🐛 Fix formatting of move-only types

Problem:
- Formatting of move-only types does not work.

Solution:
- Move the values through the formatting machinery appropriately.

Author: elbeno

include/stdx/ct_format.hpp

@@ -109,7 +109,7 @@ struct format_result<Str, Args, Spans> {
 
 template <typename Spans = type_list<>, typename Str, typename Args = tuple<>>
 constexpr auto make_format_result(Str s, Args args = {}) {
-    return format_result<Str, Args, Spans>{s, args};
+    return format_result<Str, Args, Spans>{s, std::move(args)};
 }
 
 inline namespace literals {
@@ -228,13 +228,13 @@ CONSTEVAL auto arg_type(fmt_cx_value auto a) {
 
 template <typename Str, typename Args, typename Spans, typename S>
 constexpr auto operator+(format_result<Str, Args, Spans> r, S s) {
-    return make_format_result<Spans>(r.str + s, r.args);
+    return make_format_result<Spans>(r.str + s, std::move(r.args));
 }
 
 template <typename S, typename Str, typename Args, typename Spans>
 constexpr auto operator+(S s, format_result<Str, Args, Spans> r) {
-    return make_format_result<detail::apply_offset<s.size(), Spans>>(s + r.str,
-                                                                     r.args);
+    return make_format_result<detail::apply_offset<s.size(), Spans>>(
+        s + r.str, std::move(r.args));
 }
 
 template <typename Str1, typename Args1, typename Spans1, typename Str2,
@@ -243,7 +243,7 @@ constexpr auto operator+(format_result<Str1, Args1, Spans1> r1,
                          format_result<Str2, Args2, Spans2> r2) {
     return make_format_result<boost::mp11::mp_append<
         Spans1, detail::apply_offset<r1.str.size(), Spans2>>>(
-        r1.str + r2.str, tuple_cat(r1.args, r2.args));
+        r1.str + r2.str, tuple_cat(std::move(r1.args), std::move(r2.args)));
 }
 
 template <typename T, T...> struct null_output;
@@ -271,7 +271,7 @@ CONSTEVAL auto convert_output() {
 }
 
 template <std::size_t N>
-CONSTEVAL auto perform_format(auto s, auto v) -> ct_string<N + 1> {
+CONSTEVAL auto perform_format(auto s, auto const &v) -> ct_string<N + 1> {
     ct_string<N + 1> cts{};
     fmt::format_to(cts.begin(), s, v);
     return cts;
@@ -302,10 +302,10 @@ constexpr auto format1(Arg arg) {
         auto const sub_result = format1<Fmt, Start>(arg.str);
         using Spans = typename Arg::spans_t;
         return make_format_result<detail::apply_offset<Start, Spans>>(
-            sub_result.str, arg.args);
+            sub_result.str, std::move(arg).args);
     } else {
         using Spans = type_list<format_span<Start, Fmt.size()>>;
-        return make_format_result<Spans>(cts_t<Fmt>{}, tuple{arg});
+        return make_format_result<Spans>(cts_t<Fmt>{}, tuple{std::move(arg)});
     }
 }
 
@@ -324,10 +324,20 @@ template <ct_string Fmt> struct fmt_data {
         to_ct_string<splits[N].view.size()>(splits[N].view);
 };
 
-template <typename T>
-constexpr auto ct_format_as(T const &t) -> decltype(auto) {
-    return (t);
-}
+[[maybe_unused]] constexpr inline struct format_as_t {
+    template <typename T>
+        requires true
+    constexpr auto operator()(T &&t) const
+        noexcept(noexcept(ct_format_as(std::forward<T>(t))))
+            -> decltype(ct_format_as(std::forward<T>(t))) {
+        return ct_format_as(std::forward<T>(t));
+    }
+
+    template <typename T>
+    constexpr auto operator()(T &&t) const -> decltype(auto) {
+        return T(std::forward<T>(t));
+    }
+} format_as;
 } // namespace detail
 
 template <ct_string Fmt,
@@ -349,14 +359,13 @@ constexpr auto ct_format = [](auto &&...args) {
         return detail::format1<cts, data::splits[I].start>(FWD(arg));
     };
 
-    auto const result = [&]<std::size_t... Is>(std::index_sequence<Is...>) {
-        using detail::ct_format_as;
-        return (format1.template operator()<Is>(ct_format_as(FWD(args))) + ... +
-                make_format_result(cts_t<data::last_cts>{}));
+    auto result = [&]<std::size_t... Is>(std::index_sequence<Is...>) {
+        return (format1.template operator()<Is>(detail::format_as(FWD(args))) +
+                ... + make_format_result(cts_t<data::last_cts>{}));
     }(std::make_index_sequence<data::N>{});
     constexpr auto str = detail::convert_output<result.str.value, Output>();
     using Spans = typename std::remove_cvref_t<decltype(result)>::spans_t;
-    return make_format_result<Spans>(str, result.args);
+    return make_format_result<Spans>(str, std::move(result).args);
 };
 
 template <ct_string Fmt>

test/ct_format.cpp

@@ -1,3 +1,5 @@
+#include "detail/tuple_types.hpp"
+
 #include <stdx/ct_format.hpp>
 #include <stdx/utility.hpp>
 
@@ -112,6 +114,33 @@ TEST_CASE("format a char (CX_VALUE)", "[ct_format]") {
             "Hello world"_ctst, stdx::tuple{stdx::ct_format_arg<char>{}}));
 }
 
+namespace move_test {
+struct move_only {
+    constexpr move_only() = default;
+    constexpr move_only(int x) : value{x} {}
+    constexpr move_only(move_only &&) = default;
+    constexpr auto operator=(move_only &&) noexcept -> move_only & = default;
+
+    friend constexpr auto operator==(move_only const &, move_only const &)
+        -> bool = default;
+
+    int value;
+};
+
+[[nodiscard]] constexpr auto format_as(move_only const &) -> std::string_view {
+    return "17";
+}
+} // namespace move_test
+
+TEST_CASE("format a move-only argument (CX_VALUE)", "[ct_format]") {
+    using expected_spans_t = stdx::type_list<stdx::format_span<6, 8>>;
+    STATIC_CHECK(
+        stdx::ct_format<"Hello {}">(CX_VALUE(move_test::move_only{17})) ==
+        stdx::make_format_result<expected_spans_t>(
+            "Hello 17"_ctst,
+            stdx::tuple{stdx::ct_format_arg<move_test::move_only>{}}));
+}
+
 TEST_CASE("format a compile-time integral argument (ct)", "[ct_format]") {
     using expected_spans_t = stdx::type_list<stdx::format_span<6, 8>>;
     STATIC_CHECK(stdx::ct_format<"Hello {}">(stdx::ct<42>()) ==
@@ -193,6 +222,14 @@ TEST_CASE("format a runtime argument", "[ct_format]") {
     STATIC_CHECK(stdx::ct_format<"Hello {}">(x) == expected);
 }
 
+TEST_CASE("format a move-only runtime argument", "[ct_format]") {
+    using expected_spans_t = stdx::type_list<stdx::format_span<6, 8>>;
+    constexpr auto expected = stdx::make_format_result<expected_spans_t>(
+        "Hello {}"_ctst, stdx::tuple{move_only{17}});
+    CHECK(stdx::ct_format<"Hello {}">(move_only{17}) == expected);
+    STATIC_CHECK(stdx::ct_format<"Hello {}">(move_only{17}) == expected);
+}
+
 TEST_CASE("format a compile-time and a runtime argument (1)", "[ct_format]") {
     constexpr auto x = 17;
     using expected_spans_t =