diff --git a/src/format.cc b/src/format.cc
index c26ce6a5..2e1c3a7f 100644
--- a/src/format.cc
+++ b/src/format.cc
@@ -11,8 +11,8 @@ FMT_BEGIN_NAMESPACE
 namespace internal {
 
 template <typename T>
-int format_float(char* buf, std::size_t size, const char* format, int precision,
-                 T value) {
+auto format_float(char* buf, std::size_t size, const char* format, int precision,
+                 T value) -> int {
 #ifdef FMT_FUZZ
   if (precision > 100000)
     throw std::runtime_error(
diff --git a/src/os.cc b/src/os.cc
index bd925e44..07da2b3b 100644
--- a/src/os.cc
+++ b/src/os.cc
@@ -66,7 +66,7 @@ inline unsigned convert_rwcount(std::size_t count) {
 // Return type of read and write functions.
 using RWResult = ssize_t;
 
-inline std::size_t convert_rwcount(std::size_t count) { return count; }
+inline auto convert_rwcount(std::size_t count) -> std::size_t { return count; }
 #endif
 }  // namespace
 
@@ -166,7 +166,7 @@ void buffered_file::close() {
 // A macro used to prevent expansion of fileno on broken versions of MinGW.
 #define FMT_ARGS
 
-int buffered_file::fileno() const {
+auto buffered_file::fileno() const -> int {
   int fd = FMT_POSIX_CALL(fileno FMT_ARGS(file_));
   if (fd == -1) FMT_THROW(system_error(errno, "cannot get file descriptor"));
   return fd;
@@ -201,7 +201,7 @@ void file::close() {
   if (result != 0) FMT_THROW(system_error(errno, "cannot close file"));
 }
 
-long long file::size() const {
+auto file::size() const -> long long {
 #  ifdef _WIN32
   // Use GetFileSize instead of GetFileSizeEx for the case when _WIN32_WINNT
   // is less than 0x0500 as is the case with some default MinGW builds.
@@ -227,21 +227,21 @@ long long file::size() const {
 #  endif
 }
 
-std::size_t file::read(void* buffer, std::size_t count) const {
+auto file::read(void* buffer, std::size_t count) const -> std::size_t {
   RWResult result = 0;
   FMT_RETRY(result, FMT_POSIX_CALL(read(fd_, buffer, convert_rwcount(count))));
   if (result < 0) FMT_THROW(system_error(errno, "cannot read from file"));
   return internal::to_unsigned(result);
 }
 
-std::size_t file::write(const void* buffer, std::size_t count) const {
+auto file::write(const void* buffer, std::size_t count) const -> std::size_t {
   RWResult result = 0;
   FMT_RETRY(result, FMT_POSIX_CALL(write(fd_, buffer, convert_rwcount(count))));
   if (result < 0) FMT_THROW(system_error(errno, "cannot write to file"));
   return internal::to_unsigned(result);
 }
 
-file file::dup(int fd) {
+auto file::dup(int fd) -> file {
   // Don't retry as dup doesn't return EINTR.
   // http://pubs.opengroup.org/onlinepubs/009695399/functions/dup.html
   int new_fd = FMT_POSIX_CALL(dup(fd));
@@ -287,7 +287,7 @@ void file::pipe(file& read_end, file& write_end) {
   write_end = file(fds[1]);
 }
 
-buffered_file file::fdopen(const char* mode) {
+auto file::fdopen(const char* mode) -> buffered_file {
   // Don't retry as fdopen doesn't return EINTR.
   FILE* f = FMT_POSIX_CALL(fdopen(fd_, mode));
   if (!f)
@@ -298,7 +298,7 @@ buffered_file file::fdopen(const char* mode) {
   return bf;
 }
 
-long getpagesize() {
+auto getpagesize() -> long {
 #  ifdef _WIN32
   SYSTEM_INFO si;
   GetSystemInfo(&si);