This formatter isn't in the list of required formatters in
[format.formatter.spec]/2.2
For each charT, the string type specializations template<> struct formatter<charT*, charT>; template<> struct formatter<const charT*, charT>; template<size_t N> struct formatter<const charT[N], charT>; template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;
Since remove_cvref_t<const charT[N]> is charT[N] the formatter is
required by
[format.functions]/25
Preconditions: formatter<remove_cvref_t<Ti>, charT> meets the BasicFormatter requirements ([formatter.requirements]) for each Ti in Args.
Depends on D120921
I suggest adding a static assert that _CharT is non-const.