Fix group_digits for negative integers (fmtlib#3901)

Signed-off-by: Vladislav Shchapov <[email protected]>

Author: phprus

Diff for: include/fmt/format.h

@@ -4130,9 +4130,10 @@ template <typename T> struct formatter<group_digits_view<T>> : formatter<T> {
                                                        specs.width_ref, ctx);
     detail::handle_dynamic_spec<detail::precision_checker>(
         specs.precision, specs.precision_ref, ctx);
-    return detail::write_int(ctx.out(),
-                             static_cast<detail::uint64_or_128_t<T>>(t.value),
-                             0, specs, detail::digit_grouping<char>("\3", ","));
+    auto arg = detail::make_write_int_arg(t.value, specs.sign);
+    return detail::write_int(
+        ctx.out(), static_cast<detail::uint64_or_128_t<T>>(arg.abs_value),
+        arg.prefix, specs, detail::digit_grouping<char>("\3", ","));
   }
 };
 

Diff for: test/format-test.cc

@@ -1837,6 +1837,9 @@ TEST(format_test, bytes) {
 TEST(format_test, group_digits_view) {
   EXPECT_EQ(fmt::format("{}", fmt::group_digits(10000000)), "10,000,000");
   EXPECT_EQ(fmt::format("{:8}", fmt::group_digits(1000)), "   1,000");
+  EXPECT_EQ(fmt::format("{}", fmt::group_digits(-10000000)), "-10,000,000");
+  EXPECT_EQ(fmt::format("{:8}", fmt::group_digits(-1000)), "  -1,000");
+  EXPECT_EQ(fmt::format("{:8}", fmt::group_digits(-100)), "    -100");
 }
 
 #ifdef __cpp_generic_lambdas