constexpr for compiled fmt

include/fmt/compile.h

@@ -135,7 +135,7 @@ struct udl_compiled_string : compiled_string {
 #endif
 
 template <typename T, typename... Tail>
-const T& first(const T& value, const Tail&...) {
+FMT_CONSTEXPR const T& first(const T& value, const Tail&...) {
   return value;
 }
 
@@ -504,8 +504,8 @@ FMT_BEGIN_EXPORT
 template <typename CompiledFormat, typename... Args,
           typename Char = typename CompiledFormat::char_type,
           FMT_ENABLE_IF(detail::is_compiled_format<CompiledFormat>::value)>
-FMT_INLINE std::basic_string<Char> format(const CompiledFormat& cf,
+FMT_CONSTEXPR20 FMT_INLINE std::basic_string<Char> format(
-                                          const Args&... args) {
+    const CompiledFormat& cf, const Args&... args) {
   auto s = std::basic_string<Char>();
   cf.format(std::back_inserter(s), args...);
   return s;
@@ -520,8 +520,8 @@ constexpr FMT_INLINE OutputIt format_to(OutputIt out, const CompiledFormat& cf,
 
 template <typename S, typename... Args,
           FMT_ENABLE_IF(detail::is_compiled_string<S>::value)>
-FMT_INLINE std::basic_string<typename S::char_type> format(const S&,
+FMT_INLINE FMT_CONSTEXPR20 std::basic_string<typename S::char_type> format(
-                                                           Args&&... args) {
+    const S&, Args&&... args) {
   if constexpr (std::is_same<typename S::char_type, char>::value) {
     constexpr auto str = basic_string_view<typename S::char_type>(S());
     if constexpr (str.size() == 2 && str[0] == '{' && str[1] == '}') {

include/fmt/core.h

@@ -825,11 +825,11 @@ constexpr auto has_const_formatter() -> bool {
 
 // Extracts a reference to the container from back_insert_iterator.
 template <typename Container>
-inline auto get_container(std::back_insert_iterator<Container> it)
+FMT_CONSTEXPR inline auto get_container(std::back_insert_iterator<Container> it)
     -> Container& {
   using base = std::back_insert_iterator<Container>;
   struct accessor : base {
-    accessor(base b) : base(b) {}
+    FMT_CONSTEXPR accessor(base b) : base(b) {}
     using base::container;
   };
   return *accessor(it).container;
@@ -933,7 +933,8 @@ template <typename T> class buffer {
   }
 
   /** Appends data to the end of the buffer. */
-  template <typename U> void append(const U* begin, const U* end);
+  template <typename U>
+  void FMT_CONSTEXPR append(const U* begin, const U* end);
 
   template <typename Idx> FMT_CONSTEXPR auto operator[](Idx index) -> T& {
     return ptr_[index];
@@ -1493,8 +1494,8 @@ class appender : public std::back_insert_iterator<detail::buffer<char>> {
   appender(base it) noexcept : base(it) {}
   FMT_UNCHECKED_ITERATOR(appender);
 
-  auto operator++() noexcept -> appender& { return *this; }
+  FMT_CONSTEXPR20 auto operator++() noexcept -> appender& { return *this; }
-  auto operator++(int) noexcept -> appender { return *this; }
+  FMT_CONSTEXPR20 auto operator++(int) noexcept -> appender { return *this; }
 };
 
 // A formatting argument. It is a trivially copyable/constructible type to
@@ -1604,7 +1605,8 @@ FMT_CONSTEXPR FMT_INLINE auto visit_format_arg(
 namespace detail {
 
 template <typename Char, typename InputIt>
-auto copy_str(InputIt begin, InputIt end, appender out) -> appender {
+FMT_CONSTEXPR20 auto copy_str(InputIt begin, InputIt end, appender out)
+    -> appender {
   get_container(out).append(begin, end);
   return out;
 }

include/fmt/format.h

@@ -546,11 +546,12 @@ template <typename T> using sentinel_t = decltype(std::end(std::declval<T&>()));
 
