[libc++][string] Assert resize_and_overwrite operation returns integer-like type

[chaitanyav]

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

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: NagaChaitanya Vellanki (chaitanyav)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/162030.diff

3 Files Affected:

• (modified) libcxx/include/string (+2)
• (modified) libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp (+16)
• (added) libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.verify.cpp (+39)

diff --git a/libcxx/include/string b/libcxx/include/string
index dc562e0207630..00592e61547af 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(__op(data(), auto(__n)));
+    static_assert(std::__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..aa0b32bf3f589 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
@@ -90,6 +90,18 @@ void test_value_categories() {
   LIBCPP_ASSERT(is_string_asan_correct(s));
 }
 
+void test_integer_like_return_types() {
+  std::string s;
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> int { return 5; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> unsigned int { return 5u; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> long { return 5l; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> unsigned long { return 5ul; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> long long { return 5ll; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> unsigned long long { return 5ull; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> std::size_t { return 5; });
+  s.resize_and_overwrite(10, [](char*, std::size_t) -> std::ptrdiff_t { return 5; });
+}
+
 int main(int, char**) {
   test<std::basic_string<char, std::char_traits<char>, std::allocator<char>>>();
   test<std::basic_string<char8_t, std::char_traits<char8_t>, std::allocator<char8_t>>>();
@@ -105,5 +117,9 @@ 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();
+
   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..00d04e56a3aea
--- /dev/null
+++ b/libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.verify.cpp
@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// 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}}
+  });
+}
+
+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}}
+  });
+}

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[frederick-vs-ja]

LGTM. CI failures are unrelated.

[philnik777]

Suggested change

	static_assert(std::__integer_like<__result_type>, "Operation return type must be integer-like");
	static_assert(__integer_like<__result_type>, "Operation return type must be integer-like");

[philnik777]

These calls have UB, since you haven't initialized the elements you claim to have initialized. Also, can we return either the exact type or have explicit return types? We can also use types::for_each with integer_types here instead.

[philnik777]

These tests are entirely redundant.

[philnik777]

These should also be run during constant evaluation.