[Clang] Implement C++26 P2748R5 "Disallow Binding a Returned Glvalue to a Temporary"

clang/docs/ReleaseNotes.rst

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

clang/include/clang/Basic/DiagnosticSemaKinds.td

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

clang/lib/Sema/SemaInit.cpp

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

clang/test/CXX/drs/cwg650.cpp

@@ -4,7 +4,7 @@
 // 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
 // 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
 // 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
-// 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
+// We aren't testing this since C++26 because of P2748R5 "Disallow Binding a Returned Glvalue to a Temporary".
 
 #if __cplusplus == 199711L
 #define NOTHROW throw()

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

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

clang/test/SemaCXX/type-traits.cpp

@@ -2509,6 +2509,20 @@ void is_convertible()
   static_assert(__is_convertible(FloatWrapper, IntWrapper));
   static_assert(__is_convertible(FloatWrapper, float));
   static_assert(__is_convertible(float, FloatWrapper));
+  static_assert(__is_convertible(IntWrapper, IntWrapper&&));
+  static_assert(__is_convertible(IntWrapper, const IntWrapper&));
+  static_assert(__is_convertible(IntWrapper, int&&));
+  static_assert(__is_convertible(IntWrapper, const int&));
+  static_assert(__is_convertible(int, IntWrapper&&));
+  static_assert(__is_convertible(int, const IntWrapper&));
+  static_assert(__is_convertible(IntWrapper, FloatWrapper&&));
+  static_assert(__is_convertible(IntWrapper, const FloatWrapper&));
+  static_assert(__is_convertible(FloatWrapper, IntWrapper&&));
+  static_assert(__is_convertible(FloatWrapper, const IntWrapper&&));
+  static_assert(__is_convertible(FloatWrapper, float&&));
+  static_assert(__is_convertible(FloatWrapper, const float&));
+  static_assert(__is_convertible(float, FloatWrapper&&));
+  static_assert(__is_convertible(float, const FloatWrapper&));
 }
 
 void is_nothrow_convertible()
@@ -2521,6 +2535,20 @@ void is_nothrow_convertible()
   static_assert(!__is_nothrow_convertible(FloatWrapper, IntWrapper));
   static_assert(!__is_nothrow_convertible(FloatWrapper, float));
   static_assert(__is_nothrow_convertible(float, FloatWrapper));
+  static_assert(__is_nothrow_convertible(IntWrapper, IntWrapper&&));
+  static_assert(__is_nothrow_convertible(IntWrapper, const IntWrapper&));
+  static_assert(__is_nothrow_convertible(IntWrapper, int&&));
+  static_assert(__is_nothrow_convertible(IntWrapper, const int&));
+  static_assert(!__is_nothrow_convertible(int, IntWrapper&&));
+  static_assert(!__is_nothrow_convertible(int, const IntWrapper&));
+  static_assert(!__is_nothrow_convertible(IntWrapper, FloatWrapper&&));
+  static_assert(!__is_nothrow_convertible(IntWrapper, const FloatWrapper&));
+  static_assert(!__is_nothrow_convertible(FloatWrapper, IntWrapper&&));
+  static_assert(!__is_nothrow_convertible(FloatWrapper, const IntWrapper&));
+  static_assert(!__is_nothrow_convertible(FloatWrapper, float&&));
+  static_assert(!__is_nothrow_convertible(FloatWrapper, const float&));
+  static_assert(__is_nothrow_convertible(float, FloatWrapper&&));
+  static_assert(__is_nothrow_convertible(float, const FloatWrapper&));
 }
 
 struct FromInt { FromInt(int); };

clang/www/cxx_status.html

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