fix bug and add test

Author: changkhothuychung

libcxx/include/__ranges/concat_view.h

@@ -421,29 +421,22 @@ class concat_view<_Views...>::__iterator : public __concat_view_iterator_categor
         !__it_.valueless_by_exception(), "Trying to increment a valueless iterator of concat_view.");
     size_t __active_index = __it_.index();
     if (__n > 0) {
-      __variant_detail::__visitation::__variant::__visit_value(
-          [&](auto& __active_it) {
-            __apply_at_index<tuple_size_v<decltype(__parent_->__views_)>>(__active_index, [&](auto __index_constant) {
-              constexpr size_t __i  = __index_constant.value;
-              auto& __active_view   = std::get<__i>(__parent_->__views_);
-              difference_type __idx = __active_it - ranges::begin(__active_view);
-              __advance_fwd<__i>(__idx, __n);
-            });
-          },
-          __it_);
+      __apply_at_index<tuple_size_v<decltype(__parent_->__views_)>>(__active_index, [&](auto __index_constant) {
+        constexpr size_t __i  = __index_constant.value;
+        auto& __active_view   = std::get<__i>(__parent_->__views_);
+        difference_type __idx = std::get<__i>(__it_) - ranges::begin(__active_view);
+        __advance_fwd<__i>(__idx, __n);
+      });
+
     }
 
     else if (__n < 0) {
-      __variant_detail::__visitation::__variant::__visit_value(
-          [&](auto& __active_it) {
-            __apply_at_index<tuple_size_v<decltype(__parent_->__views_)>>(__active_index, [&](auto __index_constant) {
-              constexpr size_t __i  = __index_constant.value;
-              auto& __active_view   = std::get<__i>(__parent_->__views_);
-              difference_type __idx = __active_it - ranges::begin(__active_view);
-              __advance_bwd<__i>(__idx, -__n);
-            });
-          },
-          __it_);
+      __apply_at_index<tuple_size_v<decltype(__parent_->__views_)>>(__active_index, [&](auto __index_constant) {
+        constexpr size_t __i  = __index_constant.value;
+        auto& __active_view   = std::get<__i>(__parent_->__views_);
+        difference_type __idx = std::get<__i>(__it_) - ranges::begin(__active_view);
+        __advance_bwd<__i>(__idx, -__n);
+      });
     }
 
     return *this;

libcxx/test/std/ranges/range.adaptors/range.concat/iterator/decrement.pass.cpp

@@ -194,6 +194,32 @@ constexpr bool test() {
     assert(*it == a.back());
   }
 
+  // Different range types
+  {
+    std::span<const int> sa{a};
+    auto sb = std::ranges::subrange{b.begin(), b.end()};
+    std::array<int, 2> c{9, 10};
+
+    auto v = std::views::concat(sa, sb, c);
+
+    auto it = v.begin();
+    std::ranges::advance(it, sa.size());
+    --it;
+    assert(*it == a.back());
+
+    auto it2 = v.begin();
+    std::ranges::advance(it2, sa.size() + sb.size());
+    --it2;
+    assert(*it2 == b.back());
+
+    // const-iterator.
+    const auto& cv = v;
+    auto cit       = cv.begin();
+    std::ranges::advance(cit, sa.size() + sb.size());
+    --cit;
+    assert(*cit == b.back());
+  }
+
   return true;
 }