Test ASAN SSO string copies; improve `basic_string::_Construct` preconditions (#3039)

Co-authored-by: Stephan T. Lavavej <stl@microsoft.com>

stl/inc/xstring

@@ -2696,7 +2696,9 @@ private:
     enum class _Construct_strategy : uint8_t { _From_char, _From_ptr, _From_string };
     template <_Construct_strategy _Strat, class _Char_or_ptr>
     _CONSTEXPR20 void _Construct(const _Char_or_ptr _Arg, _CRT_GUARDOVERFLOW const size_type _Count) {
-        // Pre: *this is in SSO mode; the lifetime of the SSO elements has already begun
+        auto& _My_data = _Mypair._Myval2;
+        _STL_INTERNAL_CHECK(!_My_data._Large_string_engaged());
+        _STL_INTERNAL_CHECK(_STD count(_My_data._Bx._Buf, _My_data._Bx._Buf + _BUF_SIZE, _Elem()) == _BUF_SIZE);
 
         if constexpr (_Strat == _Construct_strategy::_From_char) {
             _STL_INTERNAL_STATIC_ASSERT(is_same_v<_Char_or_ptr, _Elem>);
@@ -2708,7 +2710,6 @@ private:
             _Xlen_string(); // result too long
         }
 
-        auto& _My_data  = _Mypair._Myval2;
         auto& _Al       = _Getal();
         auto&& _Alproxy = _GET_PROXY_ALLOCATOR(_Alty, _Al);
         _Container_proxy_ptr<_Alty> _Proxy(_Alproxy, _My_data);
@@ -2718,10 +2719,8 @@ private:
             _My_data._Myres  = _BUF_SIZE - 1;
             if constexpr (_Strat == _Construct_strategy::_From_char) {
                 _Traits::assign(_My_data._Bx._Buf, _Count, _Arg);
-                _Traits::assign(_My_data._Bx._Buf[_Count], _Elem());
             } else if constexpr (_Strat == _Construct_strategy::_From_ptr) {
                 _Traits::move(_My_data._Bx._Buf, _Arg, _Count);
-                _Traits::assign(_My_data._Bx._Buf[_Count], _Elem());
             } else { // _Strat == _Construct_strategy::_From_string
 #ifdef _INSERT_STRING_ANNOTATION
                 _Traits::move(_My_data._Bx._Buf, _Arg, _Count);

tests/std/tests/GH_002030_asan_annotate_string/test.cpp

@@ -19,6 +19,7 @@
 #include <cstring>
 #include <iterator>
 #include <memory>
+#include <new>
 #include <sstream>
 #include <string>
 #if _HAS_CXX17
@@ -1822,6 +1823,28 @@ void run_allocator_matrix() {
     run_custom_allocator_matrix<CharType, implicit_allocator>();
 }
 
+void test_DevCom_10116361() {
+    // We failed to null-terminate copies of SSO strings with ASAN annotations active.
+#ifdef _WIN64
+    constexpr const char* text = "testtest";
+    constexpr size_t n         = 8;
+#else
+    constexpr const char* text = "test";
+    constexpr size_t n         = 4;
+#endif
+
+    string s0{text};
+    assert(s0.c_str()[n] == '\0');
+
+    alignas(string) unsigned char space[sizeof(string)];
+    memset(space, 0xff, sizeof(space));
+
+    string& s1 = *::new (&space) string{s0};
+    assert(s1.c_str()[n] == '\0');
+
+    s1.~string();
+}
+
 int main() {
     run_allocator_matrix<char>();
 #ifdef __cpp_char8_t
@@ -1830,6 +1853,8 @@ int main() {
     run_allocator_matrix<char16_t>();
     run_allocator_matrix<char32_t>();
     run_allocator_matrix<wchar_t>();
+
+    test_DevCom_10116361();
 }
 #endif // TRANSITION, VSO-1586016