[clang] Implement CWG1719 "Layout compatibility and cv-qualification revisited" (#82358)

This patch updates our internal notion of `layout-compatible` to ignore cv-qualification,
which in turn fixes `__is_layout_compatible` intrinsic.

Author: Endilll

clang/docs/ReleaseNotes.rst

@@ -98,9 +98,8 @@ C++20 Feature Support
 
 - Implemented the `__is_layout_compatible` intrinsic to support
   `P0466R5: Layout-compatibility and Pointer-interconvertibility Traits <https://wg21.link/P0466R5>`_.
-  Note: `CWG1719: Layout compatibility and cv-qualification revisited  <https://cplusplus.github.io/CWG/issues/1719.html>`_
-  and `CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_
-  are not yet implemented.
+  Note: `CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_
+  is not yet implemented.
 
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
@@ -120,6 +119,10 @@ Resolutions to C++ Defect Reports
   in the template parameters, but is deduced from a previous argument.
   (`#78449: <https://github.com/llvm/llvm-project/issues/78449>`_).
 
+- Type qualifications are now ignored when evaluating layout compatibility
+  of two types.
+  (`CWG1719: Layout compatibility and cv-qualification revisited <https://cplusplus.github.io/CWG/issues/1719.html>`_).
+
 C Language Changes
 ------------------
 

clang/lib/Sema/SemaChecking.cpp

@@ -19124,15 +19124,16 @@ static bool isLayoutCompatible(ASTContext &C, QualType T1, QualType T2) {
   if (T1.isNull() || T2.isNull())
     return false;
 
-  // C++11 [basic.types] p11:
-  // If two types T1 and T2 are the same type, then T1 and T2 are
-  // layout-compatible types.
-  if (C.hasSameType(T1, T2))
-    return true;
-
+  // C++20 [basic.types] p11:
+  // Two types cv1 T1 and cv2 T2 are layout-compatible types
+  // if T1 and T2 are the same type, layout-compatible enumerations (9.7.1),
+  // or layout-compatible standard-layout class types (11.4).
   T1 = T1.getCanonicalType().getUnqualifiedType();
   T2 = T2.getCanonicalType().getUnqualifiedType();
 
+  if (C.hasSameType(T1, T2))
+    return true;
+
   const Type::TypeClass TC1 = T1->getTypeClass();
   const Type::TypeClass TC2 = T2->getTypeClass();
 

clang/test/CXX/drs/dr17xx.cpp

@@ -46,7 +46,7 @@ namespace dr1715 { // dr1715: 3.9
 #endif
 }
 
-namespace dr1719 { // dr1719: no
+namespace dr1719 { // dr1719: 19
 #if __cplusplus >= 201103L
 struct CStruct {
   int one;
@@ -66,11 +66,11 @@ struct CStructWithQualifiers {
 static_assert(__is_layout_compatible(CStruct, const CStruct2), "");
 static_assert(__is_layout_compatible(CStruct, volatile CStruct2), "");
 static_assert(__is_layout_compatible(const CStruct, volatile CStruct2), "");
-// FIXME: all of the following pairs of types are layout-compatible
-static_assert(!__is_layout_compatible(int, const int), "");
-static_assert(!__is_layout_compatible(int, volatile int), "");
-static_assert(!__is_layout_compatible(const int, volatile int), "");
-static_assert(!__is_layout_compatible(CStruct, CStructWithQualifiers), "");
+static_assert(__is_layout_compatible(int, const int), "");
+static_assert(__is_layout_compatible(int, volatile int), "");
+static_assert(__is_layout_compatible(const int, volatile int), "");
+static_assert(__is_layout_compatible(CStruct, CStructWithQualifiers), "");
+static_assert(__is_layout_compatible(int[], const volatile int[]), "");
 #endif
 } // namespace dr1719
 

clang/test/SemaCXX/type-traits.cpp

@@ -1609,7 +1609,12 @@ struct CStructNoUniqueAddress2 {
   [[no_unique_address]] int two;
 };
 
-struct CStructAlignment {
+struct alignas(64) CStructAlignment {
+  int one;
+  int two;
+};
+
+struct CStructAlignedMembers {
   int one;
   alignas(16) int two;
 };
@@ -1711,13 +1716,17 @@ void is_layout_compatible(int n)
 {
   static_assert(__is_layout_compatible(void, void), "");
   static_assert(!__is_layout_compatible(void, int), "");
-  static_assert(!__is_layout_compatible(void, const void), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(void, volatile void), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(const int, volatile int), ""); // FIXME: this is CWG1719
+  static_assert(__is_layout_compatible(void, const void), "");
+  static_assert(__is_layout_compatible(void, volatile void), "");
+  static_assert(__is_layout_compatible(const int, volatile int), "");
   static_assert(__is_layout_compatible(int, int), "");
-  static_assert(!__is_layout_compatible(int, const int), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(int, volatile int), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(const int, volatile int), ""); // FIXME: this is CWG1719
+  static_assert(__is_layout_compatible(int, const int), "");
+  static_assert(__is_layout_compatible(int, volatile int), "");
+  static_assert(__is_layout_compatible(const int, volatile int), "");
+  static_assert(__is_layout_compatible(int *, int * __restrict), "");
+  // Note: atomic qualification matters for layout compatibility.
+  static_assert(!__is_layout_compatible(int, _Atomic int), "");
+  static_assert(__is_layout_compatible(_Atomic(int), _Atomic int), "");
   static_assert(!__is_layout_compatible(int, unsigned int), "");
   static_assert(!__is_layout_compatible(char, unsigned char), "");
   static_assert(!__is_layout_compatible(char, signed char), "");
@@ -1758,10 +1767,11 @@ void is_layout_compatible(int n)
   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, CStructWithQualifiers), "");
   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(CStruct, CStructAlignedMembers), ""); // FIXME: alignment of members impact common initial sequence
   static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds), "");
   static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds2), "");
   static_assert(!__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds3), "");

clang/www/cxx_dr_status.html

@@ -7812,7 +7812,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1334.html">1334</a></td>
     <td>NAD</td>
     <td>Layout compatibility and cv-qualification</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Superseded by <a href="#1719">1719</a></td>
   </tr>
   <tr id="1335">
     <td><a href="https://cplusplus.github.io/CWG/issues/1335.html">1335</a></td>
@@ -10122,7 +10122,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1719.html">1719</a></td>
     <td>CD4</td>
     <td>Layout compatibility and cv-qualification revisited</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="1720">
     <td><a href="https://cplusplus.github.io/CWG/issues/1720.html">1720</a></td>