From 9bc2dd1a7015b58c9c68e8174e5d2756d955c0ca Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2@gmail.com>
Date: Fri, 31 Jul 2026 13:21:58 -0400
Subject: [PATCH] [libc++] Add an assertion for monotonic_buffer_resource's
 initial_size (#213136)

[mem.res.monotonic.buffer.ctor] requires initial_size to be greater than
zero for the two constructors that take one, but we silently accepted
zero. Add a hardening assertion for that precondition.

Fixes #213059
---
 .../monotonic_buffer_resource.h               | 11 +++++-
 .../assert.initial_size.pass.cpp              | 38 +++++++++++++++++++
 2 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 libcxx/test/libcxx/utilities/utility/mem.res/mem.res.monotonic.buffer/mem.res.monotonic.buffer.ctor/assert.initial_size.pass.cpp

diff --git a/libcxx/include/__memory_resource/monotonic_buffer_resource.h b/libcxx/include/__memory_resource/monotonic_buffer_resource.h
index a6d292386169..2a0361165ddc 100644
--- a/libcxx/include/__memory_resource/monotonic_buffer_resource.h
+++ b/libcxx/include/__memory_resource/monotonic_buffer_resource.h
@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___MEMORY_RESOURCE_MONOTONIC_BUFFER_RESOURCE_H
 #define _LIBCPP___MEMORY_RESOURCE_MONOTONIC_BUFFER_RESOURCE_H
 
+#include <__assert>
 #include <__config>
 #include <__cstddef/size_t.h>
 #include <__memory/addressof.h>
@@ -54,7 +55,10 @@ public:
       : monotonic_buffer_resource(nullptr, __default_buffer_capacity, get_default_resource()) {}
 
   _LIBCPP_HIDE_FROM_ABI explicit monotonic_buffer_resource(size_t __initial_size)
-      : monotonic_buffer_resource(nullptr, __initial_size, get_default_resource()) {}
+      : monotonic_buffer_resource(nullptr, __initial_size, get_default_resource()) {
+    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
+        __initial_size > 0, "monotonic_buffer_resource: initial_size must be greater than zero");
+  }
 
   _LIBCPP_HIDE_FROM_ABI monotonic_buffer_resource(void* __buffer, size_t __buffer_size)
       : monotonic_buffer_resource(__buffer, __buffer_size, get_default_resource()) {}
@@ -63,7 +67,10 @@ public:
       : monotonic_buffer_resource(nullptr, __default_buffer_capacity, __upstream) {}
 
   _LIBCPP_HIDE_FROM_ABI monotonic_buffer_resource(size_t __initial_size, memory_resource* __upstream)
-      : monotonic_buffer_resource(nullptr, __initial_size, __upstream) {}
+      : monotonic_buffer_resource(nullptr, __initial_size, __upstream) {
+    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
+        __initial_size > 0, "monotonic_buffer_resource: initial_size must be greater than zero");
+  }
 
   _LIBCPP_HIDE_FROM_ABI monotonic_buffer_resource(void* __buffer, size_t __buffer_size, memory_resource* __upstream)
       : __res_(__upstream) {
diff --git a/libcxx/test/libcxx/utilities/utility/mem.res/mem.res.monotonic.buffer/mem.res.monotonic.buffer.ctor/assert.initial_size.pass.cpp b/libcxx/test/libcxx/utilities/utility/mem.res/mem.res.monotonic.buffer/mem.res.monotonic.buffer.ctor/assert.initial_size.pass.cpp
new file mode 100644
index 000000000000..61c331d9158c
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/utility/mem.res/mem.res.monotonic.buffer/mem.res.monotonic.buffer.ctor/assert.initial_size.pass.cpp
@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <memory_resource>
+
+// class monotonic_buffer_resource
+
+// explicit monotonic_buffer_resource(size_t initial_size);
+// monotonic_buffer_resource(size_t initial_size, memory_resource* upstream);
+
+// [mem.res.monotonic.buffer.ctor] requires initial_size to be greater than zero.
+// Make sure that passing zero triggers an assertion.
+
+// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: availability-pmr-missing
+// REQUIRES: has-unix-headers
+// REQUIRES: libcpp-hardening-mode={{extensive|debug}}
+// UNSUPPORTED: libcpp-assertion-semantic={{ignore|observe}}
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+#include <cstddef>
+#include <memory_resource>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  TEST_LIBCPP_ASSERT_FAILURE(std::pmr::monotonic_buffer_resource(std::size_t(0)),
+                             "monotonic_buffer_resource: initial_size must be greater than zero");
+  TEST_LIBCPP_ASSERT_FAILURE(std::pmr::monotonic_buffer_resource(std::size_t(0), std::pmr::new_delete_resource()),
+                             "monotonic_buffer_resource: initial_size must be greater than zero");
+
+  return 0;
+}