Test coverage for search_n with forward iterators

tests/std/tests/VSO_0000000_vector_algorithms_search_n/test.cpp

@@ -12,8 +12,49 @@
 
 #include "test_vector_algorithms_support.hpp"
 
+#pragma warning(disable : 4984) // if constexpr is a C++17 language extension
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wc++17-extensions"
+#endif // __clang__
+
 using namespace std;
 
+template <class UnderlyingIt>
+struct forward_iter_adaptor {
+    using iterator_category = forward_iterator_tag;
+    using reference         = typename UnderlyingIt::reference;
+    using value_type        = typename UnderlyingIt::value_type;
+    using pointer           = typename UnderlyingIt::pointer;
+    using difference_type   = ptrdiff_t;
+
+    constexpr forward_iter_adaptor() : ptr{} {}
+    constexpr explicit forward_iter_adaptor(const UnderlyingIt ptr_) : ptr(ptr_) {}
+
+    constexpr reference operator*() const {
+        return *ptr;
+    }
+    constexpr pointer operator->() const {
+        return ptr;
+    }
+    constexpr forward_iter_adaptor& operator++() {
+        ++ptr;
+        return *this;
+    }
+    constexpr forward_iter_adaptor operator++(int) {
+        forward_iter_adaptor old;
+        ++ptr;
+        return old;
+    }
+    constexpr bool operator==(const forward_iter_adaptor& o) const {
+        return ptr == o.ptr;
+    }
+    constexpr bool operator!=(const forward_iter_adaptor& o) const {
+        return ptr != o.ptr;
+    }
+
+    UnderlyingIt ptr;
+};
+
 template <class FwdIt, class T>
 auto last_known_good_search_n(FwdIt first, const FwdIt last, const size_t count, const T val) {
     // Deliberately using simple approach, not smart bidi/random iterators "check from the other end" stuff
@@ -40,24 +81,29 @@ auto last_known_good_search_n(FwdIt first, const FwdIt last, const size_t count,
     return last;
 }
 
-template <class Container, class T>
-void test_case_search_n(const Container& c, size_t count, T val) {
-    auto expected = last_known_good_search_n(c.begin(), c.end(), count, val);
-    auto actual   = search_n(c.begin(), c.end(), count, val);
-    assert(expected == actual);
+template <bool forward_only_iterators, class It, class T>
+void test_case_search_n(const It first, const It last, const size_t count, const T val) {
+    if constexpr (forward_only_iterators) {
+        using iter_type = forward_iter_adaptor<It>;
+        test_case_search_n<false>(iter_type(first), iter_type(last), count, val);
+    } else {
+        const auto expected = last_known_good_search_n(first, last, count, val);
+        const auto actual   = search_n(first, last, count, val);
+        assert(expected == actual);
 
 #if _HAS_CXX20
-    auto ranges_actual = ranges::search_n(c, static_cast<ptrdiff_t>(count), val);
-    assert(expected == begin(ranges_actual));
-    if (expected == c.end()) {
-        assert(end(ranges_actual) == c.end());
-    } else {
-        assert(distance(expected, end(ranges_actual)) == static_cast<ptrdiff_t>(count));
-    }
+        const auto ranges_actual = ranges::search_n(first, last, static_cast<ptrdiff_t>(count), val);
+        assert(expected == begin(ranges_actual));
+        if (expected == last) {
+            assert(end(ranges_actual) == last);
+        } else {
+            assert(distance(expected, end(ranges_actual)) == static_cast<ptrdiff_t>(count));
+        }
 #endif // _HAS_CXX20
+    }
 }
 
-template <class T>
+template <class T, bool forward_only_iterators = false>
 void test_search_n(mt19937_64& gen) {
     constexpr size_t lengthCount  = 70;
     constexpr size_t patternCount = 5;
@@ -74,7 +120,7 @@ void test_search_n(mt19937_64& gen) {
 
             const T val = static_cast<T>(dis(gen));
 
-            test_case_search_n(input, count, val);
+            test_case_search_n<forward_only_iterators>(input.begin(), input.end(), count, val);
 
             if (input.empty()) {
                 continue;
@@ -90,7 +136,7 @@ void test_search_n(mt19937_64& gen) {
                 if (pattern_length + pattern_pos <= input.size()) {
                     fill_n(input.begin() + static_cast<ptrdiff_t>(pattern_pos), pattern_length, val);
 
-                    test_case_search_n(input, count, val);
+                    test_case_search_n<forward_only_iterators>(input.begin(), input.end(), count, val);
                 }
             }
         }
@@ -113,6 +159,10 @@ void test_vector_algorithms(mt19937_64& gen) {
     test_search_n<unsigned int>(gen);
     test_search_n<long long>(gen);
     test_search_n<unsigned long long>(gen);
+
+    // Test only one case with forward iterators. It is a different and complex code path, hence it's worth testing,
+    // but it is not vectorized, so there's no point in trying different types.
+    test_search_n<short, true>(gen);
 }
 
 int main() {