diff --git a/include/fmt/base.h b/include/fmt/base.h
index c9b18cacb867..9d02f25a3204 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -1788,9 +1788,9 @@ template <typename T> class buffer {
 };
 
 struct buffer_traits {
-  explicit buffer_traits(size_t) {}
-  auto count() const -> size_t { return 0; }
-  auto limit(size_t size) -> size_t { return size; }
+  constexpr explicit buffer_traits(size_t) {}
+  constexpr auto count() const -> size_t { return 0; }
+  constexpr auto limit(size_t size) const -> size_t { return size; }
 };
 
 class fixed_buffer_traits {
@@ -1799,9 +1799,9 @@ class fixed_buffer_traits {
   size_t limit_;
 
  public:
-  explicit fixed_buffer_traits(size_t limit) : limit_(limit) {}
-  auto count() const -> size_t { return count_; }
-  auto limit(size_t size) -> size_t {
+  constexpr explicit fixed_buffer_traits(size_t limit) : limit_(limit) {}
+  constexpr auto count() const -> size_t { return count_; }
+  FMT_CONSTEXPR auto limit(size_t size) -> size_t {
     size_t n = limit_ > count_ ? limit_ - count_ : 0;
     count_ += size;
     return size < n ? size : n;
@@ -2224,7 +2224,7 @@ struct locale_ref {
   template <typename Locale, FMT_ENABLE_IF(sizeof(Locale::collate) != 0)>
   locale_ref(const Locale& loc);
 
-  explicit operator bool() const noexcept { return locale_ != nullptr; }
+  inline explicit operator bool() const noexcept { return locale_ != nullptr; }
 #endif  // FMT_USE_LOCALE
 
   template <typename Locale> auto get() const -> Locale;
@@ -2599,7 +2599,7 @@ class context : private detail::locale_ref {
   void operator=(const context&) = delete;
 
   FMT_CONSTEXPR auto arg(int id) const -> format_arg { return args_.get(id); }
-  auto arg(string_view name) -> format_arg { return args_.get(name); }
+  inline auto arg(string_view name) -> format_arg { return args_.get(name); }
   FMT_CONSTEXPR auto arg_id(string_view name) -> int {
     return args_.get_id(name);
   }
@@ -2608,7 +2608,7 @@ class context : private detail::locale_ref {
   FMT_CONSTEXPR auto out() -> iterator { return out_; }
 
   // Advances the begin iterator to `it`.
-  void advance_to(iterator) {}
+  FMT_CONSTEXPR void advance_to(iterator) {}
 
   FMT_CONSTEXPR auto locale() -> detail::locale_ref { return *this; }
 };
diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 5472eae09147..caf8e41a1293 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -540,24 +540,24 @@ inline auto localtime(std::time_t time) -> std::tm {
     std::time_t time_;
     std::tm tm_;
 
-    dispatcher(std::time_t t) : time_(t) {}
+    inline dispatcher(std::time_t t) : time_(t) {}
 
-    auto run() -> bool {
+    inline auto run() -> bool {
       using namespace fmt::detail;
       return handle(localtime_r(&time_, &tm_));
     }
 
-    auto handle(std::tm* tm) -> bool { return tm != nullptr; }
+    inline auto handle(std::tm* tm) -> bool { return tm != nullptr; }
 
-    auto handle(detail::null<>) -> bool {
+    inline auto handle(detail::null<>) -> bool {
       using namespace fmt::detail;
       return fallback(localtime_s(&tm_, &time_));
     }
 
-    auto fallback(int res) -> bool { return res == 0; }
+    inline auto fallback(int res) -> bool { return res == 0; }
 
 #if !FMT_MSC_VERSION
-    auto fallback(detail::null<>) -> bool {
+    inline auto fallback(detail::null<>) -> bool {
       using namespace fmt::detail;
       std::tm* tm = std::localtime(&time_);
       if (tm) tm_ = *tm;
@@ -591,24 +591,24 @@ inline auto gmtime(std::time_t time) -> std::tm {
     std::time_t time_;
     std::tm tm_;
 
