ensure that expected has no tail padding

jiixyj · Oct 15, 2023 · 9228950 · 9228950
1 parent 7b81fb5
commit 9228950
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diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
@@ -18,6 +18,7 @@
 #include <__memory/addressof.h>
 #include <__memory/construct_at.h>
 #include <__type_traits/conjunction.h>
+#include <__type_traits/datasizeof.h>
 #include <__type_traits/disjunction.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_assignable.h>
@@ -948,8 +949,17 @@ class expected {
       return __union_t<_Tp, _Err>(std::unexpect, std::move(__other.__union_.__unex_));
   }
 
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __calculate_padding() {
+    struct __calc_expected {
+      _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
+      bool __has_val_;
+    };
+    return sizeof(__calc_expected) - __libcpp_datasizeof<__calc_expected>::value;
+  }
+
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
   bool __has_val_;
+  char __padding_[__calculate_padding()]{};
 };
 
 template <class _Tp, class _Err>
@@ -1545,8 +1555,17 @@ class expected<_Tp, _Err> {
       return __union_t<_Err>(std::unexpect, std::move(__other.__union_.__unex_));
   }
 
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __calculate_padding() {
+    struct __calc_expected {
+      _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;
+      bool __has_val_;
+    };
+    return sizeof(__calc_expected) - __libcpp_datasizeof<__calc_expected>::value;
+  }
+
   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;
   bool __has_val_;
+  char __padding_[__calculate_padding()]{};
 };
 
 _LIBCPP_END_NAMESPACE_STD

diff --git a/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/assign/emplace.pass.cpp
@@ -73,6 +73,22 @@ constexpr bool test() {
     assert(e.value() == 10);
   }
 
+  // Test that `expected` has no tail padding.
+  {
+    struct BoolWithPadding {
+      explicit operator bool() { return b; }
+
+    private:
+      alignas(4) bool b = false;
+    };
+
+    static_assert(sizeof(std::expected<BoolWithPadding, bool>) ==
+                  std::__libcpp_datasizeof<std::expected<BoolWithPadding, bool>>::value);
+
+    static_assert(sizeof(std::expected<void, BoolWithPadding>) ==
+                  std::__libcpp_datasizeof<std::expected<void, BoolWithPadding>>::value);
+  }
+
   return true;
 }