Fix error handling in fmt::fprintf.

format.cc

@@ -1087,7 +1087,8 @@ void fmt::print_colored(Color c, StringRef format, ArgList args) {
 int fmt::fprintf(std::FILE *f, StringRef format, ArgList args) {
   MemoryWriter w;
   printf(w, format, args);
-  return std::fwrite(w.data(), 1, w.size(), f);
+  std::size_t size = w.size();
+  return std::fwrite(w.data(), 1, size, f) < size ? -1 : static_cast<int>(size);
 }
 
 // Explicit instantiations for char.

test/printf-test.cc

@@ -438,4 +438,11 @@ TEST(PrintfTest, Examples) {
   EXPECT_WRITE(stdout, fmt::printf("%1$s, %3$d %2$s", weekday, month, day),
                "Thursday, 21 August");
 }
+
+TEST(PrintfTest, PrintfError) {
+  fmt::File read_end, write_end;
+  fmt::File::pipe(read_end, write_end);
+  int result = fmt::fprintf(read_end.fdopen("r").get(), "test");
+  EXPECT_LT(result, 0);
+}
 #endif