Add support of most format_specs for formatting at compile-time (#2056)

include/fmt/compile.h

@@ -466,10 +466,10 @@ struct is_compiled_format<field<Char, T, N>> : std::true_type {};
 // A replacement field that refers to argument N and has format specifiers.
 template <typename Char, typename T, int N> struct spec_field {
   using char_type = Char;
-  mutable formatter<T, Char> fmt;
+  formatter<T, Char> fmt;
 
   template <typename OutputIt, typename... Args>
-  OutputIt format(OutputIt out, const Args&... args) const {
+  constexpr OutputIt format(OutputIt out, const Args&... args) const {
     // This ensures that the argument type is convertile to `const T&`.
     const T& arg = get<N>(args...);
     const auto& vargs =

include/fmt/core.h

@@ -283,6 +283,14 @@ struct monostate {};
 
 namespace detail {
 
+constexpr bool is_constant_evaluated() FMT_NOEXCEPT {
+#ifdef __cpp_lib_is_constant_evaluated
+  return std::is_constant_evaluated();
+#else
+  return false;
+#endif
+}
+
 // A helper function to suppress "conditional expression is constant" warnings.
 template <typename T> constexpr T const_check(T value) { return value; }
 
@@ -481,7 +489,7 @@ inline basic_string_view<Char> to_string_view(
 }
 
 template <typename Char>
-inline basic_string_view<Char> to_string_view(basic_string_view<Char> s) {
+constexpr basic_string_view<Char> to_string_view(basic_string_view<Char> s) {
   return s;
 }
 
@@ -938,9 +946,9 @@ struct arg_data<T, Char, NUM_ARGS, 0> {
   T args_[NUM_ARGS != 0 ? NUM_ARGS : +1];
 
   template <typename... U>
-  FMT_INLINE arg_data(const U&... init) : args_{init...} {}
+  FMT_CONSTEXPR arg_data(const U&... init) : args_{init...} {}
-  FMT_INLINE const T* args() const { return args_; }
+  FMT_CONSTEXPR const T* args() const { return args_; }
-  FMT_INLINE std::nullptr_t named_args() { return nullptr; }
+  FMT_CONSTEXPR std::nullptr_t named_args() { return nullptr; }
 };
 
 template <typename Char>
@@ -961,7 +969,7 @@ void init_named_args(named_arg_info<Char>* named_args, int arg_count,
 }
 
 template <typename... Args>
-FMT_INLINE void init_named_args(std::nullptr_t, int, int, const Args&...) {}
+FMT_CONSTEXPR void init_named_args(std::nullptr_t, int, int, const Args&...) {}
 
 template <typename T> struct is_named_arg : std::false_type {};
 
@@ -1073,17 +1081,20 @@ template <typename Context> class value {
 
   constexpr FMT_INLINE value(int val = 0) : int_value(val) {}
   constexpr FMT_INLINE value(unsigned val) : uint_value(val) {}
-  FMT_INLINE value(long long val) : long_long_value(val) {}
+  constexpr FMT_INLINE value(long long val) : long_long_value(val) {}
-  FMT_INLINE value(unsigned long long val) : ulong_long_value(val) {}
+  constexpr FMT_INLINE value(unsigned long long val) : ulong_long_value(val) {}
   FMT_INLINE value(int128_t val) : int128_value(val) {}
   FMT_INLINE value(uint128_t val) : uint128_value(val) {}
   FMT_INLINE value(float val) : float_value(val) {}
   FMT_INLINE value(double val) : double_value(val) {}
   FMT_INLINE value(long double val) : long_double_value(val) {}
-  FMT_INLINE value(bool val) : bool_value(val) {}
+  constexpr FMT_INLINE value(bool val) : bool_value(val) {}
-  FMT_INLINE value(char_type val) : char_value(val) {}
+  constexpr FMT_INLINE value(char_type val) : char_value(val) {}
-  FMT_INLINE value(const char_type* val) { string.data = val; }
+  FMT_CONSTEXPR value(const char_type* val) {
-  FMT_INLINE value(basic_string_view<char_type> val) {
+    string.data = val;
+    if (is_constant_evaluated()) string.size = {};
+  }
+  FMT_CONSTEXPR value(basic_string_view<char_type> val) {
     string.data = val.data();
     string.size = val.size();
   }
@@ -1406,7 +1417,7 @@ class locale_ref {
   const void* locale_;  // A type-erased pointer to std::locale.
 
