From a197a994c5a2363bc93bcff41bc77390802051cf Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Fri, 20 Sep 2024 07:08:47 -0700
Subject: [PATCH] Add member format_as for std

---
 include/fmt/base.h   | 38 +++++++++++++++++++++-----------------
 include/fmt/format.h | 13 +++++++++++++
 include/fmt/ranges.h |  4 ++--
 test/format-test.cc  |  2 ++
 4 files changed, 38 insertions(+), 19 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index a4af4f4c..04fbf22d 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -17,8 +17,6 @@
 #  include <stdio.h>   // FILE
 #  include <string.h>  // memcmp
 
-// <cstddef> is also included transitively from <type_traits>.
-#  include <cstddef>      // std::byte
 #  include <type_traits>  // std::enable_if
 #endif
 
@@ -309,6 +307,7 @@ using make_unsigned_t = typename std::make_unsigned<T>::type;
 template <typename T>
 using underlying_t = typename std::underlying_type<T>::type;
 template <typename T> using decay_t = typename std::decay<T>::type;
+using nullptr_t = decltype(nullptr);
 
 #if FMT_GCC_VERSION && FMT_GCC_VERSION < 500
 // A workaround for gcc 4.9 to make void_t work in a SFINAE context.
@@ -506,7 +505,7 @@ template <typename Char> class basic_string_view {
   constexpr basic_string_view(const Char* s, size_t count) noexcept
       : data_(s), size_(count) {}
 
-  constexpr basic_string_view(std::nullptr_t) = delete;
+  constexpr basic_string_view(nullptr_t) = delete;
 
   /// Constructs a string reference object from a C string.
 #if FMT_GCC_VERSION
@@ -643,15 +642,6 @@ struct formatter {
   formatter() = delete;
 };
 
-// This is defined in base.h instead of format.h to avoid injecting in std.
-// It is a template to avoid undesirable implicit conversions to std::byte.
-#ifdef __cpp_lib_byte
-template <typename T, FMT_ENABLE_IF(std::is_same<T, std::byte>::value)>
-inline auto format_as(T b) -> unsigned char {
-  return static_cast<unsigned char>(b);
-}
-#endif
-
 /// Reports a format error at compile time or, via a `format_error` exception,
 /// at runtime.
 // This function is intentionally not constexpr to give a compile-time error.
@@ -1071,14 +1061,24 @@ using ulong_type = conditional_t<long_short, unsigned, unsigned long long>;
 template <typename T>
 using format_as_result =
     remove_cvref_t<decltype(format_as(std::declval<const T&>()))>;
+template <typename T>
+using format_as_member_result =
+    remove_cvref_t<decltype(formatter<T>::format_as(std::declval<const T&>()))>;
 
 template <typename T, typename Enable = std::true_type>
 struct use_format_as : std::false_type {};
+// format_as member is only used to avoid injection into the std namespace.
+template <typename T, typename Enable = std::true_type>
+struct use_format_as_member : std::false_type {};
 
 // Only map owning types because mapping views can be unsafe.
 template <typename T>
 struct use_format_as<
-    T, bool_constant<std::is_integral<format_as_result<T>>::value>>
+    T, bool_constant<std::is_arithmetic<format_as_result<T>>::value>>
+    : std::true_type {};
+template <typename T>
+struct use_format_as_member<
+    T, bool_constant<std::is_arithmetic<format_as_member_result<T>>::value>>
     : std::true_type {};
 
 template <typename T, typename U = remove_const_t<T>>
@@ -1086,7 +1086,7 @@ using use_formatter =
     bool_constant<(std::is_class<T>::value || std::is_enum<T>::value ||
                    std::is_union<T>::value || std::is_array<T>::value) &&
                   !has_to_string_view<T>::value && !is_named_arg<T>::value &&
-                  !use_format_as<T>::value>;
+                  !use_format_as<T>::value && !use_format_as_member<T>::value>;
 
 template <typename Char, typename T, typename U = remove_const_t<T>>
 auto has_formatter_impl(T* p, buffered_context<Char>* ctx = nullptr)
@@ -1140,13 +1140,15 @@ template <typename Char> struct type_mapper {
   static auto map(const void*) -> const void*;
   static auto map(volatile void*) -> const void*;
   static auto map(const volatile void*) -> const void*;
-  static auto map(std::nullptr_t) -> const void*;
+  static auto map(nullptr_t) -> const void*;
   template <typename T, FMT_ENABLE_IF(std::is_pointer<T>::value ||
                                       std::is_member_pointer<T>::value)>
   static auto map(const T&) -> void;
 
