[clang] [clang] Implement `__is_layout_compatible` (PR #81506)

[Aaron Ballman via cfe-commits]

================
@@ -1558,6 +1558,89 @@ void is_standard_layout()
   int t71[F(__is_standard_layout(HasEmptyIndirectBaseAsSecondUnionMember))];
 }
 
+struct CStruct2 {
+  int one;
+  int two;
+};
+
+struct CEmptyStruct2 {};
+
+struct CppEmptyStruct2 : CStruct2 {};
+struct CppStructStandard2 : CEmptyStruct2 {
+  int three;
+  int four;
+};
+struct CppStructNonStandardByBase2 : CStruct2 {
+  int three;
+  int four;
+};
+struct CppStructNonStandardByVirt2 : CStruct2 {
+  virtual void method() {}
+};
+struct CppStructNonStandardByMemb2 : CStruct2 {
+  CppStructNonStandardByVirt member;
+};
+struct CppStructNonStandardByProt2 : CStruct2 {
+  int five;
+protected:
+  int six;
+};
+struct CppStructNonStandardByVirtBase2 : virtual CStruct2 {
+};
+struct CppStructNonStandardBySameBase2 : CEmptyStruct2 {
+  CEmptyStruct member;
+};
+struct CppStructNonStandardBy2ndVirtBase2 : CEmptyStruct2 {
+  CEmptyStruct member;
+};
+
+struct CStructWithQualifiers {
+  const int one;
+  volatile int two;
+};
+
+struct CStructNoUniqueAddress {
+  int one;
+  [[no_unique_address]] int two;
+};
+
+struct CStructNoUniqueAddress2 {
+  int one;
+  [[no_unique_address]] int two;
+};
+
+struct CStructAlignment {
+  int one;
+  alignas(16) int two;
+};
+
+struct CStructIncomplete;
+
+void is_layout_compatible()
+{
+  static_assert(__is_layout_compatible(void, void), "");
+  static_assert(__is_layout_compatible(int, int), "");
+  static_assert(__is_layout_compatible(int[], int[]), "");
+  static_assert(__is_layout_compatible(int[2], int[2]), "");
+  static_assert(__is_layout_compatible(CStruct, CStruct2), "");
+  static_assert(__is_layout_compatible(CEmptyStruct, CEmptyStruct2), "");
+  static_assert(__is_layout_compatible(CppEmptyStruct, CppEmptyStruct2), "");
+  static_assert(__is_layout_compatible(CppStructStandard, CppStructStandard2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardByBase, CppStructNonStandardByBase2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardByVirt, CppStructNonStandardByVirt2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardByMemb, CppStructNonStandardByMemb2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardByProt, CppStructNonStandardByProt2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardByVirtBase, CppStructNonStandardByVirtBase2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardBySameBase, CppStructNonStandardBySameBase2), "");
+  static_assert(!__is_layout_compatible(CppStructNonStandardBy2ndVirtBase, CppStructNonStandardBy2ndVirtBase2), "");
+  static_assert(!__is_layout_compatible(CStruct, CStructWithQualifiers), ""); // FIXME: this is CWG1719
+  static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
+  static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
+  static_assert(__is_layout_compatible(CStruct, CStructAlignment), "");
+  static_assert(__is_layout_compatible(CStructIncomplete, CStructIncomplete), "");
+  static_assert(!__is_layout_compatible(CStruct, CStructIncomplete), "");
+}
----------------
AaronBallman wrote:

I think we should add those tests somewhere (either here or in dr1xxx.cpp) so 1) we know we don't crash on them, 2) we're alerted to behavioral changes in this area, and 3) better documentation of existing behavior.

https://github.com/llvm/llvm-project/pull/81506