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[clang] Implement CWG2759 "[[no_unique_address] and common initial sequence" (#82607)
This patch implements said defect report resolution by adding additional check to common initial sequence evaluation. Consequently, this fixes CWG2759.
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‎clang/docs/ReleaseNotes.rst

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -83,8 +83,6 @@ C++20 Feature Support
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- Implemented the `__is_layout_compatible` intrinsic to support
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`P0466R5: Layout-compatibility and Pointer-interconvertibility Traits <https://wg21.link/P0466R5>`_.
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Note: `CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_
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is not yet implemented.
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C++23 Feature Support
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^^^^^^^^^^^^^^^^^^^^^
@@ -108,6 +106,10 @@ Resolutions to C++ Defect Reports
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of two types.
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(`CWG1719: Layout compatibility and cv-qualification revisited <https://cplusplus.github.io/CWG/issues/1719.html>`_).
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- ``[[no_unique_address]]`` is now respected when evaluating layout
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compatibility of two types.
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(`CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_).
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C Language Changes
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------------------
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‎clang/lib/Sema/SemaChecking.cpp

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@@ -19036,6 +19036,9 @@ static bool isLayoutCompatible(ASTContext &C, FieldDecl *Field1,
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return false;
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}
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if (Field1->hasAttr<clang::NoUniqueAddressAttr>() ||
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Field2->hasAttr<clang::NoUniqueAddressAttr>())
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return false;
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return true;
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}
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‎clang/test/CXX/drs/dr27xx.cpp

Lines changed: 90 additions & 7 deletions
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@@ -1,15 +1,98 @@
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// RUN: %clang_cc1 -std=c++98 -verify=expected %s
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// RUN: %clang_cc1 -std=c++11 -verify=expected %s
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// RUN: %clang_cc1 -std=c++14 -verify=expected %s
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// RUN: %clang_cc1 -std=c++17 -verify=expected %s
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// RUN: %clang_cc1 -std=c++20 -verify=expected %s
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// RUN: %clang_cc1 -std=c++23 -verify=expected,since-cxx23 %s
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// RUN: %clang_cc1 -std=c++2c -verify=expected,since-cxx23,since-cxx26 %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++98 -verify=expected %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++11 -verify=expected %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++14 -verify=expected %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++17 -verify=expected %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 -verify=expected %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -verify=expected,since-cxx23 %s
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// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++2c -verify=expected,since-cxx23,since-cxx26 %s
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#if __cplusplus <= 202002L
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// expected-no-diagnostics
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#endif
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namespace dr2759 { // dr2759: 19
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#if __cplusplus >= 201103L
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struct CStruct {
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int one;
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int two;
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};
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struct CEmptyStruct {};
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struct CEmptyStruct2 {};
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struct CStructNoUniqueAddress {
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int one;
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[[no_unique_address]] int two;
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};
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struct CStructNoUniqueAddress2 {
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int one;
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[[no_unique_address]] int two;
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};
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union UnionLayout {
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int a;
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double b;
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CStruct c;
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[[no_unique_address]] CEmptyStruct d;
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[[no_unique_address]] CEmptyStruct2 e;
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};
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union UnionLayout2 {
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CStruct c;
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int a;
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CEmptyStruct2 e;
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double b;
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[[no_unique_address]] CEmptyStruct d;
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};
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union UnionLayout3 {
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CStruct c;
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int a;
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double b;
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[[no_unique_address]] CEmptyStruct d;
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};
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struct StructWithAnonUnion {
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union {
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int a;
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double b;
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CStruct c;
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[[no_unique_address]] CEmptyStruct d;
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[[no_unique_address]] CEmptyStruct2 e;
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};
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};
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struct StructWithAnonUnion2 {
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union {
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CStruct c;
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int a;
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CEmptyStruct2 e;
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double b;
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[[no_unique_address]] CEmptyStruct d;
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};
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};
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struct StructWithAnonUnion3 {
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union {
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CStruct c;
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int a;
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CEmptyStruct2 e;
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double b;
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[[no_unique_address]] CEmptyStruct d;
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} u;
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};
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static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) != bool(__has_cpp_attribute(no_unique_address)), "");
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static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) != bool(__has_cpp_attribute(no_unique_address)), "");
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static_assert(!__is_layout_compatible(UnionLayout, UnionLayout2), "");
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static_assert(!__is_layout_compatible(UnionLayout, UnionLayout3), "");
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static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion2), "");
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static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion3), "");
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#endif
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} // namespace dr2759
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namespace dr2789 { // dr2789: 18
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#if __cplusplus >= 202302L
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template <typename T = int>

