oh-so-thoughtfully re-introduces the issue

No it doesn't. The issue is there with or without the -Wformat. Your sarcasm is misplaced since the warning isn't spurious. The issue is that the wrong format is being used. This is a real error that needs fixing.

The warning is spurious, %I64u is valid on Windows. Unfortunately with the macro the full text of the format is hidden so all you have is the misleading "%u" from the base message:

C:\Users\Tony\dev\perl\git>gcc -Wformat -oint64.exe int64.c
int64.c: In function 'main':
int64.c:6:15: warning: format '%u' expects argument of type 'unsigned 
+int', but argument 2 has type 'long long unsigned int' [-Wformat=]
    6 |   printf("%I64u\n", x);
      |           ~~~~^     ~
      |               |     |
      |               |     long long unsigned int
      |               unsigned int
      |           %I64llu

C:\Users\Tony\dev\perl\git>type int64.c
#include <stdio.h>

unsigned long long x = 0;

int main() {
  printf("%I64u\n", x);
}
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If it is a bug, it's a bug in perl, since that's where the macro OP is using comes from.

I've considered changing the Win32 config in perl to use "%llu"/"%lld" instead, but testing against the older versions of MSVCRT in older Windows that we theoretically support is difficult.

jdporter or someone - when I saw the reply in my "chatterbox", it didn't link to my article, but to a node "XS" (a Super Search link). I think that might be a bug?