Format octal 0 as 0

include/fmt/format.h

@@ -1441,7 +1441,7 @@ template <typename Range> class basic_writer {
 
     void on_oct() {
       int num_digits = internal::count_digits<3>(abs_value);
-      if (specs.alt && specs.precision <= num_digits) {
+      if (specs.alt && specs.precision <= num_digits && abs_value != 0) {
         // Octal prefix '0' is counted as a digit, so only add it if precision
         // is not greater than the number of digits.
         prefix[prefix_size++] = '0';

test/format-test.cc

@@ -1001,6 +1001,7 @@ TEST(FormatterTest, HashFlag) {
   EXPECT_EQ("0x42", format("{0:#x}", 0x42));
   EXPECT_EQ("0X42", format("{0:#X}", 0x42));
   EXPECT_EQ("-0x42", format("{0:#x}", -0x42));
+  EXPECT_EQ("0", format("{0:#o}", 0));
   EXPECT_EQ("042", format("{0:#o}", 042));
   EXPECT_EQ("-042", format("{0:#o}", -042));
   EXPECT_EQ("42", format("{0:#}", 42u));