From a2650ad43b9c8496de7c0f8e16bf85d5b753e71b Mon Sep 17 00:00:00 2001
From: Mingxin Wang <mingxwa@outlook.com>
Date: Sun, 10 May 2026 22:07:07 +0800
Subject: [PATCH 1/3] Refactor format skill

---
 CMakeLists.txt                            |   8 ++
 benchmarks/CMakeLists.txt                 |   7 +-
 docs/CMakeLists.txt                       |   6 +-
 include/proxy/v4/proxy.h                  | 156 ++++++++++++----------
 include/proxy/v4/proxy_fmt.h              |  59 ++------
 tests/CMakeLists.txt                      |  11 +-
 tests/proxy_fmt_format_no_wchar_tests.cpp |  35 +++++
 tests/proxy_fmt_format_tests.cpp          |   2 +
 8 files changed, 144 insertions(+), 140 deletions(-)
 create mode 100644 tests/proxy_fmt_format_no_wchar_tests.cpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 60e796c..29ce63b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -110,6 +110,14 @@ if (BUILD_TESTING)
   )
   FetchContent_MakeAvailable(fmt)
 
+  if(MSVC)
+    set(PROXY_WARNING_FLAGS /W4)
+    set(PROXY_STRICT_WARNING_FLAGS ${PROXY_WARNING_FLAGS})
+  else()
+    set(PROXY_WARNING_FLAGS -Wall -Wextra -Wpedantic $<$<CXX_COMPILER_ID:Clang>:-Wno-c++2b-extensions>)
+    set(PROXY_STRICT_WARNING_FLAGS ${PROXY_WARNING_FLAGS})
+  endif()
+
   add_subdirectory(tests)
   add_subdirectory(benchmarks)
   add_subdirectory(docs)
diff --git a/benchmarks/CMakeLists.txt b/benchmarks/CMakeLists.txt
index 061f7e9..f8060e2 100644
--- a/benchmarks/CMakeLists.txt
+++ b/benchmarks/CMakeLists.txt
@@ -16,9 +16,4 @@ add_executable(msft_proxy_benchmarks
 )
 target_include_directories(msft_proxy_benchmarks PRIVATE .)
 target_link_libraries(msft_proxy_benchmarks PRIVATE msft_proxy4::proxy benchmark::benchmark benchmark::benchmark_main)
-
-if (MSVC)
-  target_compile_options(msft_proxy_benchmarks PRIVATE /W4)
-else()
-  target_compile_options(msft_proxy_benchmarks PRIVATE -Wall -Wextra -Wpedantic $<$<CXX_COMPILER_ID:Clang>:-Wno-c++2b-extensions>)
-endif()
+target_compile_options(msft_proxy_benchmarks PRIVATE ${PROXY_STRICT_WARNING_FLAGS})
diff --git a/docs/CMakeLists.txt b/docs/CMakeLists.txt
index ff9dc5b..b149a12 100644
--- a/docs/CMakeLists.txt
+++ b/docs/CMakeLists.txt
@@ -18,11 +18,7 @@ foreach(SOURCE ${EXAMPLE_SOURCES})
   get_filename_component(EXECUTABLE_NAME ${SOURCE} NAME_WE)
   add_executable(${EXECUTABLE_NAME} ${SOURCE})
   target_link_libraries(${EXECUTABLE_NAME} PRIVATE msft_proxy4::proxy)
-  if (MSVC)
-    target_compile_options(${EXECUTABLE_NAME} PRIVATE /W4)
-  else()
-    target_compile_options(${EXECUTABLE_NAME} PRIVATE -Wall -Wextra -Wpedantic $<$<CXX_COMPILER_ID:Clang>:-Wno-c++2b-extensions>)
-  endif()
+  target_compile_options(${EXECUTABLE_NAME} PRIVATE ${PROXY_WARNING_FLAGS})
 endforeach()
 
 target_link_libraries(example_spec_skills_fmt_format PRIVATE fmt::fmt)
diff --git a/include/proxy/v4/proxy.h b/include/proxy/v4/proxy.h
index 89ea028..2b26da3 100644
--- a/include/proxy/v4/proxy.h
+++ b/include/proxy/v4/proxy.h
@@ -18,14 +18,14 @@
 #include <tuple>
 #include <type_traits>
 #include <utility>
+#include <version>
 
