std::is_layout_compatible(3)

NAME
std::is_layout_compatible - std::is_layout_compatible

Synopsis
   Defined in header <type_traits>
   template< class T, class U > (since C++20)
   struct is_layout_compatible;
   If T and U are layout-compatible types, provides the member constant value equal to
   true. Otherwise value is false.
   Every type is layout-compatible with its any cv-qualified versions, even if it is
   not an object type.
   If T or U is not a complete type, (possibly cv-qualified) void, or an array of
   unknown bound, the behavior is undefined.
   If an instantiation of a template above depends, directly or indirectly, on an
   incomplete type, and that instantiation could yield a different result if that type
   were hypothetically completed, the behavior is undefined.
   If the program adds specializations for std::is_layout_compatible or
   std::is_layout_compatible_v, the behavior is undefined.

Member constants
   value true if T and U are layout-compatible, false otherwise
   [static] (public static member constant)

Member functions
   operator bool converts the object to bool, returns value
                 (public member function)
   operator() returns value
   (C++14) (public member function)

Member types
   Type Definition
   value_type bool
   type std::integral_constant<bool, value>

Notes
   A signed integer type and its unsigned counterpart are not layout-compatible. char
   is layout-compatible with neither signed char nor unsigned char.
   Similar types are not layout-compatible if they are not the same type after ignoring
   top-level cv-qualification.
   An enumeration type and its underlying type are not layout-compatible.
   Array types of layout-compatible but different element types (ignoring
   cv-qualification) are not layout-compatible, even if they are of equal length.
         Feature-test macro Value Std Feature
   __cpp_lib_is_layout_compatible 201907L (C++20) std::is_layout_compatible

Example
// Run this code
 #include <iomanip>
 #include <iostream>
 #include <type_traits>
 struct Foo
 {
     int x;
     char y;
 };
 struct FooNua
 {
     int x;
     [[no_unique_address]] char y;
 };
 class Bar
 {
     const int u = 42;
     volatile char v = '*';
 };
 enum E0 : int {};
 enum class E1 : int {};
 static_assert
 (
     std::is_layout_compatible_v<const void, volatile void> == true and
     std::is_layout_compatible_v<Foo, Bar> == true and
     std::is_layout_compatible_v<Foo[2], Bar[2]> == false and
     std::is_layout_compatible_v<int, E0> == false and
     std::is_layout_compatible_v<E0, E1> == true and
     std::is_layout_compatible_v<long, unsigned long> == false and
     std::is_layout_compatible_v<char*, const char*> == false and
     std::is_layout_compatible_v<char*, char* const> == true and
     std::is_layout_compatible_v<Foo, FooNua> == false // Note [1]
 );
 // [1] MSVC erroneously fails this assert
 int main() {}

See also
   is_standard_layout checks if a type is a standard-layout type
   (C++11) (class template)