  public:
-  locale_ref() : locale_(nullptr) {}
+  constexpr locale_ref() : locale_(nullptr) {}
   template <typename Locale> explicit locale_ref(const Locale& loc);
 
   explicit operator bool() const FMT_NOEXCEPT { return locale_ != nullptr; }
@@ -1437,7 +1448,7 @@ template <typename T> int check(unformattable) {
       "formatter<T> specialization: https://fmt.dev/latest/api.html#udt");
   return 0;
 }
-template <typename T, typename U> inline const U& check(const U& val) {
+template <typename T, typename U> constexpr const U& check(const U& val) {
   return val;
 }
 
@@ -1446,7 +1457,7 @@ template <typename T, typename U> inline const U& check(const U& val) {
 // another (not recommended).
 template <bool IS_PACKED, typename Context, type, typename T,
           FMT_ENABLE_IF(IS_PACKED)>
-inline value<Context> make_arg(const T& val) {
+constexpr value<Context> make_arg(const T& val) {
   return check<T>(arg_mapper<Context>().map(val));
 }
 
@@ -1480,28 +1491,30 @@ template <typename OutputIt, typename Char> class basic_format_context {
    Constructs a ``basic_format_context`` object. References to the arguments are
    stored in the object so make sure they have appropriate lifetimes.
    */
-  basic_format_context(OutputIt out,
+  constexpr basic_format_context(
-                       basic_format_args<basic_format_context> ctx_args,
+      OutputIt out, basic_format_args<basic_format_context> ctx_args,
       detail::locale_ref loc = detail::locale_ref())
       : out_(out), args_(ctx_args), loc_(loc) {}
 
-  format_arg arg(int id) const { return args_.get(id); }
+  constexpr format_arg arg(int id) const { return args_.get(id); }
-  format_arg arg(basic_string_view<char_type> name) { return args_.get(name); }
+  FMT_CONSTEXPR format_arg arg(basic_string_view<char_type> name) {
+    return args_.get(name);
+  }
   int arg_id(basic_string_view<char_type> name) { return args_.get_id(name); }
   const basic_format_args<basic_format_context>& args() const { return args_; }
 
-  detail::error_handler error_handler() { return {}; }
+  FMT_CONSTEXPR detail::error_handler error_handler() { return {}; }
   void on_error(const char* message) { error_handler().on_error(message); }
 
   // Returns an iterator to the beginning of the output range.
-  iterator out() { return out_; }
+  FMT_CONSTEXPR iterator out() { return out_; }
 
   // Advances the begin iterator to ``it``.
   void advance_to(iterator it) {
     if (!detail::is_back_insert_iterator<iterator>()) out_ = it;
   }
 
-  detail::locale_ref locale() { return loc_; }
+  FMT_CONSTEXPR detail::locale_ref locale() { return loc_; }
 };
 
 template <typename Char>
@@ -1550,7 +1563,7 @@ class format_arg_store
            : 0);
 
  public:
-  format_arg_store(const Args&... args)
+  FMT_CONSTEXPR format_arg_store(const Args&... args)
       :
 #if FMT_GCC_VERSION && FMT_GCC_VERSION < 409
         basic_format_args<Context>(*this),
@@ -1571,7 +1584,7 @@ class format_arg_store
   \endrst
  */
 template <typename Context = format_context, typename... Args>
-inline format_arg_store<Context, Args...> make_format_args(
+constexpr format_arg_store<Context, Args...> make_format_args(
     const Args&... args) {
   return {args...};
 }
@@ -1644,25 +1657,27 @@ template <typename Context> class basic_format_args {
     const format_arg* args_;
   };
 
-  bool is_packed() const { return (desc_ & detail::is_unpacked_bit) == 0; }
+  constexpr bool is_packed() const {
+    return (desc_ & detail::is_unpacked_bit) == 0;
+  }
   bool has_named_args() const {
     return (desc_ & detail::has_named_args_bit) != 0;
   }
 
-  detail::type type(int index) const {
+  FMT_CONSTEXPR detail::type type(int index) const {
     int shift = index * detail::packed_arg_bits;
     unsigned int mask = (1 << detail::packed_arg_bits) - 1;
     return static_cast<detail::type>((desc_ >> shift) & mask);
   }
 