 #if __STDC_HOSTED__
 #include <atomic>
-#if __has_include(<format>)
+
+// LLVM libc++ 17 has usable <format> despite lacking __cpp_lib_format until 19.
+#if __cpp_lib_format >= 201907L || _LIBCPP_VERSION >= 170000
 #include <format>
-#endif // __has_include(<format>)
-#if __cpp_lib_format >= 201907L ||                                             \
-    (defined(_LIBCPP_VERSION) && _LIBCPP_VERSION >= 170000)
 #define PRO4D_HAS_FORMAT
 #endif // __cpp_lib_format || _LIBCPP_VERSION >= 170000
 #endif // __STDC_HOSTED__
@@ -714,9 +714,6 @@ struct facade_conv_traits_impl {
   template <class P>
   static constexpr bool conv_applicable_ptr =
       (conv_traits<Cs, F>::template applicable_ptr<P> && ...);
-  template <bool IsDirect, class D, class O>
-  static constexpr bool is_invocable =
-      std::is_base_of_v<invocation_meta<F, IsDirect, D, O>, conv_meta>;
 };
 template <class F, class... Rs>
 struct facade_refl_traits_impl {
@@ -2148,6 +2145,11 @@ class bad_proxy_cast : public std::bad_cast {
 
 namespace details {
 
+template <template <class...> class TT, class... Ctx>
+struct enabled_t {};
+template <class T, template <class...> class TT, class... Ctx>
+concept enabled_for = std::is_base_of_v<enabled_t<TT, Ctx...>, T>;
+
 struct view_conversion_dispatch : cast_dispatch_base<false, true> {
   template <class T>
   PRO4D_STATIC_CALL(auto, T& value) noexcept
@@ -2179,47 +2181,74 @@ struct weak_conversion_dispatch : cast_dispatch_base<false, true> {
 template <class F>
 using weak_conversion_overload = weak_proxy<F>() const noexcept;
 
+template <template <class...> class Formatter,
+          template <class...> class StringView,
+          template <class...> class ParseContext,
+          template <class...> class FormatContext>
+struct format_traits {
+  template <class CharT>
+  using overload = typename FormatContext<CharT>::iterator(
+      StringView<CharT> spec, FormatContext<CharT>& fc) const;
+
+  struct dispatch {
+    template <class T, class CharT>
+    PRO4D_STATIC_CALL(auto, const T& self, StringView<CharT> spec,
+                      FormatContext<CharT>& fc)
+      requires(std::is_default_constructible_v<Formatter<T, CharT>>)
+    {
+      Formatter<T, CharT> impl;
+      {
+        ParseContext<CharT> pc{spec};
+        impl.parse(pc);
+      }
+      return impl.format(self, fc);
+    }
+
+    template <class P, class D, class... Os>
+    struct PRO4D_ENFORCE_EBO accessor : accessor<P, D, Os>... {};
+    template <class P, class D>
+    struct accessor<P, D, overload<char>> : enabled_t<Formatter, char> {};
+    template <class P, class D>
+    struct accessor<P, D, overload<wchar_t>> : enabled_t<Formatter, wchar_t> {};
+  };
+
+  template <class CharT>
+  struct formatter {
+    constexpr auto parse(ParseContext<CharT>& pc) {
+      for (auto it = pc.begin(); it != pc.end(); ++it) {
+        if (*it == '}') {
+          spec_ = StringView<CharT>{pc.begin(), it + 1};
+          return it;
+        }
+      }
+      return pc.end();
+    }
+
+    template <class P, class CompatibleFormatContext>
+    auto format(const P& p, CompatibleFormatContext& fc) const ->
+        typename CompatibleFormatContext::iterator {
+      return invoke<dispatch, overload<CharT>>(p, spec_, fc);
+    }
+
+  private:
+    StringView<CharT> spec_;
+  };
+};
+
 #ifdef PRO4D_HAS_FORMAT
 template <class CharT>
-struct format_overload_traits;
+struct std_format_context_traits;
 template <>
-struct format_overload_traits<char>
-    : std::type_identity<std::format_context::iterator(
-          std::string_view spec, std::format_context& fc) const> {};
+struct std_format_context_traits<char>
+    : std::type_identity<std::format_context> {};
 template <>
-struct format_overload_traits<wchar_t>
-    : std::type_identity<std::wformat_context::iterator(
-          std::wstring_view spec, std::wformat_context& fc) const> {};
+struct std_format_context_traits<wchar_t>
+    : std::type_identity<std::wformat_context> {};
 template <class CharT>
-using format_overload_t = typename format_overload_traits<CharT>::type;
-
-struct format_dispatch {
-  // Note: This function requires std::formatter<T, CharT> to be well-formed.
-  // However, the standard did not provide such facility before C++23. In the
-  // "required" clause of this function, std::formattable (C++23) is preferred
-  // when available. Otherwise, when building with C++20, we simply check
-  // whether std::formatter<T, CharT> is a disabled specialization of
-  // std::formatter by std::is_default_constructible_v as per
-  // [format.formatter.spec].
-  template <class T, class CharT, class OutIt>
-  PRO4D_STATIC_CALL(OutIt, const T& self, std::basic_string_view<CharT> spec,
-                    std::basic_format_context<OutIt, CharT>& fc)
-    requires(
-#if __cpp_lib_format_ranges >= 202207L
-        std::formattable<T, CharT>
-#else
-        std::is_default_constructible_v<std::formatter<T, CharT>>
-#endif // __cpp_lib_format_ranges >= 202207L
-    )
-  {
-    std::formatter<T, CharT> impl;
-    {
-      std::basic_format_parse_context<CharT> pc{spec};
-      impl.parse(pc);
-    }
-    return impl.format(self, fc);
-  }
-};
+using std_format_context = typename std_format_context_traits<CharT>::type;
+struct std_format_traits
+    : format_traits<std::formatter, std::basic_string_view,
+                    std::basic_format_parse_context, std_format_context> {};
 #endif // PRO4D_HAS_FORMAT
 
