Compile errors with GCC 7.1 and below

[sergiud]

scan-test.cc fails to compile with GCC 7.1 and older issuing the following errors:

<source>:172:21: error: enclosing class of constexpr non-static member function 'int fmt::v9::detail::scan_handler::on_arg_id()' is not a literal type
   FMT_CONSTEXPR int on_arg_id() { return on_arg_id(next_arg_id_++); }
                     ^~~~~~~~~
<source>:125:8: note: 'fmt::v9::detail::scan_handler' is not literal because:
 struct scan_handler : error_handler {
        ^~~~~~~~~~~~
<source>:125:8: note:   'fmt::v9::detail::scan_handler' is not an aggregate, does not have a trivial default constructor, and has no constexpr constructor that is not a copy or move constructor
<source>:173:21: error: enclosing class of constexpr non-static member function 'int fmt::v9::detail::scan_handler::on_arg_id(int)' is not a literal type
   FMT_CONSTEXPR int on_arg_id(int id) {
                     ^~~~~~~~~
<source>:178:21: error: enclosing class of constexpr non-static member function 'int fmt::v9::detail::scan_handler::on_arg_id(fmt::v9::string_view)' is not a literal type
   FMT_CONSTEXPR int on_arg_id(string_view id) {
                     ^~~~~~~~~
Compiler returned: 1

Reproducer: https://godbolt.org/z/jcxnx7MeT. A workaround is to remove FMT_CONSTEXPR from failing methods.