-  basic_format_args(unsigned long long desc,
+  constexpr basic_format_args(unsigned long long desc,
                               const detail::value<Context>* values)
       : desc_(desc), values_(values) {}
-  basic_format_args(unsigned long long desc, const format_arg* args)
+  constexpr basic_format_args(unsigned long long desc, const format_arg* args)
       : desc_(desc), args_(args) {}
 
  public:
-  basic_format_args() : desc_(0) {}
+  constexpr basic_format_args() : desc_(0), args_(nullptr) {}
 
   /**
    \rst
@@ -1670,7 +1685,7 @@ template <typename Context> class basic_format_args {
    \endrst
    */
   template <typename... Args>
-  FMT_INLINE basic_format_args(const format_arg_store<Context, Args...>& store)
+  constexpr basic_format_args(const format_arg_store<Context, Args...>& store)
       : basic_format_args(store.desc, store.data_.args()) {}
 
   /**
@@ -1679,7 +1694,7 @@ template <typename Context> class basic_format_args {
    `~fmt::dynamic_format_arg_store`.
    \endrst
    */
-  FMT_INLINE basic_format_args(const dynamic_format_arg_store<Context>& store)
+  constexpr basic_format_args(const dynamic_format_arg_store<Context>& store)
       : basic_format_args(store.get_types(), store.data()) {}
 
   /**
@@ -1687,12 +1702,12 @@ template <typename Context> class basic_format_args {
    Constructs a `basic_format_args` object from a dynamic set of arguments.
    \endrst
    */
-  basic_format_args(const format_arg* args, int count)
+  constexpr basic_format_args(const format_arg* args, int count)
       : basic_format_args(detail::is_unpacked_bit | detail::to_unsigned(count),
                           args) {}
 
   /** Returns the argument with the specified id. */
-  format_arg get(int id) const {
+  FMT_CONSTEXPR format_arg get(int id) const {
     format_arg arg;
     if (!is_packed()) {
       if (id < max_size()) arg = args_[id];

include/fmt/format-inl.h

@@ -248,9 +248,6 @@ const typename basic_data<T>::digit_pair basic_data<T>::digits[] = {
     {'9', '0'}, {'9', '1'}, {'9', '2'}, {'9', '3'}, {'9', '4'}, {'9', '5'},
     {'9', '6'}, {'9', '7'}, {'9', '8'}, {'9', '9'}};
 
-template <typename T>
-const char basic_data<T>::hex_digits[] = "0123456789abcdef";
-
 #define FMT_POWERS_OF_10(factor)                                             \
   factor * 10, (factor)*100, (factor)*1000, (factor)*10000, (factor)*100000, \
       (factor)*1000000, (factor)*10000000, (factor)*100000000,               \
@@ -1071,10 +1068,13 @@ const char basic_data<T>::background_color[] = "\x1b[48;2;";
 template <typename T> const char basic_data<T>::reset_color[] = "\x1b[0m";
 template <typename T> const wchar_t basic_data<T>::wreset_color[] = L"\x1b[0m";
 template <typename T> const char basic_data<T>::signs[] = {0, '-', '+', ' '};
+
+#if __cplusplus < 201703L
+template <typename T> constexpr const char basic_data<T>::hex_digits[];
+template <typename T> constexpr const char basic_data<T>::left_padding_shifts[];
 template <typename T>
-const char basic_data<T>::left_padding_shifts[] = {31, 31, 0, 1, 0};
+constexpr const char basic_data<T>::right_padding_shifts[];
-template <typename T>
+#endif
-const char basic_data<T>::right_padding_shifts[] = {0, 31, 0, 1, 0};
 
 template <typename T> struct bits {
   static FMT_CONSTEXPR_DECL const int value =

include/fmt/format.h

@@ -283,20 +283,13 @@ FMT_END_NAMESPACE
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-#if __cplusplus >= 202002L
+#if __cplusplus >= 202002L || \
+    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
 #  define FMT_CONSTEXPR20 constexpr
 #else
 #  define FMT_CONSTEXPR20 inline
 #endif
 
-constexpr bool is_constant_evaluated() FMT_DETECTED_NOEXCEPT {
-#ifdef __cpp_lib_is_constant_evaluated
-  return std::is_constant_evaluated();
-#else
-  return false;
-#endif
-}
-
 // An equivalent of `*reinterpret_cast<Dest*>(&source)` that doesn't have
 // undefined behavior (e.g. due to type aliasing).
 // Example: uint64_t d = bit_cast<uint64_t>(2.718);
@@ -552,7 +545,7 @@ inline size_t count_code_points(basic_string_view<Char> s) {
 }
 