-    dispatcher(std::time_t t) : time_(t) {}
+    inline dispatcher(std::time_t t) : time_(t) {}
 
-    auto run() -> bool {
+    inline auto run() -> bool {
       using namespace fmt::detail;
       return handle(gmtime_r(&time_, &tm_));
     }
 
-    auto handle(std::tm* tm) -> bool { return tm != nullptr; }
+    inline auto handle(std::tm* tm) -> bool { return tm != nullptr; }
 
-    auto handle(detail::null<>) -> bool {
+    inline auto handle(detail::null<>) -> bool {
       using namespace fmt::detail;
       return fallback(gmtime_s(&tm_, &time_));
     }
 
-    auto fallback(int res) -> bool { return res == 0; }
+    inline auto fallback(int res) -> bool { return res == 0; }
 
 #if !FMT_MSC_VERSION
-    auto fallback(detail::null<>) -> bool {
+    inline auto fallback(detail::null<>) -> bool {
       std::tm* tm = std::gmtime(&time_);
       if (tm) tm_ = *tm;
       return tm != nullptr;
@@ -912,7 +912,9 @@ template <typename Derived> struct null_chrono_spec_handler {
 };
 
 struct tm_format_checker : null_chrono_spec_handler<tm_format_checker> {
-  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no format")); }
+  FMT_NORETURN inline void unsupported() {
+    FMT_THROW(format_error("no format"));
+  }
 
   template <typename Char>
   FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
@@ -1572,7 +1574,7 @@ class tm_writer {
 struct chrono_format_checker : null_chrono_spec_handler<chrono_format_checker> {
   bool has_precision_integral = false;
 
-  FMT_NORETURN void unsupported() { FMT_THROW(format_error("no date")); }
+  FMT_NORETURN inline void unsupported() { FMT_THROW(format_error("no date")); }
 
   template <typename Char>
   FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
@@ -1693,14 +1695,14 @@ class get_locale {
   bool has_locale_ = false;
 
  public:
-  get_locale(bool localized, locale_ref loc) : has_locale_(localized) {
+  inline get_locale(bool localized, locale_ref loc) : has_locale_(localized) {
     if (localized)
       ::new (&locale_) std::locale(loc.template get<std::locale>());
   }
-  ~get_locale() {
+  inline ~get_locale() {
     if (has_locale_) locale_.~locale();
   }
-  operator const std::locale&() const {
+  inline operator const std::locale&() const {
     return has_locale_ ? locale_ : get_classic_locale();
   }
 };
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 9dd68a1ecabd..e3aa9fd668b8 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -372,13 +372,14 @@ class uint128_fallback {
       -> uint128_fallback {
     return {~n.hi_, ~n.lo_};
   }
-  friend auto operator+(const uint128_fallback& lhs,
-                        const uint128_fallback& rhs) -> uint128_fallback {
+  friend FMT_CONSTEXPR auto operator+(const uint128_fallback& lhs,
+                                      const uint128_fallback& rhs)
+      -> uint128_fallback {
     auto result = uint128_fallback(lhs);
     result += rhs;
     return result;
   }
-  friend auto operator*(const uint128_fallback& lhs, uint32_t rhs)
+  friend FMT_CONSTEXPR auto operator*(const uint128_fallback& lhs, uint32_t rhs)
       -> uint128_fallback {
     FMT_ASSERT(lhs.hi_ == 0, "");
     uint64_t hi = (lhs.lo_ >> 32) * rhs;
@@ -386,7 +387,7 @@ class uint128_fallback {
     uint64_t new_lo = (hi << 32) + lo;
     return {(hi >> 32) + (new_lo < lo ? 1 : 0), new_lo};
   }
-  friend auto operator-(const uint128_fallback& lhs, uint64_t rhs)
+  friend constexpr auto operator-(const uint128_fallback& lhs, uint64_t rhs)
       -> uint128_fallback {
     return {lhs.hi_ - (lhs.lo_ < rhs ? 1 : 0), lhs.lo_ - rhs};
   }
@@ -955,8 +956,8 @@ class writer {
   FILE* file_;
 
  public:
-  writer(FILE* f) : buf_(nullptr), file_(f) {}
-  writer(detail::buffer<char>& buf) : buf_(&buf) {}
+  inline writer(FILE* f) : buf_(nullptr), file_(f) {}
+  inline writer(detail::buffer<char>& buf) : buf_(&buf) {}
 
