From 8573d060e79e34e4826546dedf3a82f90ccdd558 Mon Sep 17 00:00:00 2001
From: NagaChaitanya Vellanki <pnagato@protonmail.com>
Date: Sun, 5 Oct 2025 15:33:46 -0700
Subject: [PATCH] [libc++][string] Assert resize_and_overwrite operation
 returns integer-like type

Verify that the operation passed to resize_and_overwrite returns an
integer-like type, matching the behavior of other standard library
implementations like GCC's libstdc++

Fixes #160577
---
 libcxx/include/string                         |  2 +
 .../resize_and_overwrite.pass.cpp             | 25 +++++++++++-
 .../resize_and_overwrite.verify.cpp           | 40 +++++++++++++++++++
 3 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.verify.cpp

diff --git a/libcxx/include/string b/libcxx/include/string
index 363f27a51648c..5f1662b87f1bc 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1311,6 +1311,8 @@ public:
 #  if _LIBCPP_STD_VER >= 23
   template <class _Op>
   _LIBCPP_HIDE_FROM_ABI constexpr void resize_and_overwrite(size_type __n, _Op __op) {
+    using __result_type = decltype(std::move(__op)(data(), auto(__n)));
+    static_assert(__integer_like<__result_type>, "Operation return type must be integer-like");
     size_type __sz  = size();
     size_type __cap = capacity();
     if (__n > __cap)
diff --git a/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp b/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp
index abd284852a189..aca5644ef7094 100644
--- a/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp
@@ -17,6 +17,7 @@
 #include <cassert>
 #include <memory>
 #include <string>
+#include <type_algorithms.h>
 
 #include "make_string.h"
 #include "test_macros.h"
@@ -77,17 +78,30 @@ constexpr bool test() {
   return true;
 }
 
-void test_value_categories() {
+constexpr bool test_value_categories() {
   std::string s;
   s.resize_and_overwrite(10, [](char*&&, std::size_t&&) { return 0; });
   LIBCPP_ASSERT(is_string_asan_correct(s));
   s.resize_and_overwrite(10, [](char* const&, const std::size_t&) { return 0; });
   LIBCPP_ASSERT(is_string_asan_correct(s));
   struct RefQualified {
-    int operator()(char*, std::size_t) && { return 0; }
+    constexpr int operator()(char*, std::size_t) && { return 0; }
   };
   s.resize_and_overwrite(10, RefQualified{});
   LIBCPP_ASSERT(is_string_asan_correct(s));
+  return true;
+}
+
+constexpr bool test_integer_like_return_types() {
+  types::for_each(types::integer_types(), []<typename IntegerType> {
+    std::string s;
+    s.resize_and_overwrite(10, [](char* p, std::size_t n) -> IntegerType {
+      std::fill(p, p + n, 'f');
+      return n;
+    });
+    assert(s.size() == 10);
+  });
+  return true;
 }
 
 int main(int, char**) {
@@ -105,5 +119,12 @@ int main(int, char**) {
   test<std::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>>();
   static_assert(test<std::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>>());
 #endif
+
+  test_value_categories();
+  test_integer_like_return_types();
+
+  static_assert(test_value_categories());
+  static_assert(test_integer_like_return_types());
+
   return 0;
 }
diff --git a/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.verify.cpp b/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.verify.cpp
new file mode 100644
index 0000000000000..1345fde296a20
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.verify.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <string>
+
+// template<class Operation>
+// void resize_and_overwrite(size_type n, Operation op)
+
+// Verify that the operation's return type must be integer-like
+
+#include <string>
+
+void test_bool_return_type() {
+  std::string s;
+  s.resize_and_overwrite(10, [](char*, std::size_t) {
+    return true; // expected-error-re@*:* {{{{(static_assertion|static assertion)}}{{.*}}integer-like}}
+  });
+}
+
+void test_pointer_return_type() {
+  std::string s;
+  s.resize_and_overwrite(10, [](char* p, std::size_t) {
+    return p; // expected-error-re@*:* {{{{(static_assertion|static assertion)}}{{.*}}integer-like}}
+              // expected-error@*:* {{cannot initialize}}
+  });
+}
+
+void test_float_return_type() {
+  std::string s;
+  s.resize_and_overwrite(10, [](char*, std::size_t) {
+    return 5.0f; // expected-error-re@*:* {{{{(static_assertion|static assertion)}}{{.*}}integer-like}}
+  });
+}