 // Counts the number of code points in a UTF-8 string.
-inline size_t count_code_points(basic_string_view<char> s) {
+FMT_CONSTEXPR size_t count_code_points(basic_string_view<char> s) {
   const char* data = s.data();
   size_t num_code_points = 0;
   for (size_t i = 0, size = s.size(); i != size; ++i) {
@@ -585,12 +578,15 @@ inline size_t code_point_index(basic_string_view<char8_type> s, size_t n) {
 // <algorithm> is spectacularly slow to compile in C++20 so use a simple fill_n
 // instead (#1998).
 template <typename OutputIt, typename Size, typename T>
-OutputIt fill_n(OutputIt out, Size count, const T& value) {
+FMT_CONSTEXPR OutputIt fill_n(OutputIt out, Size count, const T& value) {
   for (Size i = 0; i < count; ++i) *out++ = value;
   return out;
 }
 template <typename T, typename Size>
-inline T* fill_n(T* out, Size count, char value) {
+FMT_CONSTEXPR20 T* fill_n(T* out, Size count, char value) {
+  if (is_constant_evaluated()) {
+    return fill_n<T*, Size, T>(out, count, value);
+  }
   std::memset(out, value, to_unsigned(count));
   return out + count;
 }
@@ -938,14 +934,14 @@ template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
   // GCC generates slightly better code for pairs than chars.
   using digit_pair = char[2];
   static const digit_pair digits[];
-  static const char hex_digits[];
+  static constexpr const char hex_digits[] = "0123456789abcdef";
   static const char foreground_color[];
   static const char background_color[];
   static const char reset_color[5];
   static const wchar_t wreset_color[5];
   static const char signs[];
-  static const char left_padding_shifts[5];
+  static constexpr const char left_padding_shifts[] = {31, 31, 0, 1, 0};
-  static const char right_padding_shifts[5];
+  static constexpr const char right_padding_shifts[] = {0, 31, 0, 1, 0};
 
   // DEPRECATED! These are for ABI compatibility.
   static const uint32_t zero_or_powers_of_10_32[];
@@ -1144,7 +1140,7 @@ inline format_decimal_result<Iterator> format_decimal(Iterator out, UInt value,
 }
 
 template <unsigned BASE_BITS, typename Char, typename UInt>
-inline Char* format_uint(Char* buffer, UInt value, int num_digits,
+FMT_CONSTEXPR Char* format_uint(Char* buffer, UInt value, int num_digits,
                                 bool upper = false) {
   buffer += num_digits;
   Char* end = buffer;
@@ -1224,8 +1220,8 @@ template <typename Char> struct fill_t {
     size_ = static_cast<unsigned char>(size);
   }
 
-  size_t size() const { return size_; }
+  constexpr size_t size() const { return size_; }
-  const Char* data() const { return data_; }
+  constexpr const Char* data() const { return data_; }
 
   FMT_CONSTEXPR Char& operator[](size_t index) { return data_[index]; }
   FMT_CONSTEXPR const Char& operator[](size_t index) const {
@@ -1544,7 +1540,8 @@ class cstring_type_checker : public ErrorHandler {
 };
 
 template <typename OutputIt, typename Char>
-FMT_NOINLINE OutputIt fill(OutputIt it, size_t n, const fill_t<Char>& fill) {
+FMT_NOINLINE FMT_CONSTEXPR OutputIt fill(OutputIt it, size_t n,
+                                         const fill_t<Char>& fill) {
   auto fill_size = fill.size();
   if (fill_size == 1) return detail::fill_n(it, n, fill[0]);
   auto data = fill.data();
@@ -1558,15 +1555,16 @@ FMT_NOINLINE OutputIt fill(OutputIt it, size_t n, const fill_t<Char>& fill) {
 // width: output display width in (terminal) column positions.
 template <align::type align = align::left, typename OutputIt, typename Char,
           typename F>
-inline OutputIt write_padded(OutputIt out,
+FMT_CONSTEXPR OutputIt write_padded(OutputIt out,
-                             const basic_format_specs<Char>& specs, size_t size,
+                                    const basic_format_specs<Char>& specs,
-                             size_t width, F&& f) {
+                                    size_t size, size_t width, F&& f) {
   static_assert(align == align::left || align == align::right, "");
   unsigned spec_width = to_unsigned(specs.width);
   size_t padding = spec_width > width ? spec_width - width : 0;
+  size_t left_padding = 0;
   auto* shifts = align == align::left ? data::left_padding_shifts
                                       : data::right_padding_shifts;
-  size_t left_padding = padding >> shifts[specs.align];
+  left_padding = padding >> shifts[specs.align];
   auto it = reserve(out, size + padding * specs.fill.size());
   it = fill(it, left_padding, specs.fill);
   it = f(it);
@@ -1576,9 +1574,9 @@ inline OutputIt write_padded(OutputIt out,
 