 #if __cpp_rtti >= 199711L
@@ -2329,14 +2358,14 @@ namespace skills {
 
 #ifdef PRO4D_HAS_FORMAT
 template <class FB>
-using format =
-    typename FB::template add_convention<details::format_dispatch,
-                                         details::format_overload_t<char>>;
+using format = typename FB::template add_convention<
+    details::std_format_traits::dispatch,
+    details::std_format_traits::overload<char>>;
 
 template <class FB>
-using wformat =
-    typename FB::template add_convention<details::format_dispatch,
-                                         details::format_overload_t<wchar_t>>;
+using wformat = typename FB::template add_convention<
+    details::std_format_traits::dispatch,
+    details::std_format_traits::overload<wchar_t>>;
 #endif // PRO4D_HAS_FORMAT
 
 #if __cpp_rtti >= 199711L
@@ -2676,31 +2705,10 @@ struct weak_dispatch : D {
 #ifdef PRO4D_HAS_FORMAT
 namespace std {
 
-template <pro::v4::facade F, class CharT>
-  requires(pro::v4::details::facade_traits<F>::template is_invocable<
-           false, pro::v4::details::format_dispatch,
-           pro::v4::details::format_overload_t<CharT>>)
-struct formatter<pro::v4::proxy_indirect_accessor<F>, CharT> {
-  constexpr auto parse(basic_format_parse_context<CharT>& pc) {
-    for (auto it = pc.begin(); it != pc.end(); ++it) {
-      if (*it == '}') {
-        spec_ = basic_string_view<CharT>{pc.begin(), it + 1};
-        return it;
-      }
-    }
-    return pc.end();
-  }
-
-  template <class OutIt>
-  OutIt format(const pro::v4::proxy_indirect_accessor<F>& p,
-               basic_format_context<OutIt, CharT>& fc) const {
-    return invoke<pro::v4::details::format_dispatch,
-                  pro::v4::details::format_overload_t<CharT>>(p, spec_, fc);
-  }
-
-private:
-  basic_string_view<CharT> spec_;
-};
+template <class T, class CharT>
+  requires(pro::v4::details::enabled_for<T, std::formatter, CharT>)
+struct formatter<T, CharT>
+    : pro::v4::details::std_format_traits::formatter<CharT> {};
 
 } // namespace std
 #endif // PRO4D_HAS_FORMAT
diff --git a/include/proxy/v4/proxy_fmt.h b/include/proxy/v4/proxy_fmt.h
index 60690e4..a35b490 100644
--- a/include/proxy/v4/proxy_fmt.h
+++ b/include/proxy/v4/proxy_fmt.h
@@ -28,37 +28,23 @@ using fmt_buffered_context = fmt::buffered_context<CharT>;
 using fmt_buffered_context = fmt::buffer_context<CharT>;
 #endif // FMT_VERSION
 
-template <class CharT>
-using fmt_format_overload_t = fmt_buffered_context<CharT>::iterator(
-    std::basic_string_view<CharT> spec, fmt_buffered_context<CharT>& fc) const;
-
-struct fmt_format_dispatch {
-  template <class T, class CharT, class FormatContext>
-  PRO4D_STATIC_CALL(auto, const T& self, std::basic_string_view<CharT> spec,
-                    FormatContext& fc)
-    requires(std::is_default_constructible_v<fmt::formatter<T, CharT>>)
-  {
-    fmt::formatter<T, CharT> impl;
-    {
-      fmt::basic_format_parse_context<CharT> pc{spec};
-      impl.parse(pc);
-    }
-    return impl.format(self, fc);
-  }
-};
+struct fmt_format_traits
+    : format_traits<fmt::formatter, std::basic_string_view,
+                    fmt::basic_format_parse_context, fmt_buffered_context> {};
 
