[clang] Enable nullptr handle with negative elemsize in (PR #155737)

[Samarth Narang via cfe-commits]

https://github.com/snarang181 created https://github.com/llvm/llvm-project/pull/155737

Fixes https://github.com/llvm/llvm-project/issues/152904 

>From 0b13b0e77e184666d46450b264e1237e6c41a1de Mon Sep 17 00:00:00 2001
From: Samarth Narang <snarang at umass.edu>
Date: Wed, 27 Aug 2025 22:45:25 -0400
Subject: [PATCH] Enable nullptr handle with negative elemsize in a dynamic
 allocation

---
 clang/lib/AST/ByteCode/Interp.h     |  7 ++++++-
 clang/test/SemaCXX/new-neg-size.cpp | 15 +++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/new-neg-size.cpp

diff --git a/clang/lib/AST/ByteCode/Interp.h b/clang/lib/AST/ByteCode/Interp.h
index 92e60b6b88e6a..e505712b60dd3 100644
--- a/clang/lib/AST/ByteCode/Interp.h
+++ b/clang/lib/AST/ByteCode/Interp.h
@@ -3490,7 +3490,12 @@ inline bool AllocN(InterpState &S, CodePtr OpPC, PrimType T, const Expr *Source,
     S.Stk.push<Pointer>(0, nullptr);
     return true;
   }
-  assert(NumElements.isPositive());
+  if (!NumElements.isPositive()) {
+    if (!IsNoThrow)
+      return false;
+    S.Stk.push<Pointer>(0, nullptr);
+    return true;
+  }
 
   if (!CheckArraySize(S, OpPC, static_cast<uint64_t>(NumElements)))
     return false;
diff --git a/clang/test/SemaCXX/new-neg-size.cpp b/clang/test/SemaCXX/new-neg-size.cpp
new file mode 100644
index 0000000000000..4b5a0d4bfe228
--- /dev/null
+++ b/clang/test/SemaCXX/new-neg-size.cpp
@@ -0,0 +1,15 @@
+// RUN: not %clang_cc1 -std=c++20 -fsyntax-only %s 2>&1 \
+// RUN:   | FileCheck %s --implicit-check-not='Assertion `NumElements.isPositive()` failed'
+
+// In C++20, constexpr dynamic allocation is permitted *only* if valid.
+// A negative element count must be diagnosed (and must not crash).
+
+constexpr void f_bad_neg() {
+  int a = -1;
+  (void) new int[a]; // triggers negative-size path in the interpreter
+}
+
+// Force evaluation so we definitely run the constexpr interpreter.
+constexpr bool force_eval = (f_bad_neg(), true);
+
+// CHECK: error: constexpr function never produces a constant expression