 template <align::type align = align::left, typename OutputIt, typename Char,
           typename F>
-inline OutputIt write_padded(OutputIt out,
+constexpr OutputIt write_padded(OutputIt out,
-                             const basic_format_specs<Char>& specs, size_t size,
+                                const basic_format_specs<Char>& specs,
-                             F&& f) {
+                                size_t size, F&& f) {
   return write_padded<align>(out, specs, size, size, f);
 }
 
@@ -1598,7 +1596,7 @@ template <typename Char> struct write_int_data {
   size_t size;
   size_t padding;
 
-  write_int_data(int num_digits, string_view prefix,
+  FMT_CONSTEXPR write_int_data(int num_digits, string_view prefix,
                                const basic_format_specs<Char>& specs)
       : size(prefix.size() + to_unsigned(num_digits)), padding(0) {
     if (specs.align == align::numeric) {
@@ -1618,7 +1616,8 @@ template <typename Char> struct write_int_data {
 //   <left-padding><prefix><numeric-padding><digits><right-padding>
 // where <digits> are written by f(it).
 template <typename OutputIt, typename Char, typename F>
-OutputIt write_int(OutputIt out, int num_digits, string_view prefix,
+FMT_CONSTEXPR OutputIt write_int(OutputIt out, int num_digits,
+                                 string_view prefix,
                                  const basic_format_specs<Char>& specs, F f) {
   auto data = write_int_data<Char>(num_digits, prefix, specs);
   using iterator = remove_reference_t<decltype(reserve(out, 0))>;
@@ -1631,7 +1630,7 @@ OutputIt write_int(OutputIt out, int num_digits, string_view prefix,
 }
 
 template <typename StrChar, typename Char, typename OutputIt>
-OutputIt write(OutputIt out, basic_string_view<StrChar> s,
+FMT_CONSTEXPR OutputIt write(OutputIt out, basic_string_view<StrChar> s,
                              const basic_format_specs<Char>& specs) {
   auto data = s.data();
   auto size = s.size();
@@ -1658,10 +1657,12 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
   using iterator =
       remove_reference_t<decltype(reserve(std::declval<OutputIt&>(), 0))>;
 
-  string_view get_prefix() const { return string_view(prefix, prefix_size); }
+  constexpr string_view get_prefix() const {
+    return string_view(prefix, prefix_size);
+  }
 
   template <typename Int>
-  int_writer(OutputIt output, locale_ref loc, Int value,
+  FMT_CONSTEXPR int_writer(OutputIt output, locale_ref loc, Int value,
                            const basic_format_specs<Char>& s)
       : out(output),
         locale(loc),
@@ -1679,7 +1680,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
     }
   }
 
-  void on_dec() {
+  FMT_CONSTEXPR void on_dec() {
     auto num_digits = count_digits(abs_value);
     out = write_int(
         out, num_digits, get_prefix(), specs, [this, num_digits](iterator it) {
@@ -1687,7 +1688,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
         });
   }
 
-  void on_hex() {
+  FMT_CONSTEXPR void on_hex() {
     if (specs.alt) {
       prefix[prefix_size++] = '0';
       prefix[prefix_size++] = specs.type;
@@ -1700,7 +1701,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
                     });
   }
 
-  void on_bin() {
+  FMT_CONSTEXPR void on_bin() {
     if (specs.alt) {
       prefix[prefix_size++] = '0';
       prefix[prefix_size++] = static_cast<char>(specs.type);
@@ -1712,7 +1713,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
                     });
   }
 
