[libc++][NFC] Refactor __libcpp_datasizeof to be a variable template (#87769)

This decreases memory consumption and compiles times slightly and
removes a bit of boilderplate.

NOKEYCHECK=True
GitOrigin-RevId: d30f6bc5cd9579204864c944f127011be70b2c74

Author: philnik777

include/__string/constexpr_c_functions.h

@@ -224,7 +224,7 @@ __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n) {
         std::__assign_trivially_copyable(__dest[__i], __src[__i]);
     }
   } else if (__count > 0) {
-    ::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __libcpp_datasizeof<_Tp>::value);
+    ::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __datasizeof_v<_Tp>);
   }
   return __dest;
 }

include/__type_traits/datasizeof.h

@@ -26,39 +26,38 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Tp>
-struct __libcpp_datasizeof {
 #if __has_extension(datasizeof)
-  static const size_t value = __datasizeof(_Tp);
+template <class _Tp>
+inline const size_t __datasizeof_v = __datasizeof(_Tp);
 #else
 // NOLINTNEXTLINE(readability-redundant-preprocessor) This is https://llvm.org/PR64825
 #  if __has_cpp_attribute(__no_unique_address__)
-  template <class = char>
-  struct _FirstPaddingByte {
-    [[__no_unique_address__]] _Tp __v_;
-    char __first_padding_byte_;
-  };
+template <class _Tp>
+struct _FirstPaddingByte {
+  [[__no_unique_address__]] _Tp __v_;
+  char __first_padding_byte_;
+};
 #  else
-  template <bool = __libcpp_is_final<_Tp>::value || !is_class<_Tp>::value>
-  struct _FirstPaddingByte : _Tp {
-    char __first_padding_byte_;
-  };
+template <class _Tp, bool = __libcpp_is_final<_Tp>::value || !is_class<_Tp>::value>
+struct _FirstPaddingByte : _Tp {
+  char __first_padding_byte_;
+};
 
-  template <>
-  struct _FirstPaddingByte<true> {
-    _Tp __v_;
-    char __first_padding_byte_;
-  };
+template <class _Tp>
+struct _FirstPaddingByte<_Tp, true> {
+  _Tp __v_;
+  char __first_padding_byte_;
+};
 #  endif // __has_cpp_attribute(__no_unique_address__)
 
-  // _FirstPaddingByte<> is sometimes non-standard layout. Using `offsetof` is UB in that case, but GCC and Clang allow
-  // the use as an extension.
-  _LIBCPP_DIAGNOSTIC_PUSH
-  _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
-  static const size_t value = offsetof(_FirstPaddingByte<>, __first_padding_byte_);
-  _LIBCPP_DIAGNOSTIC_POP
+// _FirstPaddingByte<> is sometimes non-standard layout. Using `offsetof` is UB in that case, but GCC and Clang allow
+// the use as an extension.
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
+template <class _Tp>
+inline const size_t __datasizeof_v = offsetof(_FirstPaddingByte<_Tp>, __first_padding_byte_);
+_LIBCPP_DIAGNOSTIC_POP
 #endif   // __has_extension(datasizeof)
-};
 
 _LIBCPP_END_NAMESPACE_STD
 

test/libcxx/type_traits/datasizeof.compile.pass.cpp

@@ -9,10 +9,10 @@
 #include <__type_traits/datasizeof.h>
 #include <cstdint>
 
-static_assert(std::__libcpp_datasizeof<std::int8_t>::value == 1, "");
-static_assert(std::__libcpp_datasizeof<std::int16_t>::value == 2, "");
-static_assert(std::__libcpp_datasizeof<std::int32_t>::value == 4, "");
-static_assert(std::__libcpp_datasizeof<std::int64_t>::value == 8, "");
+static_assert(std::__datasizeof_v<std::int8_t> == 1, "");
+static_assert(std::__datasizeof_v<std::int16_t> == 2, "");
+static_assert(std::__datasizeof_v<std::int32_t> == 4, "");
+static_assert(std::__datasizeof_v<std::int64_t> == 8, "");
 
 struct OneBytePadding {
   OneBytePadding() {}
@@ -22,9 +22,9 @@ struct OneBytePadding {
 };
 
 #if defined(_WIN32) && !defined(__MINGW32__)
-static_assert(std::__libcpp_datasizeof<OneBytePadding>::value == 4, "");
+static_assert(std::__datasizeof_v<OneBytePadding> == 4, "");
 #else
-static_assert(std::__libcpp_datasizeof<OneBytePadding>::value == 3, "");
+static_assert(std::__datasizeof_v<OneBytePadding> == 3, "");
 #endif
 
 struct InBetweenPadding {
@@ -35,4 +35,4 @@ struct InBetweenPadding {
   std::int16_t c;
 };
 
-static_assert(std::__libcpp_datasizeof<InBetweenPadding>::value == 8, "");
+static_assert(std::__datasizeof_v<InBetweenPadding> == 8, "");

test/libcxx/utilities/expected/expected.expected/no_unique_address.compile.pass.cpp

@@ -47,28 +47,28 @@ static_assert(sizeof(std::expected<B, B>) == sizeof(B));
 
