Linux compilation using X11's header Xos.h failed, due to macro definition "index(s, c)"

[BoneCrasher]

Using libfmt on linux in conjunction with X11 headers I encountered a name collision with the macro index(s, c) defined in Xos.h.

If fmt/format.h is included AFTER Xos.h, this code in format.h will fail to compile:

  arg_id_kind kind;
  union value {
    FMT_CONSTEXPR value() : index(0u) {}
    FMT_CONSTEXPR value(int id) : index(id) {}
    FMT_CONSTEXPR value(basic_string_view<Char> n) : name(n) {}

    int index;
    basic_string_view<Char> name;
  } val;

with
error: macro "index" requires 2 arguments, but only 1 given FMT_CONSTEXPR value() : index(0u) {}

since index(...) expects two arguments.

[vitaut]

Worked around in 213e096.