From 52fbb738cba35a709ac10871b69575647675ae19 Mon Sep 17 00:00:00 2001
From: YexuanXiao <bizwen@nykz.org>
Date: Wed, 8 Apr 2026 09:19:49 +0800
Subject: [PATCH] Implement LWG-3662 string::append/assign(NTBS, pos, n) should
 not construct a temporary string

---
 stl/inc/xstring                               | 20 +++++++++
 .../tests/VSO_0174871_string_replace/test.cpp | 45 +++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/stl/inc/xstring b/stl/inc/xstring
index 8ba06e0de21..eebd69c0543 100644
--- a/stl/inc/xstring
+++ b/stl/inc/xstring
@@ -1556,6 +1556,16 @@ public:
     }
 #endif // _HAS_CXX17
 
+    _CONSTEXPR20 basic_string& append(
+        _In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off, _CRT_GUARDOVERFLOW const size_type _Count) {
+        // append(string_view(_Ptr).substr(_Off, _Count))
+        const size_type _Length = _Convert_size<size_type>(_Traits::length(_Ptr));
+        if (_Off > _Length) {
+            _Scary_val::_Xran();
+        }
+        return _Append(_Ptr + _Off, (_STD min) (_Length - _Off, _Count));
+    }
+
     _CONSTEXPR20 basic_string& append(
         _In_reads_(_Count) const _Elem* const _Ptr, _CRT_GUARDOVERFLOW const size_type _Count) {
         // append [_Ptr, _Ptr + _Count)
@@ -1645,6 +1655,16 @@ public:
     }
 #endif // _HAS_CXX17
 
+    _CONSTEXPR20 basic_string& assign(
+        _In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off, _CRT_GUARDOVERFLOW const size_type _Count) {
+        // assign(string_view(_Ptr).substr(_Off, _Count))
+        const size_type _Length = _Convert_size<size_type>(_Traits::length(_Ptr));
+        if (_Off > _Length) {
+            _Scary_val::_Xran();
+        }
+        return _Assign(_Ptr + _Off, (_STD min) (_Length - _Off, _Count));
+    }
+
     _CONSTEXPR20 basic_string& assign(
         _In_reads_(_Count) const _Elem* const _Ptr, _CRT_GUARDOVERFLOW const size_type _Count) {
         // assign [_Ptr, _Ptr + _Count)
diff --git a/tests/std/tests/VSO_0174871_string_replace/test.cpp b/tests/std/tests/VSO_0174871_string_replace/test.cpp
index e06149b2ef1..ef16bbfbdd4 100644
--- a/tests/std/tests/VSO_0174871_string_replace/test.cpp
+++ b/tests/std/tests/VSO_0174871_string_replace/test.cpp
@@ -103,6 +103,50 @@ void test_shrink_to_fit() {
     assert(example == longerStr);
 }
 
+template <typename T = char>
+struct CharAllocator {
+    using value_type = T;
+
+    CharAllocator() = delete;
+    explicit CharAllocator(int) noexcept {}
+    template <typename U>
+    CharAllocator(const CharAllocator<U>&) noexcept {}
+
+    T* allocate(std::size_t n) {
+        return new T[n];
+    }
+    void deallocate(T* p, std::size_t) noexcept {
+        delete[] p;
+    }
+};
+
+void test_LWG3662() {
+    // append/assign(NTBS, pos, n) should not construct a temporary string
+    basic_string<char, char_traits<char>, CharAllocator<char>> s(CharAllocator<char>(0));
+
+    s.append("hello", 1, 3);
+    assert(s == "ell");
+    s.assign("world", 1, 3);
+    assert(s == "orl");
+
+    s.clear();
+
+    try {
+        s.append("hello", 10, 1);
+        puts("append with out-of-range position should throw");
+        abort();
+    } catch (const out_of_range&) {
+        // purposely do nothing on out_of_range
+    }
+    try {
+        s.assign("world", 10, 1);
+        puts("assign with out-of-range position should throw");
+        abort();
+    } catch (const out_of_range&) {
+        // purposely do nothing on out_of_range
+    }
+}
+
 int main() {
     // Plain replacements with shrinking / same size / growing
     test_replace(3, 3, "ab", "012ab6789");
@@ -142,4 +186,5 @@ int main() {
 
     test_index_boundary_cases();
     test_shrink_to_fit();
+    test_LWG3662();
 }