Here's the warning given by clang:


/Users/remi/src/cpython/Modules/_zoneinfo.c:1487:9: warning: variable 'dst_offset' is used uninitialized whenever 'if'
      condition is true [-Wsometimes-uninitialized]
    if (*p == '\0') {
        ^~~~~~~~~~
/Users/remi/src/cpython/Modules/_zoneinfo.c:1544:50: note: uninitialized use occurs here
    build_tzrule(std_abbr, dst_abbr, std_offset, dst_offset, start, end, out);
                                                 ^~~~~~~~~~
/Users/remi/src/cpython/Modules/_zoneinfo.c:1487:5: note: remove the 'if' if its condition is always false
    if (*p == '\0') {
    ^~~~~~~~~~~~~~~~~
/Users/remi/src/cpython/Modules/_zoneinfo.c:1460:32: note: initialize the variable 'dst_offset' to silence this warning
    long std_offset, dst_offset;
                               ^
                                = 0
/Users/remi/src/cpython/Modules/_zoneinfo.c:1910:19: warning: suggest braces around initialization of subobject
      [-Wmissing-braces]
    _tzrule rv = {0};
                  ^
                  {}

Looking at the code path, the unitialized dst_offset may create a ZoneInfo with a garbage value in dstoff with some inputs so this should be backported to Python3.9 too.

This is a duplicate of bpo-40714. It's a bit of an overzealous compiler warning (as far as I can tell it's not true that the uninitialized value would ever be used), but we fixed it anyway in GH-20291.

Thanks for the report!