diff --git a/AGENTS.md b/AGENTS.md
index a89089c..b97f7fa 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -42,6 +42,20 @@ ctest --output-on-failure
 ./test/integration/integration_tests
 ```
 
+### Formatting
+The project uses `clang-format` for code formatting.
+
+To format all files:
+```bash
+# From the build directory
+cmake --build . --target format
+```
+
+To format only changed files (requires `git`):
+```bash
+./scripts/format_changed.sh
+```
+
 ### Building Documentation
 If Doxygen is found, you can generate the HTML documentation:
 ```bash
@@ -63,11 +77,14 @@ Or simply copy the `include/deferred` directory to your project's include path.
   - **Indentation**: 2 spaces.
   - **Pointer Alignment**: Left (`Type* ptr`).
   - **Brace Wrapping**: Custom (check `.clang-format` for details).
+  - **Formatting**: All code must be formatted using `clang-format` before being committed. Use the `format` CMake target or the `scripts/format_changed.sh` script.
 - **Naming**: 
   - Namespaces: `deferred`
   - Files: `.hpp` for headers, `.cpp` for tests/examples.
 - **Testing**: New features must include unit tests in `test/unit/` and integration tests in `test/integration/`. Tests use the Catch2 v3 framework.
   - **Catch2 v3**: Uses `#include <catch2/catch_test_macros.hpp>` and requires a custom `main` in `main.cpp` using `Catch::Session().run()`.
+  - **Verification**: All changes must be verified by ensuring that all tests pass (`ctest`).
+- **Project Documentation**: After any code changes, `README.md` and `AGENTS.md` must be reviewed and updated if necessary to reflect the current state of the project.
 - **License**: MIT License. All source files should include the standard MIT license header.
 - **Header Guards**: Use `#ifndef DEFERRED_FILENAME_HPP` format.
 - **CI**: GitHub Actions workflow (`.github/workflows/c-cpp.yml`) builds the project on Ubuntu, Windows (MSVC), and macOS.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8d59244..8f7c0f6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -95,3 +95,18 @@ if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
   enable_testing()
   add_subdirectory(test)
 endif()
+
+# formatting
+
+find_program(CLANG_FORMAT clang-format)
+if(CLANG_FORMAT)
+  file(GLOB_RECURSE ALL_SOURCE_FILES
+    "include/*.h*"
+    "test/*.c*"
+    "examples/*.c*")
+  add_custom_target(format
+    COMMAND ${CLANG_FORMAT} -i ${ALL_SOURCE_FILES}
+    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+    COMMENT "Formatting code with clang-format"
+    VERBATIM)
+endif()
diff --git a/README.md b/README.md
index 45618d1..4d91186 100644
--- a/README.md
+++ b/README.md
@@ -92,8 +92,18 @@ To run tests after building:
 ctest --output-on-failure
 ```
 
-Known Issues
-------------
+Formatting
+----------
+
+The project uses `clang-format` for code formatting.
+
+To format all files:
+```bash
+# From the build directory
+cmake --build . --target format
+```
 
-* Using callable objects and functions with side-effects in ``invoke()`` and ``if_then_else()`` will result in a constant expression, assignable to ``constant_``.
-* Using ``deferred`` with other expression template based classes (e.g., `std::valarray`) may not be possible.
+To format only changed files (requires `git`):
+```bash
+./scripts/format_changed.sh
+```
diff --git a/examples/saxpy/saxpy_valarray.cpp b/examples/saxpy/saxpy_valarray.cpp
index 211594e..d0b42b8 100644
--- a/examples/saxpy/saxpy_valarray.cpp
+++ b/examples/saxpy/saxpy_valarray.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <algorithm>
 #include <iostream>
diff --git a/examples/saxpy/saxpy_vector.cpp b/examples/saxpy/saxpy_vector.cpp
index 5a5c3eb..95fe8e6 100644
--- a/examples/saxpy/saxpy_vector.cpp
+++ b/examples/saxpy/saxpy_vector.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <algorithm>
 #include <iostream>
diff --git a/examples/trivial/main.cpp b/examples/trivial/main.cpp
index f3b5e48..3720e27 100644
--- a/examples/trivial/main.cpp
+++ b/examples/trivial/main.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <deferred/deferred.hpp>
 #include <iostream>
diff --git a/include/deferred/apply.hpp b/include/deferred/apply.hpp
index 44412eb..22d5eeb 100644
--- a/include/deferred/apply.hpp
+++ b/include/deferred/apply.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_APPLY_HPP
 #define DEFERRED_APPLY_HPP
@@ -20,13 +14,9 @@ namespace detail {
 
 /**
  * @brief Helper function to apply a callable to the elements of a tuple.
