

            Bug ID: 91064
           Summary: __is_standard_layout incorrect for a class with
                    multiple bases
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

According to class.prop in N4800:

  A class S is a standard-layout class if it:
  ...
  (3.5)  -- has at most one base class subobject of any given type

The test case below is taken from the example in paragraph 4 in that section. 
The GCC traits reports that class U is standard layout, in violation of the
spec.  (Clang does the same.)

$ cat x.C && gcc -S -Wall x.C
struct Q { };
struct S { };
struct T { };
struct U: S, T { };   // not a standard-layout class

static_assert (!__is_standard_layout (U));
x.C:6:16: error: static assertion failed
    6 | static_assert (!__is_standard_layout (U));
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~