From 09a606d17a2c5c2a7a295f873f67d626264d2181 Mon Sep 17 00:00:00 2001
From: Nathan Chandrasekar <322284620+vera-rubin@users.noreply.github.com>
Date: Sat, 31 Oct 2026 13:15:44 -0400
Subject: [PATCH] Fix zero padding for localized integers (#4994)

---
 include/fmt/format.h |  7 +++++++
 test/format-test.cc  | 16 ++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index a2fc9621..f74ad7a9 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -2248,10 +2248,17 @@ auto write_int(OutputIt out, UInt value, unsigned prefix,
 
   unsigned size = (prefix != 0 ? prefix >> 24 : 0) + to_unsigned(num_digits) +
                   to_unsigned(grouping.count_separators(num_digits));
+  unsigned padding = 0;
+  if (specs.align() == align::numeric) {
+    auto width = to_unsigned(specs.width);
+    if (width > size) padding = width - size;
+    size += padding;
+  }
   return write_padded<Char, align::right>(
       out, specs, size, size, [&](reserve_iterator<OutputIt> it) {
         for (unsigned p = prefix & 0xffffff; p != 0; p >>= 8)
           *it++ = static_cast<Char>(p & 0xff);
+        it = detail::fill_n(it, padding, static_cast<Char>('0'));
         return grouping.apply(it, string_view(buffer.data(), buffer.size()));
       });
 }
diff --git a/test/format-test.cc b/test/format-test.cc
index 34b0dfe1..c44db6ab 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -2686,6 +2686,22 @@ TEST(format_test, format_locale) {
   EXPECT_EQ(fmt::format(loc, "{:10Lo}", 12345), "    30,071");
 }
 
+TEST(format_test, format_int_locale_zero_padding) {
+  auto loc = std::locale::classic();
+  EXPECT_EQ(fmt::format(loc, "{:08L}", -12), "-0000012");
+  EXPECT_EQ(fmt::format(loc, "{:+08L}", 12), "+0000012");
+  EXPECT_EQ(fmt::format(loc, "{: 08L}", 12), " 0000012");
+  EXPECT_EQ(fmt::format(loc, "{:08L}", 12), "00000012");
+  EXPECT_EQ(fmt::format(loc, "{:02L}", -12), "-12");
+  EXPECT_EQ(fmt::format(loc, "{:>08L}", -12), "     -12");
+  EXPECT_EQ(fmt::format(loc, "{:0>8L}", -12), "00000-12");
+  loc = std::locale(loc, new fmt::format_facet<std::locale>(","));
+  EXPECT_EQ(fmt::format(loc, "{:08L}", -1234), "-001,234");
+  EXPECT_EQ(fmt::format(loc, "{:#010Lx}", -0x1234), "-0x001,234");
+  EXPECT_EQ(fmt::format(loc, "{:#010Lb}", -10), "-0b001,010");
+  EXPECT_EQ(fmt::format(loc, "{:#08Lo}", -01234), "-001,234");
+}
+
 #endif  // FMT_USE_LOCALE
 
 struct convertible_to_nonconst_cstring {