fix test

Author: changkhothuychung

libcxx/include/__ranges/concat_view.h

@@ -111,7 +111,7 @@ concept __concat_is_bidirectional =
     (__all_bidirectional<_Const, _Rs...>) && (__all_common_ignore_last<_Const, _Rs...>::value);
 
 template <input_range... _Views>
-  requires(view<_Views> && ...) && (sizeof...(_Views) > 0) && __concatable<_Views...>
+  requires((view<_Views> && ...) && (sizeof...(_Views) > 0) && __concatable<_Views...>)
 class concat_view : public view_interface<concat_view<_Views...>> {
   tuple<_Views...> __views_;
 
@@ -210,7 +210,7 @@ struct __concat_view_iterator_category<_Const, _Views...> {
 };
 
 template <input_range... _Views>
-  requires(view<_Views> && ...) && (sizeof...(_Views) > 0) && __concatable<_Views...>
+  requires((view<_Views> && ...) && (sizeof...(_Views) > 0) && __concatable<_Views...>)
 template <bool _Const>
 class concat_view<_Views...>::__iterator : public __concat_view_iterator_category<_Const, _Views...> {
 public:

libcxx/test/std/ranges/range.adaptors/range.concat/ctor.verify.cpp

@@ -10,14 +10,44 @@
 
 #include <cassert>
 #include <ranges>
+#include <string>
 #include <vector>
 #include "test_macros.h"
 
+struct NoSizeRange : std::ranges::view_base {
+  constexpr auto size() { return 0; }
+};
+
 int main(int, char**) {
-  std::vector<int> v{1, 2, 3};
-  auto r = std::views::counted(std::back_inserter(v), 3);
-  auto c = std::views::concat(r);
-  // expected-error@*:* {{}}
+
+  {
+    // LWG 4082
+    std::vector<int> v{1, 2, 3};
+    auto r = std::views::counted(std::back_inserter(v), 3);
+    auto c = std::views::concat(r);
+    // expected-error@*:* {{}}
+  }
+
+  {
+    // input is not a view
+    int x = 1;
+    auto c = std::views::concat(x);
+    // expected-error@*:* {{}}
+  }
+
+  {
+    // input is a view but has 0 size
+    auto c = std::views::concat(NoSizeRange{});
+    // expected-error@*:* {{}}
+  }
+
+  {
+    // inputs are non-concatable
+    std::vector<int> v1{1,2};
+    std::vector<std::string> v2{"Hello", "World"};
+    auto c = std::views::concat(v1, v2);
+    // expected-error@*:* {{}}
+  }
 
   return 0;
 }