-  void on_oct() {
+  FMT_CONSTEXPR void on_oct() {
     int num_digits = count_digits<3>(abs_value);
     if (specs.alt && specs.precision <= num_digits && abs_value != 0) {
       // Octal prefix '0' is counted as a digit, so only add it if precision
@@ -2041,7 +2042,7 @@ inline OutputIt write(OutputIt out, T value) {
 }
 
 template <typename Char, typename OutputIt>
-OutputIt write_char(OutputIt out, Char value,
+constexpr OutputIt write_char(OutputIt out, Char value,
                               const basic_format_specs<Char>& specs) {
   using iterator = remove_reference_t<decltype(reserve(out, 0))>;
   return write_padded(out, specs, 1, [=](iterator it) {
@@ -2205,7 +2206,8 @@ class arg_formatter_base {
   using reserve_iterator = remove_reference_t<decltype(
       detail::reserve(std::declval<iterator&>(), 0))>;
 
-  template <typename T> void write_int(T value, const format_specs& spec) {
+  template <typename T>
+  FMT_CONSTEXPR void write_int(T value, const format_specs& spec) {
     using uint_type = uint32_or_64_or_128_t<T>;
     int_writer<iterator, Char, uint_type> w(out_, locale_, value, spec);
     handle_int_type_spec(spec.type, w);
@@ -2243,7 +2245,8 @@ class arg_formatter_base {
   }
 
   template <typename Ch>
-  void write(basic_string_view<Ch> s, const format_specs& specs = {}) {
+  FMT_CONSTEXPR void write(basic_string_view<Ch> s,
+                           const format_specs& specs = {}) {
     out_ = detail::write(out_, s, specs);
   }
 
@@ -2255,14 +2258,14 @@ class arg_formatter_base {
     arg_formatter_base& formatter;
     Char value;
 
-    char_spec_handler(arg_formatter_base& f, Char val)
+    constexpr char_spec_handler(arg_formatter_base& f, Char val)
         : formatter(f), value(val) {}
 
-    void on_int() {
+    FMT_CONSTEXPR void on_int() {
       // char is only formatted as int if there are specs.
       formatter.write_int(static_cast<int>(value), *formatter.specs_);
     }
-    void on_char() {
+    FMT_CONSTEXPR void on_char() {
       if (formatter.specs_)
         formatter.out_ = write_char(formatter.out_, value, *formatter.specs_);
       else
@@ -2285,7 +2288,7 @@ class arg_formatter_base {
   iterator out() { return out_; }
   format_specs* specs() { return specs_; }
 
-  void write(bool value) {
+  FMT_CONSTEXPR void write(bool value) {
     if (specs_)
       write(string_view(value ? "true" : "false"), *specs_);
     else
@@ -2303,7 +2306,7 @@ class arg_formatter_base {
   }
 
  public:
-  arg_formatter_base(OutputIt out, format_specs* s, locale_ref loc)
+  constexpr arg_formatter_base(OutputIt out, format_specs* s, locale_ref loc)
       : out_(out), locale_(loc), specs_(s) {}
 
   iterator operator()(monostate) {
@@ -2312,7 +2315,7 @@ class arg_formatter_base {
   }
 
   template <typename T, FMT_ENABLE_IF(is_integral<T>::value)>
-  FMT_INLINE iterator operator()(T value) {
+  FMT_CONSTEXPR iterator operator()(T value) {
     if (specs_)
       write_int(value, *specs_);
     else
@@ -2320,13 +2323,13 @@ class arg_formatter_base {
     return out_;
   }
 
-  iterator operator()(Char value) {
+  FMT_CONSTEXPR iterator operator()(Char value) {
     handle_char_specs(specs_,
                       char_spec_handler(*this, static_cast<Char>(value)));
     return out_;
   }
 
-  iterator operator()(bool value) {
+  FMT_CONSTEXPR iterator operator()(bool value) {
     if (specs_ && specs_->type) return (*this)(value ? 1 : 0);
     write(value != 0);
     return out_;
@@ -2348,7 +2351,7 @@ class arg_formatter_base {
     return out_;
   }
 
