[clang] [Clang][ExprConst] Accept prvalue for nothrow new (PR #226753)

[via cfe-commits]

https://github.com/aokblast created https://github.com/llvm/llvm-project/pull/226753

A user defined operator new can accept prvalue for nothrow. There is no guarantee in spec that it should be lvalue. Relax the check with side effect only.

>From ff5453f705924f0409df46cc523fcaceedcf459b Mon Sep 17 00:00:00 2001
From: ShengYi Hung <aokblast at FreeBSD.org>
Date: Sat, 26 Sep 2026 23:52:05 -0500
Subject: [PATCH] [Clang][ExprConst] Accept prvalue for nothrow new

A user defined operator new can accept prvalue for nothrow. There is
no guarantee in spec that it should be lvalue. Relax the check with
side effect only.
---
 clang/lib/AST/ExprConstant.cpp              |  7 +++++--
 clang/test/SemaCXX/new-nothrow-by-value.cpp | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/SemaCXX/new-nothrow-by-value.cpp

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 2df754dc9007f..578ce94e93e3e 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11030,8 +11030,11 @@ bool PointerExprEvaluator::VisitCXXNewExpr(const CXXNewExpr *E) {
     // (which should presumably be valid only if N is a multiple of
     // alignof(int), and in any case can't be deallocated unless N is
     // alignof(X) and X has new-extended alignment).
-    LValue Nothrow;
-    if (!EvaluateLValue(E->getPlacementArg(0), Nothrow, Info))
+    //
+    // The argument is usually an lvalue (the standard signature takes a
+    // 'const std::nothrow_t &'), but a user-declared allocation function may
+    // take 'std::nothrow_t' by value, in which case it is a prvalue.
+    if (!EvaluateIgnoredValue(Info, E->getPlacementArg(0)))
       return false;
     IsNothrow = true;
   } else if (OperatorNew->isReservedGlobalPlacementOperator()) {
diff --git a/clang/test/SemaCXX/new-nothrow-by-value.cpp b/clang/test/SemaCXX/new-nothrow-by-value.cpp
new file mode 100644
index 0000000000000..acb70803b020c
--- /dev/null
+++ b/clang/test/SemaCXX/new-nothrow-by-value.cpp
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+// The constant evaluator used to assume the (std::nothrow) placement argument
+// is always an lvalue and crashed when a user-declared allocation function
+// takes std::nothrow_t by value, making the argument a prvalue.
+
+namespace std {
+inline constexpr struct nothrow_t {
+} nothrow;
+} // namespace std
+
+void *operator new[](unsigned long, std::nothrow_t) noexcept;
+void *operator new(unsigned long, std::nothrow_t) noexcept;
+
+void set(int *p) {
+  p = (1 ? new (std::nothrow) int[1] : nullptr);
+  p = (1 ? new (std::nothrow) int : nullptr);
+}