 } // namespace details
 
 namespace skills {
 
 template <class FB>
-using fmt_format =
-    typename FB::template add_convention<details::fmt_format_dispatch,
-                                         details::fmt_format_overload_t<char>>;
+using fmt_format = typename FB::template add_convention<
+    details::fmt_format_traits::dispatch,
+    details::fmt_format_traits::overload<char>>;
 
 template <class FB>
 using fmt_wformat = typename FB::template add_convention<
-    details::fmt_format_dispatch, details::fmt_format_overload_t<wchar_t>>;
+    details::fmt_format_traits::dispatch,
+    details::fmt_format_traits::overload<wchar_t>>;
 
 } // namespace skills
 
@@ -66,31 +52,10 @@ using fmt_wformat = typename FB::template add_convention<
 
 namespace fmt {
 
-template <pro::v4::facade F, class CharT>
-  requires(pro::v4::details::facade_traits<F>::template is_invocable<
-           false, pro::v4::details::fmt_format_dispatch,
-           pro::v4::details::fmt_format_overload_t<CharT>>)
-struct formatter<pro::v4::proxy_indirect_accessor<F>, CharT> {
-  constexpr auto parse(basic_format_parse_context<CharT>& pc) {
-    for (auto it = pc.begin(); it != pc.end(); ++it) {
-      if (*it == '}') {
-        spec_ = std::basic_string_view<CharT>{pc.begin(), it + 1};
-        return it;
-      }
-    }
-    return pc.end();
-  }
-
-  template <class FormatContext>
-  auto format(const pro::v4::proxy_indirect_accessor<F>& p,
-              FormatContext& fc) const -> typename FormatContext::iterator {
-    return invoke<pro::v4::details::fmt_format_dispatch,
-                  pro::v4::details::fmt_format_overload_t<CharT>>(p, spec_, fc);
-  }
-
-private:
-  std::basic_string_view<CharT> spec_;
-};
+template <class T, class CharT>
+  requires(pro::v4::details::enabled_for<T, fmt::formatter, CharT>)
+struct formatter<T, CharT>
+    : pro::v4::details::fmt_format_traits::formatter<CharT> {};
 
 } // namespace fmt
 
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ff508a3..07580b2 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -14,6 +14,7 @@ add_executable(msft_proxy_tests
   proxy_creation_tests.cpp
   proxy_details_tests.cpp
   proxy_dispatch_tests.cpp
+  proxy_fmt_format_no_wchar_tests.cpp
   proxy_fmt_format_tests.cpp
   proxy_format_tests.cpp
   proxy_integration_tests.cpp
@@ -27,18 +28,12 @@ add_executable(msft_proxy_tests
 )
 target_include_directories(msft_proxy_tests PRIVATE .)
 target_link_libraries(msft_proxy_tests PRIVATE msft_proxy4::proxy gtest_main fmt::fmt)
-
-if(MSVC)
-  target_compile_options(msft_proxy_tests PRIVATE /W4 /WX)
-else()
-  target_compile_options(msft_proxy_tests PRIVATE -Wall -Wextra -Wpedantic -Werror $<$<CXX_COMPILER_ID:Clang>:-Wno-c++2b-extensions>)
-endif()
-
+target_compile_options(msft_proxy_tests PRIVATE ${PROXY_STRICT_WARNING_FLAGS})
 gtest_discover_tests(msft_proxy_tests)
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
   add_executable(msft_proxy_freestanding_tests freestanding/proxy_freestanding_tests.cpp)
-  target_compile_options(msft_proxy_freestanding_tests PRIVATE -ffreestanding -fno-exceptions -fno-rtti -Wall -Wextra -Wpedantic -Werror $<$<CXX_COMPILER_ID:Clang>:-Wno-c++2b-extensions>)
+  target_compile_options(msft_proxy_freestanding_tests PRIVATE -ffreestanding -fno-exceptions -fno-rtti ${PROXY_STRICT_WARNING_FLAGS})
   target_link_options(msft_proxy_freestanding_tests PRIVATE -nodefaultlibs -lc)
   target_link_libraries(msft_proxy_freestanding_tests PRIVATE msft_proxy4::proxy)
   add_test(NAME ProxyFreestandingTests COMMAND msft_proxy_freestanding_tests)
diff --git a/tests/proxy_fmt_format_no_wchar_tests.cpp b/tests/proxy_fmt_format_no_wchar_tests.cpp
new file mode 100644
index 0000000..677c57d
--- /dev/null
+++ b/tests/proxy_fmt_format_no_wchar_tests.cpp
@@ -0,0 +1,35 @@
+// Copyright (c) 2022-2026 Microsoft Corporation.
+// Copyright (c) 2026-Present Next Gen C++ Foundation.
+// Licensed under the MIT License.
+
+#include <gtest/gtest.h>
+
+#include <fmt/format.h> // <fmt/xchar.h> is intentionally not included in this TU
+#include <proxy/proxy.h>
+#include <proxy/proxy_fmt.h>
+
+namespace proxy_fmt_format_no_wchar_tests_details {
+
+struct NonFormattable : pro::facade_builder::build {};
+
+static_assert(
+    !std::is_default_constructible_v<
+        fmt::formatter<pro::proxy_indirect_accessor<NonFormattable>, char>>);
+
+struct Formattable : pro::facade_builder                  //
+                     ::add_skill<pro::skills::fmt_format> //
+                     ::build {};
+
+static_assert(std::is_default_constructible_v<
+              fmt::formatter<pro::proxy_indirect_accessor<Formattable>, char>>);
+
+} // namespace proxy_fmt_format_no_wchar_tests_details
+
+namespace details = proxy_fmt_format_no_wchar_tests_details;
+
+TEST(ProxyFmtFormatNoWcharTests, TestFormat) {
+  int v = 123;
+  pro::proxy<details::Formattable> p = &v;
+  ASSERT_EQ(fmt::format("{}", *p), "123");
+  ASSERT_EQ(fmt::format("{:*<6}", *p), "123***");
+}
diff --git a/tests/proxy_fmt_format_tests.cpp b/tests/proxy_fmt_format_tests.cpp
index 7a4a93e..2206b63 100644
--- a/tests/proxy_fmt_format_tests.cpp
+++ b/tests/proxy_fmt_format_tests.cpp
@@ -5,7 +5,9 @@
 #include <gtest/gtest.h>
 