   template <typename T, FMT_ENABLE_IF(use_format_as<T>::value)>
   static auto map(const T& x) -> decltype(map(format_as(x)));
+  template <typename T, FMT_ENABLE_IF(use_format_as_member<T>::value)>
+  static auto map(const T& x) -> decltype(map(formatter<T>::format_as(x)));
 
   template <typename T, FMT_ENABLE_IF(use_formatter<T>::value)>
   static auto map(T&) -> conditional_t<has_formatter<T, Char>(), T&, void>;
@@ -2130,7 +2132,7 @@ template <typename Context> class value {
       : pointer(const_cast<const void*>(x)) {}
   FMT_INLINE value(const volatile void* x FMT_BUILTIN)
       : pointer(const_cast<const void*>(x)) {}
-  FMT_INLINE value(std::nullptr_t) : pointer(nullptr) {}
+  FMT_INLINE value(nullptr_t) : pointer(nullptr) {}
 
   template <typename T, FMT_ENABLE_IF(std::is_pointer<T>::value ||
                                       std::is_member_pointer<T>::value)>
@@ -2144,6 +2146,8 @@ template <typename Context> class value {
 
   template <typename T, FMT_ENABLE_IF(use_format_as<T>::value)>
   value(const T& x) : value(format_as(x)) {}
+  template <typename T, FMT_ENABLE_IF(use_format_as_member<T>::value)>
+  value(const T& x) : value(formatter<T>::format_as(x)) {}
 
   template <typename T, FMT_ENABLE_IF(is_named_arg<T>::value)>
   value(const T& named_arg) : value(named_arg.value) {}
@@ -2376,9 +2380,9 @@ template <typename T> class basic_appender {
  public:
   using iterator_category = int;
   using value_type = T;
-  using difference_type = ptrdiff_t;
   using pointer = T*;
   using reference = T&;
+  using difference_type = decltype(pointer() - pointer());
   using container_type = detail::buffer<T>;
 
   FMT_CONSTEXPR basic_appender(detail::buffer<T>& buf) : container(&buf) {}
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 58f52990..45cab4cd 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -42,6 +42,7 @@
 
 #ifndef FMT_MODULE
 #  include <cmath>             // std::signbit
+#  include <cstddef>           // std::byte
 #  include <cstdint>           // uint32_t
 #  include <cstring>           // std::memcpy
 #  include <initializer_list>  // std::initializer_list
@@ -3919,6 +3920,18 @@ constexpr auto format_as(Enum e) noexcept -> underlying_t<Enum> {
 }
 }  // namespace enums
 
+#ifdef __cpp_lib_byte
+template <> struct formatter<std::byte> : formatter<unsigned> {
+  static auto format_as(std::byte b) -> unsigned char {
+    return static_cast<unsigned char>(b);
+  }
+  template <typename Context>
+  auto format(std::byte b, Context& ctx) const -> decltype(ctx.out()) {
+    return formatter<unsigned>::format(format_as(b), ctx);
+  }
+};
+#endif
+
 class bytes {
  private:
   string_view data_;
diff --git a/include/fmt/ranges.h b/include/fmt/ranges.h
index 572aa407..c6020abe 100644
--- a/include/fmt/ranges.h
+++ b/include/fmt/ranges.h
@@ -115,8 +115,8 @@ struct is_range_<T, void>
 
 // tuple_size and tuple_element check.
 template <typename T> class is_tuple_like_ {
-  template <typename U>
-  static auto check(U* p) -> decltype(std::tuple_size<U>::value, int());
+  template <typename U, typename V = typename std::remove_cv<U>::type>
+  static auto check(U* p) -> decltype(std::tuple_size<V>::value, 0);
   template <typename> static void check(...);
 
  public:
diff --git a/test/format-test.cc b/test/format-test.cc
index 73186d02..1c907c87 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -2030,6 +2030,8 @@ enum big_enum : unsigned long long { big_enum_value = 5000000000ULL };
 auto format_as(big_enum e) -> unsigned long long { return e; }
 
 TEST(format_test, strong_enum) {
+  auto arg = fmt::basic_format_arg<fmt::context>(big_enum_value);
+  EXPECT_EQ(arg.type(), fmt::detail::type::ulong_long_type);
   EXPECT_EQ(fmt::format("{}", big_enum_value), "5000000000");
 }
 #endif