Fix flushing C++ iostreams before calling write_console()

This change correctly implements https://wg21.link/P2539/ for both
C streams and C++ iostreams.

Fixes fmtlib#3688.

Author: dimztimz

include/fmt/format-inl.h

@@ -1425,16 +1425,13 @@ FMT_FUNC std::string vformat(string_view fmt, format_args args) {
 
 namespace detail {
 #if !defined(_WIN32) || defined(FMT_WINDOWS_NO_WCHAR)
-FMT_FUNC bool write_console(std::FILE*, string_view) { return false; }
+FMT_FUNC bool write_console(int, string_view) { return false; }
 #else
 using dword = conditional_t<sizeof(long) == 4, unsigned long, unsigned>;
 extern "C" __declspec(dllimport) int __stdcall WriteConsoleW(  //
     void*, const void*, dword, dword*, void*);
 
-FMT_FUNC bool write_console(std::FILE* f, string_view text) {
-  int fd = _fileno(f);
-  if (!_isatty(fd)) return false;
-  std::fflush(f);
+FMT_FUNC bool write_console(int fd, string_view text) {
   auto u16 = utf8_to_utf16(text);
   return WriteConsoleW(reinterpret_cast<void*>(_get_osfhandle(fd)), u16.c_str(),
                        static_cast<dword>(u16.size()), nullptr, nullptr) != 0;
@@ -1451,7 +1448,14 @@ FMT_FUNC void vprint_mojibake(std::FILE* f, string_view fmt, format_args args) {
 #endif
 
 FMT_FUNC void print(std::FILE* f, string_view text) {
-  if (!write_console(f, text)) fwrite_fully(text.data(), text.size(), f);
+#ifdef _WIN32
+  int fd = _fileno(f);
+  if (_isatty(fd)) {
+    std::fflush(f);
+    if (write_console(fd, text)) return;
+  }
+#endif
+  fwrite_fully(text.data(), text.size(), f);
 }
 }  // namespace detail
 

include/fmt/format.h

@@ -1038,7 +1038,7 @@ struct is_contiguous<basic_memory_buffer<T, SIZE, Allocator>> : std::true_type {
 
 FMT_END_EXPORT
 namespace detail {
-FMT_API bool write_console(std::FILE* f, string_view text);
+FMT_API bool write_console(int fd, string_view text);
 FMT_API void print(std::FILE*, string_view);
 }  // namespace detail
 

include/fmt/ostream.h

@@ -10,10 +10,13 @@
 
 #include <fstream>  // std::filebuf
 
-#if defined(_WIN32) && defined(__GLIBCXX__)
+#ifdef _WIN32
+#ifdef __GLIBCXX__
 #  include <ext/stdio_filebuf.h>
 #  include <ext/stdio_sync_filebuf.h>
 #endif
+#include <io.h>
+#endif
 
 #include "format.h"
 
@@ -38,21 +41,31 @@ auto get_file(std::filebuf&) -> FILE*;
 #endif
 
 inline bool write_ostream_unicode(std::ostream& os, fmt::string_view data) {
+  FILE* f = nullptr;
 #if FMT_MSC_VERSION
   if (auto* buf = dynamic_cast<std::filebuf*>(os.rdbuf()))
-    if (FILE* f = get_file(*buf)) return write_console(f, data);
+    f = get_file(*buf);
+  else
+    return false;
 #elif defined(_WIN32) && defined(__GLIBCXX__)
   auto* rdbuf = os.rdbuf();
-  FILE* c_file;
   if (auto* sfbuf = dynamic_cast<__gnu_cxx::stdio_sync_filebuf<char>*>(rdbuf))
-    c_file = sfbuf->file();
+    f = sfbuf->file();
   else if (auto* fbuf = dynamic_cast<__gnu_cxx::stdio_filebuf<char>*>(rdbuf))
-    c_file = fbuf->file();
+    f = fbuf->file();
   else
     return false;
-  if (c_file) return write_console(c_file, data);
 #else
-  ignore_unused(os, data);
+  ignore_unused(os, data, f);
+#endif
+#ifdef _WIN32
+  if (f) {
+    int fd = _fileno(f);
+    if (_isatty(fd)) {
+      os.flush();
+      return write_console(fd, data);
+    }
+  }
 #endif
   return false;
 }