 // Check that `expected`'s datasize is large enough for the parameter type(s).
 static_assert(sizeof(std::expected<BoolWithPadding, Empty>) ==
-              std::__libcpp_datasizeof<std::expected<BoolWithPadding, Empty>>::value);
+              std::__datasizeof_v<std::expected<BoolWithPadding, Empty>>);
 static_assert(sizeof(std::expected<Empty, BoolWithPadding>) ==
-              std::__libcpp_datasizeof<std::expected<Empty, BoolWithPadding>>::value);
+              std::__datasizeof_v<std::expected<Empty, BoolWithPadding>>);
 
 // In this case, there should be tail padding in the `expected` because `A`
 // itself does _not_ have tail padding.
-static_assert(sizeof(std::expected<A, A>) > std::__libcpp_datasizeof<std::expected<A, A>>::value);
+static_assert(sizeof(std::expected<A, A>) > std::__datasizeof_v<std::expected<A, A>>);
 
 // Test with some real types.
 static_assert(sizeof(std::expected<std::optional<int>, int>) == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<std::optional<int>, int>>::value == 8);
+static_assert(std::__datasizeof_v<std::expected<std::optional<int>, int>> == 8);
 
 static_assert(sizeof(std::expected<int, std::optional<int>>) == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<int, std::optional<int>>>::value == 8);
+static_assert(std::__datasizeof_v<std::expected<int, std::optional<int>>> == 8);
 
 static_assert(sizeof(std::expected<int, int>) == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<int, int>>::value == 5);
+static_assert(std::__datasizeof_v<std::expected<int, int>> == 5);
 
 // clang-format off
-static_assert(std::__libcpp_datasizeof<int>::value == 4);
-static_assert(std::__libcpp_datasizeof<std::expected<int, int>>::value == 5);
-static_assert(std::__libcpp_datasizeof<std::expected<std::expected<int, int>, int>>::value == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<std::expected<std::expected<int, int>, int>, int>>::value == 9);
-static_assert(std::__libcpp_datasizeof<std::expected<std::expected<std::expected<std::expected<int, int>, int>, int>, int>>::value == 12);
+static_assert(std::__datasizeof_v<int> == 4);
+static_assert(std::__datasizeof_v<std::expected<int, int>> == 5);
+static_assert(std::__datasizeof_v<std::expected<std::expected<int, int>, int>> == 8);
+static_assert(std::__datasizeof_v<std::expected<std::expected<std::expected<int, int>, int>, int>> == 9);
+static_assert(std::__datasizeof_v<std::expected<std::expected<std::expected<std::expected<int, int>, int>, int>, int>> == 12);
 // clang-format on

test/libcxx/utilities/expected/expected.void/no_unique_address.compile.pass.cpp

@@ -45,23 +45,23 @@ static_assert(sizeof(std::expected<void, B>) == sizeof(B));
 
 // Check that `expected`'s datasize is large enough for the parameter type(s).
 static_assert(sizeof(std::expected<void, BoolWithPadding>) ==
-              std::__libcpp_datasizeof<std::expected<void, BoolWithPadding>>::value);
+              std::__datasizeof_v<std::expected<void, BoolWithPadding>>);
 
 // In this case, there should be tail padding in the `expected` because `A`
 // itself does _not_ have tail padding.
-static_assert(sizeof(std::expected<void, A>) > std::__libcpp_datasizeof<std::expected<void, A>>::value);
+static_assert(sizeof(std::expected<void, A>) > std::__datasizeof_v<std::expected<void, A>>);
 
 // Test with some real types.
 static_assert(sizeof(std::expected<void, std::optional<int>>) == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<void, std::optional<int>>>::value == 8);
+static_assert(std::__datasizeof_v<std::expected<void, std::optional<int>>> == 8);
 
 static_assert(sizeof(std::expected<void, int>) == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<void, int>>::value == 5);
+static_assert(std::__datasizeof_v<std::expected<void, int>> == 5);
 
 // clang-format off
-static_assert(std::__libcpp_datasizeof<int>::value == 4);
-static_assert(std::__libcpp_datasizeof<std::expected<void, int>>::value == 5);
-static_assert(std::__libcpp_datasizeof<std::expected<void, std::expected<void, int>>>::value == 8);
-static_assert(std::__libcpp_datasizeof<std::expected<void, std::expected<void, std::expected<void, int>>>>::value == 9);
-static_assert(std::__libcpp_datasizeof<std::expected<void, std::expected<void, std::expected<void, std::expected<void, int>>>>>::value == 12);
+static_assert(std::__datasizeof_v<int> == 4);
+static_assert(std::__datasizeof_v<std::expected<void, int>> == 5);
+static_assert(std::__datasizeof_v<std::expected<void, std::expected<void, int>>> == 8);
+static_assert(std::__datasizeof_v<std::expected<void, std::expected<void, std::expected<void, int>>>> == 9);
+static_assert(std::__datasizeof_v<std::expected<void, std::expected<void, std::expected<void, std::expected<void, int>>>>> == 12);
 // clang-format on

test/std/containers/sequences/array/size_and_alignment.compile.pass.cpp

@@ -46,7 +46,7 @@ void test_type() {
     static_assert(!std::is_empty<Array>::value, "");
 
     // Make sure empty arrays don't have padding bytes
-    LIBCPP_STATIC_ASSERT(std::__libcpp_datasizeof<Array>::value == sizeof(Array), "");
+    LIBCPP_STATIC_ASSERT(std::__datasizeof_v<Array> == sizeof(Array), "");
   }
 
   {