diff --git a/stl/inc/format b/stl/inc/format
index 105307688f..3d7807dcdb 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -744,12 +744,15 @@ public:
     template <_Formattable_with<_Context> _Ty>
     static basic_format_arg _Make_from(_Ty& _Val) noexcept {
         using _Erased_type = _Format_arg_traits<_Context>::template _Storage_type<_Ty>;
+        if constexpr (is_same_v<remove_const_t<_Ty>, char> && is_same_v<_CharType, wchar_t>) {
+            return basic_format_arg(static_cast<_Erased_type>(static_cast<unsigned char>(_Val)));
+        }
 #if !_HAS_CXX23
-        if constexpr (is_same_v<_Erased_type, basic_string_view<_CharType>>) {
+        else if constexpr (is_same_v<_Erased_type, basic_string_view<_CharType>>) {
             return basic_format_arg(_Erased_type{_Val.data(), _Val.size()});
-        } else
+        }
 #endif // !_HAS_CXX23
-        {
+        else {
             return basic_format_arg(static_cast<_Erased_type>(_Val));
         }
     }
@@ -2047,13 +2050,17 @@ private:
             _Arg_type = _Basic_format_arg_type::_Custom_type;
         }
 
+        if constexpr (is_same_v<remove_const_t<remove_reference_t<_Ty>>, char> && is_same_v<_CharType, wchar_t>) {
+            _Store_impl<_Erased_type>(
+                _Arg_index, _Arg_type, static_cast<_Erased_type>(static_cast<unsigned char>(_Val)));
+        }
 #if !_HAS_CXX23
         // Workaround towards N4950 [format.arg]/6.8 in C++20
-        if constexpr (is_same_v<_Erased_type, basic_string_view<_CharType>>) {
+        else if constexpr (is_same_v<_Erased_type, basic_string_view<_CharType>>) {
             _Store_impl<_Erased_type>(_Arg_index, _Arg_type, _Erased_type{_Val.data(), _Val.size()});
-        } else
+        }
 #endif // !_HAS_CXX23
-        {
+        else {
             _Store_impl<_Erased_type>(_Arg_index, _Arg_type, static_cast<_Erased_type>(_Val));
         }
     }
@@ -2941,7 +2948,7 @@ _NODISCARD _OutputIt _Fmt_write(
         && _Specs._Type != '?'
 #endif // _HAS_CXX23
     ) {
-        return _Write_integral(_STD move(_Out), _Value, _Specs, _Locale);
+        return _Write_integral(_STD move(_Out), static_cast<make_unsigned_t<_CharT>>(_Value), _Specs, _Locale);
     }
 
     _STL_INTERNAL_CHECK(_Specs._Precision == -1);
diff --git a/stl/inc/yvals_core.h b/stl/inc/yvals_core.h
index 1c494ad110..5abef80106 100644
--- a/stl/inc/yvals_core.h
+++ b/stl/inc/yvals_core.h
@@ -306,6 +306,7 @@
 // P2711R1 Making Multi-Param Constructors Of Views explicit
 // P2736R2 Referencing The Unicode Standard
 // P2770R0 Stashing Stashing Iterators For Proper Flattening
+// P2909R4 Fix Formatting Of Code Units As Integers
 
 // _HAS_CXX20 indirectly controls:
 // P0619R4 Removing C++17-Deprecated Features
@@ -1748,6 +1749,7 @@ _EMIT_STL_ERROR(STL1004, "C++98 unexpected() is incompatible with C++23 unexpect
 
 #ifdef __cpp_lib_concepts
 #define __cpp_lib_format              202207L
+#define __cpp_lib_format_uchar        202311L
 #define __cpp_lib_freestanding_ranges 202306L
 #endif // defined(__cpp_lib_concepts)
 
diff --git a/tests/std/tests/P0645R10_text_formatting_formatting/test.cpp b/tests/std/tests/P0645R10_text_formatting_formatting/test.cpp
index 9954c85d77..4fa15eefa3 100644
--- a/tests/std/tests/P0645R10_text_formatting_formatting/test.cpp
+++ b/tests/std/tests/P0645R10_text_formatting_formatting/test.cpp
@@ -725,6 +725,17 @@ void test_char_specs() {
     test_type(STR("{:X}"), charT{'X'});
 
     test_type(STR("{:+d}"), charT{'X'});
+
+    // P2909R4 Fix formatting of code units as integers
+    constexpr charT irregular_code_unit     = static_cast<charT>(255);
+    constexpr int irregular_code_unit_value = static_cast<make_unsigned_t<charT>>(irregular_code_unit);
+    assert(format(STR("{:b}"), irregular_code_unit) == format(STR("{:b}"), irregular_code_unit_value));
+    assert(format(STR("{:B}"), irregular_code_unit) == format(STR("{:B}"), irregular_code_unit_value));
+    assert(format(STR("{:d}"), irregular_code_unit) == format(STR("{:d}"), irregular_code_unit_value));
+    assert(format(STR("{:o}"), irregular_code_unit) == format(STR("{:o}"), irregular_code_unit_value));
+    assert(format(STR("{:x}"), irregular_code_unit) == format(STR("{:x}"), irregular_code_unit_value));
+    assert(format(STR("{:X}"), irregular_code_unit) == format(STR("{:X}"), irregular_code_unit_value));
+    assert(format(STR("{:+d}"), irregular_code_unit) == format(STR("{:+d}"), irregular_code_unit_value));
 }
 
 template <class charT, class Float>
diff --git a/tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp b/tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp
index 3ab7a981fe..4b4c118f69 100644
--- a/tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp
+++ b/tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp
@@ -361,6 +361,12 @@ STATIC_ASSERT(__cpp_lib_format == 202207L);
 #error __cpp_lib_format is defined
 #endif
 
+#if _HAS_CXX20 && defined(__cpp_lib_concepts) // TRANSITION, GH-395
+STATIC_ASSERT(__cpp_lib_format_uchar == 202311L);
+#elif defined(__cpp_lib_format_uchar)
+#error __cpp_lib_format_uchar is defined
+#endif
+
 #if _HAS_CXX23 && defined(__cpp_lib_concepts) // TRANSITION, GH-395
 STATIC_ASSERT(__cpp_lib_formatters == 202302L);
 #elif defined(__cpp_lib_formatters)