   /// Formats `args` according to specifications in `fmt` and writes the
   /// output to the file.
@@ -974,10 +975,10 @@ class string_buffer {
   detail::container_buffer<std::string> buf_;
 
  public:
-  string_buffer() : buf_(str_) {}
+  inline string_buffer() : buf_(str_) {}
 
-  operator writer() { return buf_; }
-  std::string& str() { return str_; }
+  inline operator writer() { return buf_; }
+  inline std::string& str() { return str_; }
 };
 
 template <typename T, size_t SIZE, typename Allocator>
@@ -1418,10 +1419,12 @@ class utf8_to_utf16 {
 
  public:
   FMT_API explicit utf8_to_utf16(string_view s);
-  operator basic_string_view<wchar_t>() const { return {&buffer_[0], size()}; }
-  auto size() const -> size_t { return buffer_.size() - 1; }
-  auto c_str() const -> const wchar_t* { return &buffer_[0]; }
-  auto str() const -> std::wstring { return {&buffer_[0], size()}; }
+  inline operator basic_string_view<wchar_t>() const {
+    return {&buffer_[0], size()};
+  }
+  inline auto size() const -> size_t { return buffer_.size() - 1; }
+  inline auto c_str() const -> const wchar_t* { return &buffer_[0]; }
+  inline auto str() const -> std::wstring { return {&buffer_[0], size()}; }
 };
 
 enum class to_utf8_error_policy { abort, replace };
@@ -3927,7 +3930,7 @@ template <> struct formatter<std::byte> : formatter<unsigned> {
 struct bytes {
   string_view data;
 
-  explicit bytes(string_view s) : data(s) {}
+  inline explicit bytes(string_view s) : data(s) {}
 };
 
 template <> struct formatter<bytes> {
@@ -4131,7 +4134,7 @@ class format_int {
   }
 
   /// Returns the content of the output buffer as an `std::string`.
-  auto str() const -> std::string { return {str_, size()}; }
+  inline auto str() const -> std::string { return {str_, size()}; }
 };
 
 #define FMT_STRING_IMPL(s, base)                                           \
diff --git a/include/fmt/os.h b/include/fmt/os.h
index 75b47c6908e3..b2cc5e4b85ff 100644
--- a/include/fmt/os.h
+++ b/include/fmt/os.h
@@ -176,24 +176,24 @@ class buffered_file {
 
   friend class file;
 
-  explicit buffered_file(FILE* f) : file_(f) {}
+  inline explicit buffered_file(FILE* f) : file_(f) {}
 
  public:
   buffered_file(const buffered_file&) = delete;
   void operator=(const buffered_file&) = delete;
 
   // Constructs a buffered_file object which doesn't represent any file.
-  buffered_file() noexcept : file_(nullptr) {}
+  inline buffered_file() noexcept : file_(nullptr) {}
 
   // Destroys the object closing the file it represents if any.
   FMT_API ~buffered_file() noexcept;
 
  public:
-  buffered_file(buffered_file&& other) noexcept : file_(other.file_) {
+  inline buffered_file(buffered_file&& other) noexcept : file_(other.file_) {
     other.file_ = nullptr;
   }
 
-  auto operator=(buffered_file&& other) -> buffered_file& {
+  inline auto operator=(buffered_file&& other) -> buffered_file& {
     close();
     file_ = other.file_;
     other.file_ = nullptr;
@@ -207,7 +207,7 @@ class buffered_file {
   FMT_API void close();
 
