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[Clang] Implement C++26 P2748R5 "Disallow Binding a Returned Glvalue to a Temporary" (llvm#89942)
Implement P2748R5 "Disallow Binding a Returned Glvalue to a Temporary" https://wg21.link/P2748R5 --------- Signed-off-by: yronglin <[email protected]>
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‎clang/docs/ReleaseNotes.rst

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@@ -145,6 +145,7 @@ C++2c Feature Support
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- Implemented `P0609R3: Attributes for Structured Bindings <https://wg21.link/P0609R3>`_
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- Implemented `P2748R5 Disallow Binding a Returned Glvalue to a Temporary <https://wg21.link/P2748R5>`_.
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Resolutions to C++ Defect Reports
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

‎clang/include/clang/Basic/DiagnosticSemaKinds.td

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@@ -9950,6 +9950,8 @@ def warn_ret_stack_addr_ref : Warning<
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def warn_ret_local_temp_addr_ref : Warning<
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"returning %select{address of|reference to}0 local temporary object">,
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InGroup<ReturnStackAddress>;
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def err_ret_local_temp_ref : Error<
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"returning reference to local temporary object">;
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def warn_ret_addr_label : Warning<
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"returning address of label, which is local">,
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InGroup<ReturnStackAddress>;

‎clang/lib/Sema/SemaInit.cpp

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@@ -8340,8 +8340,17 @@ void Sema::checkInitializerLifetime(const InitializedEntity &Entity,
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<< Entity.getType()->isReferenceType() << CLE->getInitializer() << 2
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<< DiagRange;
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} else {
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Diag(DiagLoc, diag::warn_ret_local_temp_addr_ref)
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<< Entity.getType()->isReferenceType() << DiagRange;
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// P2748R5: Disallow Binding a Returned Glvalue to a Temporary.
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// [stmt.return]/p6: In a function whose return type is a reference,
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// other than an invented function for std::is_convertible ([meta.rel]),
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// a return statement that binds the returned reference to a temporary
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// expression ([class.temporary]) is ill-formed.
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if (getLangOpts().CPlusPlus26 && Entity.getType()->isReferenceType())
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Diag(DiagLoc, diag::err_ret_local_temp_ref)
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<< Entity.getType()->isReferenceType() << DiagRange;
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else
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Diag(DiagLoc, diag::warn_ret_local_temp_addr_ref)
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<< Entity.getType()->isReferenceType() << DiagRange;
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}
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break;
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}

‎clang/test/CXX/drs/cwg650.cpp

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@@ -4,7 +4,7 @@
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// RUN: %clang_cc1 -std=c++17 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | llvm-cxxfilt -n | FileCheck %s --check-prefixes CHECK
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// RUN: %clang_cc1 -std=c++20 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | llvm-cxxfilt -n | FileCheck %s --check-prefixes CHECK
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// RUN: %clang_cc1 -std=c++23 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | llvm-cxxfilt -n | FileCheck %s --check-prefixes CHECK
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// RUN: %clang_cc1 -std=c++2c %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | llvm-cxxfilt -n | FileCheck %s --check-prefixes CHECK
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// We aren't testing this since C++26 because of P2748R5 "Disallow Binding a Returned Glvalue to a Temporary".
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#if __cplusplus == 199711L
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#define NOTHROW throw()

‎clang/test/CXX/stmt.stmt/stmt.return/p6.cpp

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@@ -0,0 +1,25 @@
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// RUN: %clang_cc1 -std=c++26 -fsyntax-only -verify %s
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auto&& f1() {
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return 42; // expected-error{{returning reference to local temporary object}}
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}
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const double& f2() {
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static int x = 42;
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return x; // expected-error{{returning reference to local temporary object}}
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}
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auto&& id(auto&& r) {
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return static_cast<decltype(r)&&>(r);
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}
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auto&& f3() {
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return id(42); // OK, but probably a bug
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}
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void unevaluated() {
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using a = decltype ([] () -> const int & {
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const int &i = 0; // expected-note {{binding reference variable 'i' here}}
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return i; // expected-error{{returning reference to local temporary object}}
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} ());
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}
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static_assert(__is_convertible(int, const int &));
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static_assert(__is_nothrow_convertible(int, const int &));

‎clang/test/SemaCXX/type-traits.cpp

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@@ -2509,6 +2509,20 @@ void is_convertible()
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static_assert(__is_convertible(FloatWrapper, IntWrapper));
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static_assert(__is_convertible(FloatWrapper, float));
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static_assert(__is_convertible(float, FloatWrapper));
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static_assert(__is_convertible(IntWrapper, IntWrapper&&));
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static_assert(__is_convertible(IntWrapper, const IntWrapper&));
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static_assert(__is_convertible(IntWrapper, int&&));
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static_assert(__is_convertible(IntWrapper, const int&));
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static_assert(__is_convertible(int, IntWrapper&&));
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static_assert(__is_convertible(int, const IntWrapper&));
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static_assert(__is_convertible(IntWrapper, FloatWrapper&&));
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static_assert(__is_convertible(IntWrapper, const FloatWrapper&));
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static_assert(__is_convertible(FloatWrapper, IntWrapper&&));
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static_assert(__is_convertible(FloatWrapper, const IntWrapper&&));
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static_assert(__is_convertible(FloatWrapper, float&&));
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static_assert(__is_convertible(FloatWrapper, const float&));
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static_assert(__is_convertible(float, FloatWrapper&&));
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static_assert(__is_convertible(float, const FloatWrapper&));
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}
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void is_nothrow_convertible()
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static_assert(!__is_nothrow_convertible(FloatWrapper, IntWrapper));
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static_assert(!__is_nothrow_convertible(FloatWrapper, float));
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static_assert(__is_nothrow_convertible(float, FloatWrapper));
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static_assert(__is_nothrow_convertible(IntWrapper, IntWrapper&&));
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static_assert(__is_nothrow_convertible(IntWrapper, const IntWrapper&));
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static_assert(__is_nothrow_convertible(IntWrapper, int&&));
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static_assert(__is_nothrow_convertible(IntWrapper, const int&));
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static_assert(!__is_nothrow_convertible(int, IntWrapper&&));
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static_assert(!__is_nothrow_convertible(int, const IntWrapper&));
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static_assert(!__is_nothrow_convertible(IntWrapper, FloatWrapper&&));
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static_assert(!__is_nothrow_convertible(IntWrapper, const FloatWrapper&));
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static_assert(!__is_nothrow_convertible(FloatWrapper, IntWrapper&&));
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static_assert(!__is_nothrow_convertible(FloatWrapper, const IntWrapper&));
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static_assert(!__is_nothrow_convertible(FloatWrapper, float&&));
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static_assert(!__is_nothrow_convertible(FloatWrapper, const float&));
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static_assert(__is_nothrow_convertible(float, FloatWrapper&&));
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static_assert(__is_nothrow_convertible(float, const FloatWrapper&));
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}
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struct FromInt { FromInt(int); };

‎clang/www/cxx_status.html

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@@ -167,7 +167,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
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<tr>
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<td>Disallow Binding a Returned Glvalue to a Temporary</td>
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<td><a href="https://wg21.link/P2748R5">P2748R5</a></td>
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<td class="none" align="center">No</td>
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<td class="full" align="center">Clang 19</td>
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</tr>
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<tr>
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<td>Clarifying rules for brace elision in aggregate initialization</td>

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