[Bug c++/79301] New: With -Werror=pedantic outside C++17 mode, __has_cpp_attribute(fallthrough) is nonzero but [[fallthrough]] fails

[andersk at mit dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79301

            Bug ID: 79301
           Summary: With -Werror=pedantic outside C++17 mode,
                    __has_cpp_attribute(fallthrough) is nonzero but
                    [[fallthrough]] fails
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andersk at mit dot edu
  Target Milestone: ---

One would expect code like this to silence the new -Wimplicit-fallthrough
warning that’s been added to -Wextra:

    switch (x) {
    case 0:
        printf("zero\n");
#if __has_cpp_attribute(fallthrough)
        [[fallthrough]];
#elif __has_cpp_attribute(gnu::fallthrough)
        [[gnu::falthrough]];
#endif
    case 1:
        printf("zero or one\n");
    }

However, this fails to compile with -Werror=pedantic in C++14 mode or earlier:
“error: ‘fallthrough’ is a C++17 feature; use ‘gnu::fallthrough’
[-Werror=pedantic]”.

__has_cpp_attribute(fallthrough) should evaluate to 0 if [[fallthrough]] is not
going to work.