Given the following source:

typedef unsigned char uint8_t;
typedef unsigned short int uint16_t;
typedef unsigned int uint32_t;

struct a {
  uint8_t f0;
  uint8_t f1;
  uint16_t f2;
} __attribute__((packed));

struct b {
  uint16_t f0;
};

void
test (struct a *p)
{
  struct b *q= (struct b *)p;
}

when compiled with -w, gcc 9 emits just these stray notes:

rhbz1696441.c: In function ‘test’:
../../src/rhbz1696441.c:5:8: note: defined here
    5 | struct a {
      |        ^
rhbz1696441.c:11:8: note: defined here
   11 | struct b {
      |        ^

(from downstream report https://bugzilla.redhat.com/show_bug.cgi?id=1696441 )

The notes are followups to -Waddress-of-packed-member warnings, which aren't being properly guarded by emission of that warning in c-family/c-warn.c:

2776		      warning_at (location, OPT_Waddress_of_packed_member,
2777				  "converting a packed %qT pointer (alignment %d) "
2778				  "to a %qT pointer (alignment %d) may result in an "
2779				  "unaligned pointer value",
2780				  rhstype, rhs_align, type, type_align);
2781		      tree decl = TYPE_STUB_DECL (rhstype);
2782		      if (decl)
2783			inform (DECL_SOURCE_LOCATION (decl), "defined here");
2784		      decl = TYPE_STUB_DECL (type);
2785		      if (decl)
2786			inform (DECL_SOURCE_LOCATION (decl), "defined here");

which is normally guarded by:

2886	warn_for_address_or_pointer_of_packed_member (tree type, tree rhs)
2887	{
2888	  if (!warn_address_of_packed_member)
2889	    return;

but the "-w" sidesteps this (via diagnostic_report_warnings_p).

Appears to be new with gcc 9.

I'm working on a fix.