use SFINAE

Author: changkhothuychung

libcxx/test/std/ranges/range.adaptors/range.concat/constraints.pass.cpp

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+#include <cassert>
+#include <list>
+#include <ranges>
+#include <vector>
+#include "test_iterators.h"
+#include "test_macros.h"
+
+// This tests https://cplusplus.github.io/LWG/issue4082
+// views::concat(r) is well-formed when r is an output_range
+
+template<typename T>
+concept WellFormedView = requires(T& a) {
+  std::views::concat(a);
+};
+
+struct InputRange {
+  using Iterator = cpp17_input_iterator<int*>;
+  using Sentinel = sentinel_wrapper<Iterator>;
+  constexpr InputRange(int* b, int *e): begin_(b), end_(e) {}
+  constexpr Iterator begin() { return Iterator(begin_); }
+  constexpr Sentinel end() { return Sentinel(Iterator(end_)); }
+
+private:
+  int* begin_;
+  int* end_;
+};
+
+
+int main(int, char**) {
+
+  // rejects when it is an output range
+  {
+    std::vector<int> v{1,2,3};
+    static_assert(!WellFormedView<decltype(std::views::counted(std::back_inserter(v), 3))>);
+  }
+
+  // input range
+  {
+    static_assert(WellFormedView<InputRange>);
+  }
+
+  // bidirectional range
+  {
+    static_assert(WellFormedView<std::list<int>>);
+  }
+
+  // random access range
+  {
+    static_assert(WellFormedView<std::vector<int>>);
+  }
+
+  return 0;
+}