Fix sign handling in 'L'

include/fmt/format.h

@@ -1552,6 +1552,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
     char digits[40];
     format_decimal(digits, abs_value, num_digits);
     basic_memory_buffer<Char> buffer;
+    size += prefix_size;
     buffer.resize(size);
     basic_string_view<Char> s(&sep, sep_size);
     // Index of a decimal digit with the least significant digit having index 0.
@@ -1571,6 +1572,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
       std::uninitialized_copy(s.data(), s.data() + s.size(),
                               make_checked(p, s.size()));
     }
+    if (prefix_size != 0) p[-1] = static_cast<Char>('-');
     write(out, basic_string_view<Char>(buffer.data(), buffer.size()), specs);
   }
 

test/locale-test.cc

@@ -49,6 +49,7 @@ TEST(LocaleTest, Format) {
   std::locale loc(std::locale(), new numpunct<char>());
   EXPECT_EQ("1234567", fmt::format(std::locale(), "{:n}", 1234567));
   EXPECT_EQ("1~234~567", fmt::format(loc, "{:n}", 1234567));
+  EXPECT_EQ("-1~234~567", fmt::format(loc, "{:n}", -1234567));
   fmt::format_arg_store<fmt::format_context, int> as{1234567};
   EXPECT_EQ("1~234~567", fmt::vformat(loc, "{:n}", fmt::format_args(as)));
   std::string s;