 #include <fmt/format.h>
+#if FMT_VERSION >= 80000
 #include <fmt/xchar.h>
+#endif // FMT_VERSION >= 80000
 #include <proxy/proxy.h>
 #include <proxy/proxy_fmt.h>
 

From 97eca26eed6fcf9a4fe3f27c5c8caac44bbb1e92 Mon Sep 17 00:00:00 2001
From: Mingxin Wang <mingxwa@outlook.com>
Date: Sun, 10 May 2026 22:56:20 +0800
Subject: [PATCH 2/3] Fix compiler options

---
 CMakeLists.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 29ce63b..28db85c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -112,10 +112,10 @@ if (BUILD_TESTING)
 
   if(MSVC)
     set(PROXY_WARNING_FLAGS /W4)
-    set(PROXY_STRICT_WARNING_FLAGS ${PROXY_WARNING_FLAGS})
+    set(PROXY_STRICT_WARNING_FLAGS ${PROXY_WARNING_FLAGS} /WX)
   else()
     set(PROXY_WARNING_FLAGS -Wall -Wextra -Wpedantic $<$<CXX_COMPILER_ID:Clang>:-Wno-c++2b-extensions>)
-    set(PROXY_STRICT_WARNING_FLAGS ${PROXY_WARNING_FLAGS})
+    set(PROXY_STRICT_WARNING_FLAGS ${PROXY_WARNING_FLAGS} -Werror)
   endif()
 
   add_subdirectory(tests)

From b0d4dfd72a8b94fed3288466e156969d74311c50 Mon Sep 17 00:00:00 2001
From: Mingxin Wang <mingxwa@outlook.com>
Date: Sun, 10 May 2026 23:11:04 +0800
Subject: [PATCH 3/3] Fix compile error

---
 benchmarks/CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/benchmarks/CMakeLists.txt b/benchmarks/CMakeLists.txt
index f8060e2..b6c0306 100644
--- a/benchmarks/CMakeLists.txt
+++ b/benchmarks/CMakeLists.txt
@@ -16,4 +16,4 @@ add_executable(msft_proxy_benchmarks
 )
 target_include_directories(msft_proxy_benchmarks PRIVATE .)
 target_link_libraries(msft_proxy_benchmarks PRIVATE msft_proxy4::proxy benchmark::benchmark benchmark::benchmark_main)
-target_compile_options(msft_proxy_benchmarks PRIVATE ${PROXY_STRICT_WARNING_FLAGS})
+target_compile_options(msft_proxy_benchmarks PRIVATE ${PROXY_WARNING_FLAGS})