Starting to require C++20 broke MSVC support on 2026-07-16

[hartwork]

Hi!

I noticed that the MSVC Windows CI is red on main and has been since run https://github.com/taocpp/PEGTL/actions/runs/29514279131 for commit cfe26fd where C++20 was activated and required. Back then the issue was…

2026-07-16T16:08:56.3694974Z D:\a\PEGTL\PEGTL\include\tao/pegtl/action/add_guard.hpp(61): error C2220: the following warning is treated as an error
2026-07-16T16:08:56.3712165Z D:\a\PEGTL\PEGTL\include\tao/pegtl/action/add_guard.hpp(61): warning C4100: '<st_0>': unreferenced parameter

…with MSVC 2022 and…

2026-07-16T16:10:09.9101956Z D:\a\PEGTL\PEGTL\include\tao\pegtl\control\internal/rewind_state_control.hpp(48): error C2220: the following warning is treated as an error
2026-07-16T16:10:09.9103972Z D:\a\PEGTL\PEGTL\include\tao\pegtl\control\internal/rewind_state_control.hpp(48): warning C4864: expected 'template' keyword before dependent template name

…with MSVC 2025.

I'm opening this issue because I think it's important to know and maybe it motivates someone to have a closer look and help out with a fix (that is more versed in C++20 than me).

Best, Sebastian

[ColinH]

Yes, while working on new features I tend to ignore the Windows build until things settle down - no use fixing MSVC issues if the code is still changing. The new "builders" are triggering another completely bogus error...