Dev11_0000000_null_forward_iterators: Avoid Clang warnings, guard headers for C++17/20 (#5654)

Co-authored-by: Stephan T. Lavavej <[email protected]>

Author: YexuanXiao

tests/std/tests/Dev11_0000000_null_forward_iterators/test.cpp

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#define _SILENCE_CXX23_ALIGNED_UNION_DEPRECATION_WARNING
 #define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING
 #define _SILENCE_STDEXT_ARR_ITERS_DEPRECATION_WARNING
 
@@ -10,21 +9,28 @@
 #include <cstring>
 #include <deque>
 #include <experimental/filesystem>
-#include <filesystem>
 #include <forward_list>
 #include <iterator>
 #include <list>
 #include <map>
 #include <new>
 #include <regex>
 #include <set>
-#include <span>
 #include <string>
 #include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
 
+#if _HAS_CXX17
+#include <filesystem>
+#include <string_view>
+#endif // _HAS_CXX17
+
+#if _HAS_CXX20
+#include <span>
+#endif // _HAS_CXX20
+
 using namespace std;
 
 // N3797 24.2.5 [forward.iterators]/2:
@@ -91,14 +97,14 @@ void test_iterator() {
     }
 
     {
-        aligned_union_t<0, FwdIt> au3;
-        aligned_union_t<0, FwdIt> au4;
+        alignas(FwdIt) unsigned char buf3[sizeof(FwdIt)];
+        alignas(FwdIt) unsigned char buf4[sizeof(FwdIt)];
 
-        FwdIt* p3 = reinterpret_cast<FwdIt*>(&au3);
-        FwdIt* p4 = reinterpret_cast<FwdIt*>(&au4);
+        memset(buf3, 0xCC, sizeof(FwdIt));
+        memset(buf4, 0xDD, sizeof(FwdIt));
 
-        memset(p3, 0xCC, sizeof(FwdIt));
-        memset(p4, 0xDD, sizeof(FwdIt));
+        FwdIt* p3 = reinterpret_cast<FwdIt*>(buf3);
+        FwdIt* p4 = reinterpret_cast<FwdIt*>(buf4);
 
         new (p3) FwdIt{};
         new (p4) FwdIt{};