[clang] [Clang] Implement LWG3819 for `__reference_meows_from_temporary` (PR #142554)

[A. Jiang via cfe-commits]

https://github.com/frederick-vs-ja updated https://github.com/llvm/llvm-project/pull/142554

>From 4a98b75bcabd0f524988fbd63caedd0ecbad5810 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Tue, 3 Jun 2025 17:40:30 +0800
Subject: [PATCH] [Clang] Implement LWG3819 for
 `__reference_meows_from_temporary`

Also fix use cases for function reference binding (
`__reference_binds_to_temporary` is also affected despite being
deprecated).
---
 clang/docs/ReleaseNotes.rst        |  9 +++++++++
 clang/lib/Sema/SemaTypeTraits.cpp  | 15 +++++++++++++-
 clang/test/SemaCXX/type-traits.cpp | 32 ++++++++++++++++++++++++++++++
 3 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a73c71cdd7201..eccee1aeef846 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -703,6 +703,15 @@ Bug Fixes to Compiler Builtins
   are ``__builtin_is_cpp_trivially_relocatable``. It is recommended to use
   ``__builtin_trivially_relocate`` instead.
 
+- ``__reference_binds_to_temporary``, ``__reference_constructs_from_temporary``
+  and ``__reference_converts_from_temporary`` intrinsics no longer consider
+  function references can bind to temporary objects. (#GH114344)
+
+- ``__reference_constructs_from_temporary`` and
+  ``__reference_converts_from_temporary`` intrinsics detect reference binding
+  to prvalue instead of xvalue now if the second operand is an object type, per
+  `LWG3819 <https://cplusplus.github.io/LWG/issue3819>`_.
+
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  - Fixed crash when a parameter to the ``clang::annotate`` attribute evaluates to ``void``. See #GH119125
diff --git a/clang/lib/Sema/SemaTypeTraits.cpp b/clang/lib/Sema/SemaTypeTraits.cpp
index 04f54d7044e4f..b849bbe94af62 100644
--- a/clang/lib/Sema/SemaTypeTraits.cpp
+++ b/clang/lib/Sema/SemaTypeTraits.cpp
@@ -1343,12 +1343,21 @@ static bool EvaluateBooleanTypeTrait(Sema &S, TypeTrait Kind,
     if (RD && RD->isAbstract())
       return false;
 
+    // LWG3819: For reference_meows_from_temporary traits, && is not added to
+    // the source object type.
+    // Otherwise, compute the result of add_rvalue_reference_t.
+    bool UseRawObjectType =
+        Kind == clang::BTT_ReferenceBindsToTemporary ||
+        Kind == clang::BTT_ReferenceConstructsFromTemporary ||
+        Kind == clang::BTT_ReferenceConvertsFromTemporary;
+
     llvm::BumpPtrAllocator OpaqueExprAllocator;
     SmallVector<Expr *, 2> ArgExprs;
     ArgExprs.reserve(Args.size() - 1);
     for (unsigned I = 1, N = Args.size(); I != N; ++I) {
       QualType ArgTy = Args[I]->getType();
-      if (ArgTy->isObjectType() || ArgTy->isFunctionType())
+      if ((ArgTy->isObjectType() && !UseRawObjectType) ||
+          ArgTy->isFunctionType())
         ArgTy = S.Context.getRValueReferenceType(ArgTy);
       ArgExprs.push_back(
           new (OpaqueExprAllocator.Allocate<OpaqueValueExpr>())
@@ -1386,6 +1395,10 @@ static bool EvaluateBooleanTypeTrait(Sema &S, TypeTrait Kind,
       if (!T->isReferenceType())
         return false;
 
+      // A function reference never binds to a temporary object.
+      if (T.getNonReferenceType()->isFunctionType())
+        return false;
+
       if (!Init.isDirectReferenceBinding())
         return true;
 
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 56cca05402f45..3f0124755c674 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -3143,6 +3143,10 @@ void reference_binds_to_temporary_checks() {
   static_assert(!(__reference_binds_to_temporary(int, long)));
 
   static_assert((__reference_binds_to_temporary(const int &, long)));
+
+  // Test that function references are never considered bound to temporaries.
+  static_assert(!__reference_binds_to_temporary(void(&)(), void()));
+  static_assert(!__reference_binds_to_temporary(void(&&)(), void()));
 }
 
 
@@ -3156,6 +3160,14 @@ struct ExplicitConversionRef {
     explicit operator int&();
 };
 
+struct NonMovable {
+  NonMovable(NonMovable&&) = delete;
+};
+
+struct ConvertsFromNonMovable {
+  ConvertsFromNonMovable(NonMovable);
+};
+
 void reference_constructs_from_temporary_checks() {
   static_assert(!__reference_constructs_from_temporary(int &, int &));
   static_assert(!__reference_constructs_from_temporary(int &, int &&));
@@ -3193,6 +3205,16 @@ void reference_constructs_from_temporary_checks() {
 
   static_assert(__reference_constructs_from_temporary(const int &, long));
 
+  // Test that function references are never considered bound to temporaries.
+  static_assert(!__reference_constructs_from_temporary(void(&&)(), void()));
+  static_assert(!__reference_constructs_from_temporary(void(&)(), void()));
+
+  // LWG3819: reference_meows_from_temporary should not use is_meowible
+  static_assert(__reference_constructs_from_temporary(ConvertsFromNonMovable&&, NonMovable) == __cplusplus >= 201703L);
+  // For scalar types, cv-qualifications are dropped first for prvalues.
+  static_assert(__reference_constructs_from_temporary(int&&, const int));
+  static_assert(__reference_constructs_from_temporary(int&&, volatile int));
+
   // Additional checks
   static_assert(__reference_constructs_from_temporary(POD const&, Derives));
   static_assert(__reference_constructs_from_temporary(int&&, int));
@@ -3250,6 +3272,16 @@ void reference_converts_from_temporary_checks() {
 
   static_assert(__reference_converts_from_temporary(const int &, long));
 
+  // Test that function references are never considered bound to temporaries.
+  static_assert(!__reference_converts_from_temporary(void(&)(), void()));
+  static_assert(!__reference_converts_from_temporary(void(&&)(), void()));
+
+  // LWG3819: reference_meows_from_temporary should not use is_meowible
+  static_assert(__reference_converts_from_temporary(ConvertsFromNonMovable&&, NonMovable) == __cplusplus >= 201703L);
+  // For scalar types, cv-qualifications are dropped first for prvalues.
+  static_assert(__reference_converts_from_temporary(int&&, const int));
+  static_assert(__reference_converts_from_temporary(int&&, volatile int));
+
   // Additional checks
   static_assert(__reference_converts_from_temporary(POD const&, Derives));
   static_assert(__reference_converts_from_temporary(int&&, int));