fix compilation issue (#4613)

* fix compilation issue

Signed-off-by: Harinath Nampally <harinath922@gmail.com>

* add unit test

Signed-off-by: Harinath Nampally <harinath922@gmail.com>

* fix ci failure check

Signed-off-by: Harinath Nampally <harinath922@gmail.com>

---------

Signed-off-by: Harinath Nampally <harinath922@gmail.com>

include/nlohmann/detail/json_pointer.hpp

@@ -750,7 +750,7 @@ class json_pointer
                     // iterate array and use index as reference string
                     for (std::size_t i = 0; i < value.m_data.m_value.array->size(); ++i)
                     {
-                        flatten(detail::concat(reference_string, '/', std::to_string(i)),
+                        flatten(detail::concat<string_t>(reference_string, '/', std::to_string(i)),
                                 value.m_data.m_value.array->operator[](i), result);
                     }
                 }
@@ -769,7 +769,7 @@ class json_pointer
                     // iterate object and use keys as reference string
                     for (const auto& element : *value.m_data.m_value.object)
                     {
-                        flatten(detail::concat(reference_string, '/', detail::escape(element.first)), element.second, result);
+                        flatten(detail::concat<string_t>(reference_string, '/', detail::escape(element.first)), element.second, result);
                     }
                 }
                 break;

single_include/nlohmann/json.hpp

@@ -15239,7 +15239,7 @@ class json_pointer
                     // iterate array and use index as reference string
                     for (std::size_t i = 0; i < value.m_data.m_value.array->size(); ++i)
                     {
-                        flatten(detail::concat(reference_string, '/', std::to_string(i)),
+                        flatten(detail::concat<string_t>(reference_string, '/', std::to_string(i)),
                                 value.m_data.m_value.array->operator[](i), result);
                     }
                 }
@@ -15258,7 +15258,7 @@ class json_pointer
                     // iterate object and use keys as reference string
                     for (const auto& element : *value.m_data.m_value.object)
                     {
-                        flatten(detail::concat(reference_string, '/', detail::escape(element.first)), element.second, result);
+                        flatten(detail::concat<string_t>(reference_string, '/', detail::escape(element.first)), element.second, result);
                     }
                 }
                 break;

tests/src/unit-alt-string.cpp

@@ -359,4 +359,12 @@ TEST_CASE("alternative string type")
         alt_json const j2 = {"foo", "bam"};
         CHECK(alt_json::diff(j1, j2).dump() == "[{\"op\":\"replace\",\"path\":\"/1\",\"value\":\"bam\"},{\"op\":\"remove\",\"path\":\"/2\"}]");
     }
+
+    SECTION("flatten")
+    {
+        // a JSON value
+        const alt_json j = alt_json::parse(R"({"foo": ["bar", "baz"]})");
+        const auto j2 = j.flatten();
+        CHECK(j2.dump() == R"({"/foo/0":"bar","/foo/1":"baz"})");
+    }
 }