Fix CI: bind j_nested_const by reference in the json_pointer test

clang-tidy (ci_clang_tidy) flagged the new test with
performance-unnecessary-copy-initialization: the local copy
j_nested_const of j_nested is never modified. Bind it as a const
reference instead; it still exercises the const overloads of at() and
contains().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/json_pointer.hpp

@@ -746,6 +746,12 @@ class json_pointer
                         // "-" always fails the range check
                         return false;
                     }
+                    if (JSON_HEDLEY_UNLIKELY(reference_token.empty()))
+                    {
+                        // an empty reference token is not an array index; array_index()
+                        // would throw out_of_range.404 -- contains() must not throw (see #5395)
+                        return false;
+                    }
                     if (JSON_HEDLEY_UNLIKELY(reference_token.size() == 1 && !("0" <= reference_token && reference_token <= "9")))
                     {
                         // invalid char

single_include/nlohmann/json.hpp

@@ -19588,6 +19588,12 @@ class json_pointer
                         // "-" always fails the range check
                         return false;
                     }
+                    if (JSON_HEDLEY_UNLIKELY(reference_token.empty()))
+                    {
+                        // an empty reference token is not an array index; array_index()
+                        // would throw out_of_range.404 -- contains() must not throw (see #5395)
+                        return false;
+                    }
                     if (JSON_HEDLEY_UNLIKELY(reference_token.size() == 1 && !("0" <= reference_token && reference_token <= "9")))
                     {
                         // invalid char

tests/src/unit-json_pointer.cpp

@@ -380,6 +380,26 @@ TEST_CASE("JSON pointers")
 
             DOCTEST_MSVC_SUPPRESS_WARNING_POP
 
+            {
+                // contains() must not throw for an empty reference token if the current
+                // value is an array (cf. #5395) -- at() still reports out_of_range.404
+                json j_nested = {{"a", {1, 2}}};
+                const json& j_nested_const = j_nested;
+                json::json_pointer const jp("/a/");
+                std::string const throw_msg = "[json.exception.out_of_range.404] unresolved reference token ''";
+
+                CHECK_THROWS_WITH_AS(j_nested.at(jp), throw_msg.c_str(), json::out_of_range&);
+                CHECK_THROWS_WITH_AS(j_nested_const.at(jp), throw_msg.c_str(), json::out_of_range&);
+
+                CHECK(j_nested.contains(json::json_pointer("/a/1")));
+                CHECK(!j_nested.contains(jp));
+                CHECK(!j_nested_const.contains(jp));
+
+                // same for an empty reference token on a top-level array
+                CHECK(!j.contains(json::json_pointer("/")));
+                CHECK(!j_const.contains(json::json_pointer("/")));
+            }
+
             CHECK_THROWS_WITH_AS(j.at("/one"_json_pointer) = 1,
                                  "[json.exception.parse_error.109] parse error: array index 'one' is not a number", json::parse_error&);
             CHECK_THROWS_WITH_AS(j_const.at("/one"_json_pointer) == 1,