test for GH issue

impl/concat/test/issues.cpp

@@ -0,0 +1,18 @@
+
+#include <catch2/catch_test_macros.hpp>
+
+#include "concat.hpp"
+
+#define TEST_POINT(x) TEST_CASE(x, "[issues]")
+
+TEST_POINT("#4") {
+  std::ranges::concat_view cv{std::vector{1, 2}, std::vector{3, 4}};
+
+  auto it = cv.begin();
+  auto cit = std::as_const(cv).begin();
+
+  [[maybe_unused]] decltype(it) it_copy(it);     // OK
+  [[maybe_unused]] decltype(cit) cit_copy(cit);  // OK
+  // [[maybe_unused]] decltype(it) it_copy2(cit);   // NOK by design
+  [[maybe_unused]] decltype(cit) cit_copy2(it);  // NOK  bug!
+}