[Bug c++/121524] New: [15 Regression] Trailing alignas leaks into subsequent types, sometimes

[headch at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121524

            Bug ID: 121524
           Summary: [15 Regression] Trailing alignas leaks into subsequent
                    types, sometimes
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: headch at gmail dot com
  Target Milestone: ---

Consider the following code:

using UT = unsigned int;
struct Foo {
        UT foo[8] alignas(32);
};
struct Bar {
        UT a[8];
        UT b;
};
static_assert(sizeof(Bar) == 36);

This code causes the static_assert to fail. Various slight permutations don’t
(such as putting the alignas at the beginning of the declaration instead of the
end, not using a type alias, or changing the size of foo). Clang rejects
alignas at the back like this, so this might be invalid code and might be
related to #69572 or #90847, but it’s a slightly different structure than any
of the code quoted there. Also the alignas isn’t ignored or applied; instead,
it leaks out and affects an unrelated type.