Undef true and false macros

Replacing `true` with `1` changes `JsonString("ABC", true)` into `JsonString("ABC", 1)`, so all static strings had a length of 1.

This is a workaround for arduino/ArduinoCore-sam#50
Fixes #2181

extras/tests/Misc/CMakeLists.txt

@@ -29,3 +29,23 @@ set_tests_properties(Misc
 	PROPERTIES
 		LABELS "Catch"
 )
+
+add_executable(Issue2181
+	issue2181.cpp # Cannot be linked with other tests
+)
+
+set_target_properties(Issue2181 PROPERTIES UNITY_BUILD OFF)
+
+add_test(Issue2181 Issue2181)
+
+set_tests_properties(Issue2181
+	PROPERTIES
+		LABELS "Catch"
+)
+
+if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+	target_compile_options(Issue2181
+		PRIVATE
+			-Wno-keyword-macro # keyword is hidden by macro definition
+	)
+endif()

extras/tests/Misc/issue2181.cpp

@@ -0,0 +1,15 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2025, Benoit BLANCHON
+// MIT License
+
+#define true 0x1
+#define false 0x0
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("Issue #2181") {
+  JsonDocument doc;
+  doc["hello"] = "world";
+  REQUIRE(doc.as<std::string>() == "{\"hello\":\"world\"}");
+}