[Bug lto/106020] New: Spurious warnings about stringop overflows only with LTO

["matt at godbolt dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106020

            Bug ID: 106020
           Summary: Spurious warnings about stringop overflows only with
                    LTO
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at godbolt dot org
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

When using Howard Hinnant's date library, and GCC 12.1 on x86, and then with
LTO enabled, there are many apparently spurious errors after the read.constprop
pass:

/opt/compiler-explorer/libs/date/v3.0.1/include/date/date.h: In function
'read.constprop':
/opt/compiler-explorer/libs/date/v3.0.1/include/date/date.h:6506:15: note:
destination object 'buf' of size 11
 6506 |         CharT buf[std::numeric_limits<unsigned>::digits10+2u] = {};

It has proven tricky to get a smaller case but https://godbolt.org/z/hPaqz7TY1
demonstrates it with an inline version of date.h - a smaller version that
doesn't inline the date library is https://godbolt.org/z/e75YG6ano and the code
amounts to:

std::chrono::system_clock::time_point parse_datetime(const std::string &sv) {
    std::istringstream ss{sv};
    std::chrono::system_clock::time_point tp;
    ss >> date::parse("%FT%T%Z", tp); // ignoring errors for simplicity
    return tp;
}

(though it has to be linked in a separate TU with LTO enabled to trigger the
issues).

I initially filed this as a bug in the date library itself:
https://github.com/HowardHinnant/date/issues/740 but we now believe this is a
compiler issue.

Compiling with GCC 11 or earlier, or with clang, or without LTO doesn't trigger
these warnings.