-  iterator operator()(basic_string_view<Char> value) {
+  FMT_CONSTEXPR iterator operator()(basic_string_view<Char> value) {
     if (specs_) {
       check_string_type_spec(specs_->type, error_handler());
       write(value, *specs_);
@@ -2388,7 +2391,7 @@ class arg_formatter : public arg_formatter_base<OutputIt, Char> {
     *specs* contains format specifier information for standard argument types.
     \endrst
    */
-  explicit arg_formatter(
+  constexpr explicit arg_formatter(
       context_type& ctx,
       basic_format_parse_context<char_type>* parse_ctx = nullptr,
       format_specs* specs = nullptr, const Char* ptr = nullptr)
@@ -3296,7 +3299,8 @@ void check_format_string(S format_str) {
 }
 
 template <template <typename> class Handler, typename Context>
-void handle_dynamic_spec(int& value, arg_ref<typename Context::char_type> ref,
+FMT_CONSTEXPR void handle_dynamic_spec(int& value,
+                                       arg_ref<typename Context::char_type> ref,
                                        Context& ctx) {
   switch (ref.kind) {
   case arg_id_kind::none:
@@ -3529,14 +3533,16 @@ struct formatter<T, Char,
   }
 
   template <typename FormatContext>
-  auto format(const T& val, FormatContext& ctx) -> decltype(ctx.out()) {
+  FMT_CONSTEXPR auto format(const T& val, FormatContext& ctx) const
-    detail::handle_dynamic_spec<detail::width_checker>(specs_.width,
+      -> decltype(ctx.out()) {
-                                                       specs_.width_ref, ctx);
+    auto specs = specs_;
+    detail::handle_dynamic_spec<detail::width_checker>(specs.width,
+                                                       specs.width_ref, ctx);
     detail::handle_dynamic_spec<detail::precision_checker>(
-        specs_.precision, specs_.precision_ref, ctx);
+        specs.precision, specs.precision_ref, ctx);
     using af = detail::arg_formatter<typename FormatContext::iterator,
                                      typename FormatContext::char_type>;
-    return visit_format_arg(af(ctx, nullptr, &specs_),
+    return visit_format_arg(af(ctx, nullptr, &specs),
                             detail::make_arg<FormatContext>(val));
   }
 
@@ -3548,7 +3554,8 @@ struct formatter<T, Char,
   template <typename Char>                                               \
   struct formatter<Type, Char> : formatter<Base, Char> {                 \
     template <typename FormatContext>                                    \
-    auto format(Type const& val, FormatContext& ctx) -> decltype(ctx.out()) { \
+    auto format(Type const& val, FormatContext& ctx) const               \
+        -> decltype(ctx.out()) {                                         \
       return formatter<Base, Char>::format(static_cast<Base>(val), ctx); \
     }                                                                    \
   }
@@ -3570,7 +3577,7 @@ FMT_FORMAT_AS(std::byte, unsigned);
 template <typename Char>
 struct formatter<void*, Char> : formatter<const void*, Char> {
   template <typename FormatContext>
-  auto format(void* val, FormatContext& ctx) -> decltype(ctx.out()) {
+  auto format(void* val, FormatContext& ctx) const -> decltype(ctx.out()) {
     return formatter<const void*, Char>::format(val, ctx);
   }
 };
@@ -3578,7 +3585,8 @@ struct formatter<void*, Char> : formatter<const void*, Char> {
 template <typename Char, size_t N>
 struct formatter<Char[N], Char> : formatter<basic_string_view<Char>, Char> {
   template <typename FormatContext>
-  auto format(const Char* val, FormatContext& ctx) -> decltype(ctx.out()) {
+  FMT_CONSTEXPR auto format(const Char* val, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
     return formatter<basic_string_view<Char>, Char>::format(val, ctx);
   }
 };

test/compile-test.cc

@@ -7,9 +7,6 @@
 
 #include <string>
 #include <type_traits>
-#if __cplusplus >= 202002L
-#  include <string_view>
-#endif
 
 // Check that fmt/compile.h compiles with windows.h included before it.
 #ifdef _WIN32
@@ -187,17 +184,18 @@ TEST(CompileTest, CompileFormatStringLiteral) {
 }
 #endif
 
-#if __cplusplus >= 202002L
+#if __cplusplus >= 202002L || \
-template <size_t max_string_length> struct test_string {
+    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
+template <size_t max_string_length, typename Char = char> struct test_string {
   template <typename T> constexpr bool operator==(const T& rhs) const noexcept {
-    return (std::string_view(rhs).compare(buffer.data()) == 0);
+    return fmt::basic_string_view<Char>(rhs).compare(buffer.data()) == 0;
   }
-  std::array<char, max_string_length> buffer{};
+  std::array<Char, max_string_length> buffer{};
 };
 