‎clang/test/SemaCXX/cxx2a-ms-no-unique-address.cpp

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Original file line numberDiff line numberDiff line change
@@ -17,3 +17,28 @@ struct [[msvc::no_unique_address]] S { // expected-error {{only applies to non-b
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int [[msvc::no_unique_address]] c; // expected-error {{cannot be applied to types}} unsupported-error {{cannot be applied to types}}
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};
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struct CStructNoUniqueAddress {
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int one;
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[[no_unique_address]] int two;
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// expected-warning@-1 {{unknown attribute 'no_unique_address' ignored}}
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};
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struct CStructMSVCNoUniqueAddress {
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int one;
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[[msvc::no_unique_address]] int two;
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// unsupported-warning@-1 {{unknown attribute 'no_unique_address' ignored}}
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};
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struct CStructMSVCNoUniqueAddress2 {
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int one;
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[[msvc::no_unique_address]] int two;
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// unsupported-warning@-1 {{unknown attribute 'no_unique_address' ignored}}
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};
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static_assert(__has_cpp_attribute(no_unique_address) == 0);
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// unsupported-error@-1 {{static assertion failed due to requirement '201803L == 0'}}
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static_assert(!__is_layout_compatible(CStructNoUniqueAddress, CStructMSVCNoUniqueAddress), "");
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static_assert(__is_layout_compatible(CStructMSVCNoUniqueAddress, CStructMSVCNoUniqueAddress), "");
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static_assert(!__is_layout_compatible(CStructMSVCNoUniqueAddress, CStructMSVCNoUniqueAddress2), "");
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// unsupported-error@-1 {{static assertion failed due to requirement '!__is_layout_compatible(CStructMSVCNoUniqueAddress, CStructMSVCNoUniqueAddress2)':}}

‎clang/test/SemaCXX/type-traits.cpp

Lines changed: 5 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1768,8 +1768,8 @@ void is_layout_compatible(int n)
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static_assert(!__is_layout_compatible(CppStructNonStandardBySameBase, CppStructNonStandardBySameBase2), "");
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static_assert(!__is_layout_compatible(CppStructNonStandardBy2ndVirtBase, CppStructNonStandardBy2ndVirtBase2), "");
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static_assert(__is_layout_compatible(CStruct, CStructWithQualifiers), "");
1771-
static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
1772-
static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
1771+
static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) != bool(__has_cpp_attribute(no_unique_address)), "");
1772+
static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) != bool(__has_cpp_attribute(no_unique_address)), "");
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static_assert(__is_layout_compatible(CStruct, CStructAlignment), "");
17741774
static_assert(__is_layout_compatible(CStruct, CStructAlignedMembers), ""); // FIXME: alignment of members impact common initial sequence
17751775
static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds), "");
@@ -1782,10 +1782,10 @@ void is_layout_compatible(int n)
17821782
static_assert(!__is_layout_compatible(void(CStruct2::*)(int), void(CStruct2::*)(char)), "");
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static_assert(__is_layout_compatible(CStructNested, CStructNested2), "");
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static_assert(__is_layout_compatible(UnionLayout, UnionLayout), "");
1785-
static_assert(__is_layout_compatible(UnionLayout, UnionLayout2), "");
1785+
static_assert(!__is_layout_compatible(UnionLayout, UnionLayout2), "");
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static_assert(!__is_layout_compatible(UnionLayout, UnionLayout3), "");
1787-
static_assert(__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion2), "");
1788-
static_assert(__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion3), "");
1787+
static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion2), "");
1788+
static_assert(!__is_layout_compatible(StructWithAnonUnion, StructWithAnonUnion3), "");
17891789
static_assert(__is_layout_compatible(EnumLayout, EnumClassLayout), "");
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static_assert(__is_layout_compatible(EnumForward, EnumForward), "");
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static_assert(__is_layout_compatible(EnumForward, EnumClassForward), "");

‎clang/www/cxx_dr_status.html

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@@ -16362,7 +16362,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
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<td><a href="https://cplusplus.github.io/CWG/issues/2759.html">2759</a></td>
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<td>DR</td>
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<td>[[no_unique_address] and common initial sequence</td>
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<td class="unknown" align="center">Unknown</td>
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<td class="unreleased" align="center">Clang 19</td>
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</tr>
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<tr id="2760">
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<td><a href="https://cplusplus.github.io/CWG/issues/2760.html">2760</a></td>

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