In toolset update#5783 I added test coverage for<filesystem> long path support. This requires a manifest to be embedded into the executable with a special linker option. The MSVC-internal test harness uses Perl for this, and I used a singlecl command. It turns out that in certain configurations (of course only run in CI checks, not in PR checks where I would have discovered this earlier), the MSVC-internal test harness adds/FastFailDump. When compiling with Clang, that gets sent to lld-link, which emits

lld-link: error: could not open '/FastFailDump': no such file or directory

I should have followed precedent and used separate compile and link commands, which avoids confusing lld-link (as in this case I believe we invoke MSVC link.exe):