Proposed resolution:

This wording is relative to N5046.

[Drafting Note: Three options are prepared, depicted below by Option A, Option B, and Option C, respectively, where option C is mutually exclusive with option A and B.

Note that libstdc++ effectively implemented option A recently (in commit 23b9ac257cf508bc4fa479c82fcfc768c4abcd79).]

Option A: Just marking the partial specialization hosted

1. Modify [optional.syn], header <optional> synopsis, as indicated:

   […]
   template<class T>
     constexpr bool ranges::enable_view<optional<T>> = true;
   template<class T>
     constexpr auto format_kind<optional<T>> = range_format::disabled; // hosted
   template<class T>
     constexpr bool ranges::enable_borrowed_range<optional<T&>> = true;
   […]
   

Option B: Adding a general rule that infers hosted-ness of specializations

1. Modify [freestanding.item] as indicated:

   -4- A declaration in a synopsis is a freestanding item if […]

   -5- An entity or deduction guide is a freestanding item if its introducing declaration is not followed by a comment that includes hosted, and is: […]

   -6- A macro is a freestanding item if it is defined in a header synopsis and […]

   -?- A partial or full template specialization is not a freestanding item if its primary template is not a freestanding item.

Option C: Making `range_format` and `format_kind` freestanding

1. Modify Table 27 [tab:headers.cpp.fs] as indicated:

   Table 27: C++ headers for freestanding implementations [tab:headers.cpp.fs]

   	Subclause	Header
   […]
   [charconv]	Primitive numeric conversions	<charconv>
   [format]	Formatting	<format>
   [rand]	Random number generation	<random>
   […]

2. Modify [format.syn], header <format> synopsis, as indicated:

   […]
   // [format.range], formatting of ranges
   // [format.range.fmtkind], variable template format_kind
   enum class range_format {
     disabled,
     map,
     set,
     sequence,
     string,
     debug_string
   };
   
   template<class R>
     constexpr unspecified format_kind = unspecified;         // freestanding
   
   template<ranges::input_range R>
     requires same_as<R, remove_cvref_t<R>>
     constexpr range_format format_kind<R> = see below;       // freestanding
   […]
   

Currently, <optional> is a mostly freestanding header and provides a partial specialization of `std::format_kind` which isn't mark `// hosted` in the synopsis ([optional.syn]). The current rules in [freestanding.item] p4 seemingly requires the partial specialization to be a freestanding item, while nothing seems to requires the primary template (provided by <format>) to be freestanding.

The status quo looks quite weird. Perhaps we should make the freestanding status same for the primary template and the partial specialization.