On Fri, May 22, 2020 at 7:22 AM Eli Zaretskii <eliz@gnu.org> wrote:
>   #0  PSEUDOVECTORP (code=<optimized out>, a=<optimized out>) at lisp.h:1720
>   #1  MARKERP (x=<optimized out>) at lisp.h:2618
>   #2  CHECK_MARKER (x=XIL(0xa000000018ac0518)) at marker.c:133
>   #3  0x010f073c in Fmarker_position (marker=XIL(0xa000000018ac0518))
>       at marker.c:452

I think I've worked it out: it's this mingw bug:
https://sourceforge.net/p/mingw-w64/bugs/778/

On mingw, if <stdint.h> is included before/instead of stddef.h,
alignof (max_align_t) == 16. However, as can be seen by the backtrace
above, Eli's malloc only returned an 8-byte-aligned block. That's not
normally a problem, because mark_maybe_object doesn't care about
alignment; but in conjunction with the gcc behavior change, we rely or
mark_maybe_pointer to mark the pointer, and it doesn't, because the
pointer is not aligned to a LISP_ALIGNMENT = 16-byte boundary.

Brute-force patch attached until we can work out how to fix this properly.