Fix wstring::find_first_of infinite loop on ARM64

stl/src/vector_algorithms.cpp

@@ -6321,7 +6321,7 @@ namespace {
                     } else if (_Count1 < 96) {
                         return _Count2 >= 8;
                     } else {
-                        return _Count2 >= 4;
+                        return _Count2 >= 4; // if these thresholds are adjusted, review test_gh_6342_find_first_of()
                     }
                 } else if constexpr (sizeof(_Ty) == 4) {
                     if (_Count1 < 32) {
@@ -6390,9 +6390,7 @@ namespace {
                                 return _Ix;
                             }
                         }
-
-                        ++_Ix;
-                    } while (_Ix != _Haystack_length);
+                    } while (++_Ix != _Haystack_length);
                 }
 
                 return static_cast<size_t>(-1);

tests/std/tests/VSO_0000000_vector_algorithms/test.cpp

@@ -1873,6 +1873,44 @@ void test_gh_5757_find_first_of() {
     assert(pos == 0);
 }
 
+// GH-6342 <string>: wstring::find_first_of infinite-loops on ARM64
+void test_gh_6342_find_first_of() {
+    const wstring needle{L"abcd"}; // Important size: 4-element needle
+    wstring haystack(150, L'x'); // Important size: 150-element haystack
+
+    // See _Find_meow_of::_Bitmap_impl::_Use_bitmap_neon() in vector_algorithms.cpp.
+    // For 2-byte wchar_t, when the haystack is >= 96 elements and the needle is >= 4 elements, the bitmap is used.
+    // The vectorized code consumes 16-element chunks. 150 % 16 is 6, so the scalar tail is [144, 150).
+    // No matching elements are present in [0, 144), so we'll look at the scalar tail.
+
+    constexpr wchar_t non_ascii_character{L'\u044F'}; // U+044F CYRILLIC SMALL LETTER YA, any value >= 256 works
+
+    {
+        const auto pos1{haystack.find_first_of(needle)};
+        assert(pos1 == wstring::npos);
+    }
+    {
+        haystack[146] = non_ascii_character;
+        const auto pos2{haystack.find_first_of(needle)};
+        assert(pos2 == wstring::npos);
+        haystack[146] = L'x';
+    }
+    {
+        haystack[148] = L'b';
+        const auto pos3{haystack.find_first_of(needle)};
+        assert(pos3 == 148);
+        haystack[148] = L'x';
+    }
+    {
+        haystack[146] = non_ascii_character;
+        haystack[148] = L'b';
+        const auto pos4{haystack.find_first_of(needle)};
+        assert(pos4 == 148);
+        haystack[146] = L'x';
+        haystack[148] = L'x';
+    }
+}
+
 void test_string(mt19937_64& gen) {
     test_basic_string<char>(gen);
     test_basic_string<wchar_t>(gen);
@@ -1884,6 +1922,7 @@ void test_string(mt19937_64& gen) {
     test_basic_string<unsigned long long>(gen);
 
     test_gh_5757_find_first_of();
+    test_gh_6342_find_first_of();
 }
 
 void test_various_containers() {