-template <size_t max_string_length, typename... Args>
+template <size_t max_string_length, typename Char = char, typename... Args>
 consteval auto test_format(auto format, const Args&... args) {
-  test_string<max_string_length> string{};
+  test_string<max_string_length, Char> string{};
   fmt::format_to(string.buffer.data(), format, args...);
   return string;
 }
@@ -205,6 +203,8 @@ consteval auto test_format(auto format, const Args&... args) {
 TEST(CompileTimeFormattingTest, Bool) {
   EXPECT_EQ("true", test_format<5>(FMT_COMPILE("{}"), true));
   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));
 }
 
 TEST(CompileTimeFormattingTest, Integer) {
@@ -213,16 +213,54 @@ TEST(CompileTimeFormattingTest, Integer) {
   EXPECT_EQ("42 42", test_format<6>(FMT_COMPILE("{} {}"), 42, 42));
   EXPECT_EQ("42 42",
             test_format<6>(FMT_COMPILE("{} {}"), uint32_t{42}, uint64_t{42}));
+
+  EXPECT_EQ("+42", test_format<4>(FMT_COMPILE("{:+}"), 42));
+  EXPECT_EQ("42", test_format<3>(FMT_COMPILE("{:-}"), 42));
+  EXPECT_EQ(" 42", test_format<4>(FMT_COMPILE("{: }"), 42));
+
+  EXPECT_EQ("-0042", test_format<6>(FMT_COMPILE("{:05}"), -42));
+
+  EXPECT_EQ("101010", test_format<7>(FMT_COMPILE("{:b}"), 42));
+  EXPECT_EQ("0b101010", test_format<9>(FMT_COMPILE("{:#b}"), 42));
+  EXPECT_EQ("0B101010", test_format<9>(FMT_COMPILE("{:#B}"), 42));
+  EXPECT_EQ("042", test_format<4>(FMT_COMPILE("{:#o}"), 042));
+  EXPECT_EQ("0x4a", test_format<5>(FMT_COMPILE("{:#x}"), 0x4a));
+  EXPECT_EQ("0X4A", test_format<5>(FMT_COMPILE("{:#X}"), 0x4a));
+
+  EXPECT_EQ("   42", test_format<6>(FMT_COMPILE("{:5}"), 42));
+  EXPECT_EQ("   42", test_format<6>(FMT_COMPILE("{:5}"), 42ll));
+  EXPECT_EQ("   42", test_format<6>(FMT_COMPILE("{:5}"), 42ull));
+
+  EXPECT_EQ("42  ", test_format<5>(FMT_COMPILE("{:<4}"), 42));
+  EXPECT_EQ("  42", test_format<5>(FMT_COMPILE("{:>4}"), 42));
+  EXPECT_EQ(" 42 ", test_format<5>(FMT_COMPILE("{:^4}"), 42));
+  EXPECT_EQ("**-42", test_format<6>(FMT_COMPILE("{:*>5}"), -42));
+}
+
+TEST(CompileTimeFormattingTest, Char) {
+  EXPECT_EQ("c", test_format<2>(FMT_COMPILE("{}"), 'c'));
+
+  EXPECT_EQ("c  ", test_format<4>(FMT_COMPILE("{:3}"), 'c'));
+  EXPECT_EQ("99", test_format<3>(FMT_COMPILE("{:d}"), 'c'));
 }
 
 TEST(CompileTimeFormattingTest, String) {
   EXPECT_EQ("42", test_format<3>(FMT_COMPILE("{}"), "42"));
   EXPECT_EQ("The answer is 42",
             test_format<17>(FMT_COMPILE("{} is {}"), "The answer", "42"));
+
+  EXPECT_EQ("abc**", test_format<6>(FMT_COMPILE("{:*<5}"), "abc"));
+  EXPECT_EQ("**🤡**", test_format<9>(FMT_COMPILE("{:*^5}"), "🤡"));
 }
 
 TEST(CompileTimeFormattingTest, Combination) {
   EXPECT_EQ("420, true, answer",
             test_format<18>(FMT_COMPILE("{}, {}, {}"), 420, true, "answer"));
+
+  EXPECT_EQ(" -42", test_format<5>(FMT_COMPILE("{:{}}"), -42, 4));
+}
+
+TEST(CompileTimeFormattingTest, MultiByteFill) {
+  EXPECT_EQ("жж42", test_format<8>(FMT_COMPILE("{:ж>4}"), 42));
 }
 #endif