   // Returns the pointer to a FILE object representing this file.
-  auto get() const noexcept -> FILE* { return file_; }
+  inline auto get() const noexcept -> FILE* { return file_; }
 
   FMT_API auto descriptor() const -> int;
 
@@ -248,7 +248,7 @@ class FMT_API file {
   };
 
   // Constructs a file object which doesn't represent any file.
-  file() noexcept : fd_(-1) {}
+  inline file() noexcept : fd_(-1) {}
 
   // Opens a file and constructs a file object representing this file.
   file(cstring_view path, int oflag);
@@ -257,10 +257,10 @@ class FMT_API file {
   file(const file&) = delete;
   void operator=(const file&) = delete;
 
-  file(file&& other) noexcept : fd_(other.fd_) { other.fd_ = -1; }
+  inline file(file&& other) noexcept : fd_(other.fd_) { other.fd_ = -1; }
 
   // Move assignment is not noexcept because close may throw.
-  auto operator=(file&& other) -> file& {
+  inline auto operator=(file&& other) -> file& {
     close();
     fd_ = other.fd_;
     other.fd_ = -1;
@@ -271,7 +271,7 @@ class FMT_API file {
   ~file() noexcept;
 
   // Returns the file descriptor.
-  auto descriptor() const noexcept -> int { return fd_; }
+  inline auto descriptor() const noexcept -> int { return fd_; }
 
   // Closes the file.
   void close();
@@ -324,9 +324,9 @@ auto getpagesize() -> long;
 namespace detail {
 
 struct buffer_size {
-  buffer_size() = default;
+  constexpr buffer_size() = default;
   size_t value = 0;
-  auto operator=(size_t val) const -> buffer_size {
+  FMT_CONSTEXPR auto operator=(size_t val) const -> buffer_size {
     auto bs = buffer_size();
     bs.value = val;
     return bs;
@@ -337,7 +337,7 @@ struct ostream_params {
   int oflag = file::WRONLY | file::CREATE | file::TRUNC;
   size_t buffer_size = BUFSIZ > 32768 ? BUFSIZ : 32768;
 
-  ostream_params() {}
+  constexpr ostream_params() {}
 
   template <typename... T>
   ostream_params(T... params, int new_oflag) : ostream_params(params...) {
@@ -381,7 +381,7 @@ class FMT_API ostream : private detail::buffer<char> {
     return buf;
   }
 
-  void flush() {
+  inline void flush() {
     if (size() == 0) return;
     file_.write(data(), size() * sizeof(data()[0]));
     clear();
@@ -390,7 +390,7 @@ class FMT_API ostream : private detail::buffer<char> {
   template <typename... T>
   friend auto output_file(cstring_view path, T... params) -> ostream;
 
-  void close() {
+  inline void close() {
     flush();
     file_.close();
   }
diff --git a/include/fmt/printf.h b/include/fmt/printf.h
index 7a1c50815f36..e72684018546 100644
--- a/include/fmt/printf.h
+++ b/include/fmt/printf.h
@@ -79,7 +79,7 @@ template <bool IsSigned> struct int_checker {
     unsigned max = to_unsigned(max_value<int>());
     return value <= max;
   }
-  static auto fits_in_int(bool) -> bool { return true; }
+  inline static auto fits_in_int(bool) -> bool { return true; }
 };
 
 template <> struct int_checker<true> {
@@ -87,7 +87,7 @@ template <> struct int_checker<true> {
     return value >= (std::numeric_limits<int>::min)() &&
            value <= max_value<int>();
   }
-  static auto fits_in_int(int) -> bool { return true; }
+  inline static auto fits_in_int(int) -> bool { return true; }
 };
 
 struct printf_precision_handler {
@@ -205,7 +205,7 @@ class printf_width_handler {
   format_specs& specs_;
 
  public:
-  explicit printf_width_handler(format_specs& specs) : specs_(specs) {}
+  inline explicit printf_width_handler(format_specs& specs) : specs_(specs) {}
 
   template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
   auto operator()(T value) -> unsigned {