[clang] [clang] Implement CWG2759 "`[[no_unique_address]` and common initial sequence" (PR #82607)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Vlad Serebrennikov (Endilll)

<details>
<summary>Changes</summary>

This patch implements said defect report resolution by adding additional check to common initial sequence evaluation. Consequently, this fixes CWG2759.

No special handling of `[[msvc::no_unique_address]]` is performed, so it should follow `[[no_unique_address]]` behavior.

---
Full diff: https://github.com/llvm/llvm-project/pull/82607.diff


5 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+4-2) 
- (modified) clang/lib/Sema/SemaChecking.cpp (+5) 
- (modified) clang/test/CXX/drs/dr27xx.cpp (+83) 
- (modified) clang/test/SemaCXX/type-traits.cpp (+5-5) 
- (modified) clang/www/cxx_dr_status.html (+1-1) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index bac166e6c35627..a7c0f2fbbe7604 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -83,8 +83,6 @@ C++20 Feature Support
 
 - Implemented the `__is_layout_compatible` intrinsic to support
   `P0466R5: Layout-compatibility and Pointer-interconvertibility Traits <https://wg21.link/P0466R5>`_.
-  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
 ^^^^^^^^^^^^^^^^^^^^^
@@ -108,6 +106,10 @@ Resolutions to C++ Defect Reports
   of two types.
   (`CWG1719: Layout compatibility and cv-qualification revisited <https://cplusplus.github.io/CWG/issues/1719.html>`_).
 
+- ``[[no_unique_address]]`` is now respected when evaluating layout
+  compatibility of two types.
+  (`CWG2759: [[no_unique_address] and common initial sequence  <https://cplusplus.github.io/CWG/issues/2759.html>`_).
+
 C Language Changes
 ------------------
 
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index e8bfb215a5b4c5..aba8852f6cfca4 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -19033,6 +19033,11 @@ static bool isLayoutCompatible(ASTContext &C, FieldDecl *Field1,
       return false;
   }
 
+  if (Field1->hasAttr<clang::NoUniqueAddressAttr>() ||
+      Field2->hasAttr<clang::NoUniqueAddressAttr>()) {
+    return false;
+  }
+
   return true;
 }
 
diff --git a/clang/test/CXX/drs/dr27xx.cpp b/clang/test/CXX/drs/dr27xx.cpp
index dd3fd5a20163fa..3fdf384bef27a7 100644
--- a/clang/test/CXX/drs/dr27xx.cpp
+++ b/clang/test/CXX/drs/dr27xx.cpp
@@ -10,6 +10,89 @@
 // expected-no-diagnostics
 #endif
 
+namespace dr2759 { // dr2759: 19
+#if __cplusplus >= 201103L
+
+struct CStruct {
+  int one;
+  int two;
+};
+
+struct CEmptyStruct {};
+struct CEmptyStruct2 {};
+
+struct CStructNoUniqueAddress {
+  int one;
+  [[no_unique_address]] int two;
+};
+
+struct CStructNoUniqueAddress2 {
+  int one;
+  [[no_unique_address]] int two;
+};
+
+union UnionLayout {
+  int a;
+  double b;
+  CStruct c;
+  [[no_unique_address]] CEmptyStruct d;
+  [[no_unique_address]] CEmptyStruct2 e;
+};
+
+union UnionLayout2 {
+  CStruct c;
+  int a;
+  CEmptyStruct2 e;
+  double b;
+  [[no_unique_address]] CEmptyStruct d;
+};
+
+union UnionLayout3 {
+  CStruct c;
+  int a;
+  double b;
+  [[no_unique_address]] CEmptyStruct d;
+};
+
+struct StructWithAnonUnion {
+  union {
+    int a;
+    double b;
+    CStruct c;
+    [[no_unique_address]] CEmptyStruct d;
+    [[no_unique_address]] CEmptyStruct2 e;
+  };
+};
+
+struct StructWithAnonUnion2 {
+  union {
+    CStruct c;
+    int a;
+    CEmptyStruct2 e;
+    double b;
+    [[no_unique_address]] CEmptyStruct d;
+  };
+};
+
+struct StructWithAnonUnion3 {
+  union {
+    CStruct c;
+    int a;
+    CEmptyStruct2 e;
+    double b;
+    [[no_unique_address]] CEmptyStruct d;
+  } u;
+};
+
+static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) != bool(__has_cpp_attribute(no_unique_address)), "");
+static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) != bool(__has_cpp_attribute(no_unique_address)), "");
+static_assert(!__is_layout_compatible(UnionLayout, UnionLayout2), "");
+static_assert(!__is_layout_compatible(UnionLayout, UnionLayout3), "");
+static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion2), "");
+static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion3), "");
+#endif
+} // namespace dr2759
+
 namespace dr2789 { // dr2789: 18
 #if __cplusplus >= 202302L
 template <typename T = int>
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 2c35d5ee19a4c6..23c339ebdf0826 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -1768,8 +1768,8 @@ void is_layout_compatible(int n)
   static_assert(!__is_layout_compatible(CppStructNonStandardBySameBase, CppStructNonStandardBySameBase2), "");
   static_assert(!__is_layout_compatible(CppStructNonStandardBy2ndVirtBase, CppStructNonStandardBy2ndVirtBase2), "");
   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, CStructNoUniqueAddress) != bool(__has_cpp_attribute(no_unique_address)), "");
+  static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) != bool(__has_cpp_attribute(no_unique_address)), "");
   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), "");
@@ -1782,10 +1782,10 @@ void is_layout_compatible(int n)
   static_assert(!__is_layout_compatible(void(CStruct2::*)(int), void(CStruct2::*)(char)), "");
   static_assert(__is_layout_compatible(CStructNested, CStructNested2), "");
   static_assert(__is_layout_compatible(UnionLayout, UnionLayout), "");
-  static_assert(__is_layout_compatible(UnionLayout, UnionLayout2), "");
+  static_assert(!__is_layout_compatible(UnionLayout, UnionLayout2), "");
   static_assert(!__is_layout_compatible(UnionLayout, UnionLayout3), "");
-  static_assert(__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion2), "");
-  static_assert(__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion3), "");
+  static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion2), "");
+  static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion3), "");
   static_assert(__is_layout_compatible(EnumLayout, EnumClassLayout), "");
   static_assert(__is_layout_compatible(EnumForward, EnumForward), "");
   static_assert(__is_layout_compatible(EnumForward, EnumClassForward), "");
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 38e2cb63142662..8b638e06f4aab6 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -16362,7 +16362,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2759.html">2759</a></td>
     <td>DR</td>
     <td>[[no_unique_address] and common initial sequence</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="2760">
     <td><a href="https://cplusplus.github.io/CWG/issues/2760.html">2760</a></td>

``````````

</details>


https://github.com/llvm/llvm-project/pull/82607