diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2c1fc2e8..4d62f799 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ HEAD
 ----
 
 * Fixed error `'dummy' may be used uninitialized` on GCC 11
+* Fixed error `expected unqualified-id before 'const'` on GCC 11 (issue #1622)
 
 v6.18.3 (2021-07-27)
 -------
diff --git a/extras/tests/CMakeLists.txt b/extras/tests/CMakeLists.txt
index c88d3000..2c0b3a11 100644
--- a/extras/tests/CMakeLists.txt
+++ b/extras/tests/CMakeLists.txt
@@ -12,6 +12,7 @@ link_libraries(ArduinoJson catch)
 include_directories(Helpers)
 add_subdirectory(Cpp11)
 add_subdirectory(Cpp17)
+add_subdirectory(Cpp20)
 add_subdirectory(FailingBuilds)
 add_subdirectory(IntegrationTests)
 add_subdirectory(JsonArray)
diff --git a/extras/tests/Cpp20/CMakeLists.txt b/extras/tests/Cpp20/CMakeLists.txt
new file mode 100644
index 00000000..89534096
--- /dev/null
+++ b/extras/tests/Cpp20/CMakeLists.txt
@@ -0,0 +1,29 @@
+# ArduinoJson - https://arduinojson.org
+# Copyright Benoit Blanchon 2014-2021
+# MIT License
+
+if(MSVC_VERSION LESS 1910)
+	return()
+endif()
+
+if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10)
+	return()
+endif()
+
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10)
+	return()
+endif()
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+add_executable(Cpp20Tests
+	smoke_test.cpp
+)
+
+add_test(Cpp20 Cpp20Tests)
+
+set_tests_properties(Cpp20
+	PROPERTIES
+		LABELS 		"Catch"
+)
diff --git a/extras/tests/Cpp20/smoke_test.cpp b/extras/tests/Cpp20/smoke_test.cpp
new file mode 100644
index 00000000..dc41e875
--- /dev/null
+++ b/extras/tests/Cpp20/smoke_test.cpp
@@ -0,0 +1,15 @@
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+#include <string>
+
+TEST_CASE("C++20 smoke test") {
+  StaticJsonDocument<128> doc;
+
+  deserializeJson(doc, "{\"hello\":\"world\"}");
+  REQUIRE(doc["hello"] == "world");
+
+  std::string json;
+  serializeJson(doc, json);
+  REQUIRE(json == "{\"hello\":\"world\"}");
+}
diff --git a/src/ArduinoJson/Strings/Adapters/ArduinoStringAdapter.hpp b/src/ArduinoJson/Strings/Adapters/ArduinoStringAdapter.hpp
index 5e4b6240..3d520406 100644
--- a/src/ArduinoJson/Strings/Adapters/ArduinoStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/Adapters/ArduinoStringAdapter.hpp
@@ -45,8 +45,7 @@ class StringAdapter< ::String> {
 template <>
 class StringAdapter< ::StringSumHelper> : public StringAdapter< ::String> {
  public:
-  StringAdapter< ::StringSumHelper>(const ::String& s)
-      : StringAdapter< ::String>(s) {}
+  StringAdapter(const ::String& s) : StringAdapter< ::String>(s) {}
 };
 
 }  // namespace ARDUINOJSON_NAMESPACE
diff --git a/src/ArduinoJson/Strings/Adapters/ConstRamStringAdapter.hpp b/src/ArduinoJson/Strings/Adapters/ConstRamStringAdapter.hpp
index 1ca7c02c..b4a696e2 100644
--- a/src/ArduinoJson/Strings/Adapters/ConstRamStringAdapter.hpp
+++ b/src/ArduinoJson/Strings/Adapters/ConstRamStringAdapter.hpp
@@ -45,7 +45,7 @@ class StringAdapter<const char*> {
 template <int N>
 class StringAdapter<const char[N]> : public StringAdapter<const char*> {
  public:
-  StringAdapter<const char[N]>(const char* s) : StringAdapter<const char*>(s) {}
+  StringAdapter(const char* s) : StringAdapter<const char*>(s) {}
 };
 
 }  // namespace ARDUINOJSON_NAMESPACE