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[clang] Implement CWG1878 "operator auto template" (#78103)
C++14 introduced deduced return type for regular functions, but shortly after [CWG1878](https://wg21.link/cwg1878) was filed and resolved to disallow deduced return types in conversion function templates. So this patch diagnoses such usage of deduced return type in C++14 mode onwards. Fixes #51776
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6 files changed

+102
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‎clang/docs/ReleaseNotes.rst

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@@ -575,6 +575,8 @@ Improvements to Clang's diagnostics
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- Clang now diagnoses unexpanded packs within the template argument lists of function template specializations.
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- Clang now diagnoses attempts to bind a bitfield to an NTTP of a reference type as erroneous
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converted constant expression and not as a reference to subobject.
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- Clang now diagnoses ``auto`` and ``decltype(auto)`` in declarations of conversion function template
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(`CWG1878: <https://cplusplus.github.io/CWG/issues/1878.html>`_)
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- Clang now diagnoses the requirement that non-template friend declarations with requires clauses
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and template friend declarations with a constraint that depends on a template parameter from an
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enclosing template must be a definition.

‎clang/include/clang/Basic/DiagnosticSemaKinds.td

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@@ -2411,7 +2411,8 @@ def err_auto_not_allowed : Error<
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"|in type allocated by 'new'|in K&R-style function parameter"
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"|in template parameter|in friend declaration|in function prototype that is "
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"not a function declaration|in requires expression parameter"
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"|in array declaration}1">;
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"|in array declaration"
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"|in declaration of conversion function template}1">;
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def err_dependent_deduced_tst : Error<
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"typename specifier refers to "
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"%select{class template|function template|variable template|alias template|"

‎clang/lib/Sema/SemaDeclCXX.cpp

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@@ -11321,9 +11321,20 @@ Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
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<< ClassType << ConvType;
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}
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if (FunctionTemplateDecl *ConversionTemplate
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= Conversion->getDescribedFunctionTemplate())
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if (FunctionTemplateDecl *ConversionTemplate =
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Conversion->getDescribedFunctionTemplate()) {
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if (const auto *ConvTypePtr = ConvType->getAs<PointerType>()) {
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ConvType = ConvTypePtr->getPointeeType();
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}
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if (ConvType->isUndeducedAutoType()) {
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Diag(Conversion->getTypeSpecStartLoc(), diag::err_auto_not_allowed)
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<< getReturnTypeLoc(Conversion).getSourceRange()
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<< llvm::to_underlying(ConvType->getAs<AutoType>()->getKeyword())
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<< /* in declaration of conversion function template= */ 24;
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}
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return ConversionTemplate;
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}
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return Conversion;
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}

‎clang/test/CXX/drs/dr18xx.cpp

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@@ -1,10 +1,10 @@
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// RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify=expected,cxx98-14,cxx98 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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// RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify=expected,cxx98-14,cxx11-17,since-cxx11 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
3-
// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify=expected,cxx98-14,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
4-
// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
5-
// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
6-
// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
7-
// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,cxx98-14,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
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#if __cplusplus == 199711L
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#define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
@@ -319,6 +319,41 @@ namespace dr1872 { // dr1872: 9
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#endif
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}
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namespace dr1878 { // dr1878: 18
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#if __cplusplus >= 201402L
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#if __cplusplus >= 202002L
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template <typename T>
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concept C = true;
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#endif
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struct S {
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template <typename T>
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operator auto() const { return short(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator const auto() const { return int(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator const auto&() const { return char(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator const auto*() const { return long(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator decltype(auto)() const { return unsigned(); }
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// since-cxx14-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
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#if __cplusplus >= 202002L
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template <typename T>
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operator C auto() const { return float(); }
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// since-cxx20-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator C decltype(auto)() const { return double(); }
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// since-cxx20-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
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#endif
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};
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#endif
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}
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namespace dr1881 { // dr1881: 7
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struct A { int a : 4; };
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struct B : A { int b : 3; };

‎clang/test/SemaCXX/deduced-return-type-cxx14.cpp

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@@ -1,11 +1,11 @@
1-
// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,cxx20_23,cxx23 %s
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// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,cxx20_23,cxx23 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
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// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx20_23,cxx23 %s
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// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx20_23,cxx23 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
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// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,cxx14_20,cxx20_23 %s
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// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,cxx14_20,cxx20_23 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
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// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx14_20,cxx20_23 %s
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// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx14_20,cxx20_23 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
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// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,cxx14_20,cxx14 %s
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// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,cxx14_20,cxx14 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
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// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,since-cxx14,cxx14_20,cxx14 %s
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// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,since-cxx14,cxx14_20,cxx14 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
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auto f(); // expected-note {{previous}}
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int f(); // expected-error {{differ only in their return type}}
@@ -686,3 +686,41 @@ auto f(auto x) { // cxx14-error {{'auto' not allowed in function prototype}}
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}
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}
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#if __cplusplus >= 202002L
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template <typename T>
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concept C = true;
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#endif
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struct DeducedTargetTypeOfConversionFunction {
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operator auto() const { return char(); }
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operator const auto() const { return float(); }
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operator const auto&() const { return int(); }
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// expected-warning@-1 {{returning reference to local temporary object}}
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operator decltype(auto)() const { return double(); }
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#if __cplusplus >= 202002L
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operator C auto() const { return unsigned(); }
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operator C decltype(auto)() const { return long(); }
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#endif
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template <typename T>
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operator auto() const { return short(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator const auto() const { return int(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator const auto&() const { return char(); }
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// since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator decltype(auto)() const { return unsigned(); }
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// since-cxx14-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
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#if __cplusplus >= 202002L
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template <typename T>
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operator C auto() const { return float(); }
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// since-cxx20-error@-1 {{'auto' not allowed in declaration of conversion function template}}
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template <typename T>
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operator C decltype(auto)() const { return double(); }
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// since-cxx20-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
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#endif
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};

‎clang/www/cxx_dr_status.html

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@@ -11076,7 +11076,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
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<td><a href="https://cplusplus.github.io/CWG/issues/1878.html">1878</a></td>
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<td>CD4</td>
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<td><TT>operator auto</TT> template</td>
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<td class="unknown" align="center">Unknown</td>
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<td class="unreleased" align="center">Clang 18</td>
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</tr>
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<tr id="1879">
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<td><a href="https://cplusplus.github.io/CWG/issues/1879.html">1879</a></td>

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