 // A workaround for std::string not having mutable data() until C++17.
 template <typename Char>
-inline auto get_data(std::basic_string<Char>& s) -> Char* {
+inline FMT_CONSTEXPR auto get_data(std::basic_string<Char>& s) -> Char* {
   return &s[0];
 }
 template <typename Container>
-inline auto get_data(Container& c) -> typename Container::value_type* {
+inline FMT_CONSTEXPR auto get_data(Container& c) ->
+    typename Container::value_type* {
   return c.data();
 }
 
@@ -574,7 +575,7 @@ template <typename Container, FMT_ENABLE_IF(is_contiguous<Container>::value)>
 #if FMT_CLANG_VERSION >= 307 && !FMT_ICC_VERSION
 __attribute__((no_sanitize("undefined")))
 #endif
-inline auto
+inline FMT_CONSTEXPR auto
 reserve(std::back_insert_iterator<Container> it, size_t n)
     -> checked_ptr<typename Container::value_type> {
   Container& c = get_container(it);
@@ -612,8 +613,9 @@ template <typename T> auto to_pointer(buffer_appender<T> it, size_t n) -> T* {
 }
 
 template <typename Container, FMT_ENABLE_IF(is_contiguous<Container>::value)>
-inline auto base_iterator(std::back_insert_iterator<Container>& it,
+inline FMT_CONSTEXPR auto base_iterator(
-                          checked_ptr<typename Container::value_type>)
+    std::back_insert_iterator<Container>& it,
+    checked_ptr<typename Container::value_type>)
     -> std::back_insert_iterator<Container> {
   return it;
 }
@@ -875,13 +877,14 @@ using is_double_double = bool_constant<std::numeric_limits<T>::digits == 106>;
 
 template <typename T>
 template <typename U>
-void buffer<T>::append(const U* begin, const U* end) {
+FMT_CONSTEXPR void buffer<T>::append(const U* begin, const U* end) {
   while (begin != end) {
     auto count = to_unsigned(end - begin);
     try_reserve(size_ + count);
     auto free_cap = capacity_ - size_;
     if (free_cap < count) count = free_cap;
-    std::uninitialized_copy_n(begin, count, make_checked(ptr_ + size_, count));
+    auto out = make_checked(ptr_ + size_, count);
+    for (size_t i = 0; i < count; ++i) *out++ = begin[i];
     size_ += count;
     begin += count;
   }
@@ -4462,14 +4465,15 @@ auto join(Range&& range, string_view sep)
   \endrst
  */
 template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
-inline auto to_string(const T& value) -> std::string {
+FMT_NODISCARD FMT_CONSTEXPR20 inline auto to_string(const T& value)
+    -> std::string {
   auto buffer = memory_buffer();
   detail::write<char>(appender(buffer), value);
   return {buffer.data(), buffer.size()};
 }
 
 template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
-FMT_NODISCARD inline auto to_string(T value) -> std::string {
+FMT_NODISCARD FMT_CONSTEXPR20 inline auto to_string(T value) -> std::string {
   // The buffer should be large enough to store the number including the sign
   // or "false" for bool.
   constexpr int max_size = detail::digits10<T>() + 2;

test/compile-test.cc

@@ -327,6 +327,10 @@ TEST(compile_time_formatting_test, bool) {
   EXPECT_EQ("false", test_format<6>(FMT_COMPILE("{}"), false));
   EXPECT_EQ("true ", test_format<6>(FMT_COMPILE("{:5}"), true));
   EXPECT_EQ("1", test_format<2>(FMT_COMPILE("{:d}"), true));
+  static_assert(fmt::format(FMT_COMPILE("{}"), true) == "true");
+  static_assert(fmt::format(FMT_COMPILE("{}"), false) == "false");
+  static_assert(fmt::format(FMT_COMPILE("{:5}"), true) == "true ");
+  static_assert(fmt::format(FMT_COMPILE("{:d}"), true) == "1");
 }
 
 TEST(compile_time_formatting_test, integer) {