- * @tparam F Type of the callable.
- * @tparam Tuple Type of the tuple.
+ * @copydoc apply(F&&, Tuple&&)
  * @tparam I Index sequence for tuple elements.
- * @param f The callable to apply.
- * @param t The tuple containing deferred objects.
  * @param I Index sequence for tuple elements.
- * @return The result of applying @p f to the evaluated elements of @p t.
  */
 template<typename F, typename Tuple, std::size_t... I>
 constexpr decltype(auto) apply_impl(F&& f, Tuple&& t, std::index_sequence<I...>)
diff --git a/include/deferred/conditional.hpp b/include/deferred/conditional.hpp
index b3c3eed..9b8deee 100644
--- a/include/deferred/conditional.hpp
+++ b/include/deferred/conditional.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_CONDITIONAL_HPP
 #define DEFERRED_CONDITIONAL_HPP
@@ -31,9 +25,6 @@ namespace deferred {
 template<typename ConditionExpression, typename ThenExpression, typename ElseExpression>
 class if_then_else_expression
 {
-  [[no_unique_address]] std::tuple<ConditionExpression, ThenExpression, ElseExpression>
-    m_expressions;
-
 public:
   using condition_expression_type = ConditionExpression;
   using then_expression_type      = ThenExpression;
@@ -42,6 +33,10 @@ class if_then_else_expression
                                                        decltype(std::declval<ElseExpression>()())>;
   using subexpression_types       = std::tuple<ConditionExpression, ThenExpression, ElseExpression>;
 
+private:
+  [[no_unique_address]] subexpression_types m_expressions;
+
+public:
   /**
    * @brief Constructs an if_then_else_expression.
    * @tparam Condition Type of the condition expression.
@@ -60,7 +55,11 @@ class if_then_else_expression
                   std::forward<ElseEx>(else_))
   { }
 
-  constexpr result_type operator()() const
+  /**
+   * @brief Evaluates the conditional expression.
+   * @return The result of the conditional expression.
+   */
+  [[nodiscard]] constexpr result_type operator()() const
   {
     if (evaluate(std::get<0>(m_expressions)))
     {
@@ -72,7 +71,8 @@ class if_then_else_expression
     }
   }
 
-  constexpr result_type operator()()
+  /// @copydoc if_then_else_expression::operator()() const
+  [[nodiscard]] constexpr result_type operator()()
   {
     if (evaluate(std::get<0>(m_expressions)))
     {
@@ -116,7 +116,7 @@ class if_then_else_expression
  * @return An \ref if_then_else_expression that will perform the deferred evaluation.
  */
 template<typename ConditionExpression, typename ThenExpression, typename ElseExpression>
-constexpr auto
+[[nodiscard]] constexpr auto
 if_then_else(ConditionExpression&& condition, ThenExpression&& then_, ElseExpression&& else_)
 {
   using condition_expression = make_deferred_t<ConditionExpression>;
diff --git a/include/deferred/constant.hpp b/include/deferred/constant.hpp
index 9452ca5..7ca0c0d 100644
--- a/include/deferred/constant.hpp
+++ b/include/deferred/constant.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_CONSTANT_HPP
 #define DEFERRED_CONSTANT_HPP
@@ -29,7 +23,7 @@ class constant_
   using subexpression_types = std::tuple<>;
 
 private:
-  T m_t;
+  [[no_unique_address]] T m_t;
 
 public:
   /**
@@ -48,13 +42,13 @@ class constant_
   constant_& operator=(constant_&&)      = delete;
 
   /// Returns the stored value.
-  constexpr T const& operator()() const& noexcept
+  [[nodiscard]] constexpr T const& operator()() const& noexcept
   {
     return m_t;
   }
 
   /// @copydoc operator()()
-  constexpr T operator()() && noexcept
+  [[nodiscard]] constexpr T operator()() && noexcept
   {
     return std::move(m_t);
   }
@@ -83,7 +77,7 @@ class constant_
  * @return A constant_ object containing the evaluated value.
  */
 template<typename T>
-constexpr auto constant(T&& t)
+[[nodiscard]] constexpr auto constant(T&& t)
 {
   using result_type = std::decay_t<decltype(recursive_evaluate(std::forward<T>(t)))>;
   return constant_<result_type>(recursive_evaluate(std::forward<T>(t)));
diff --git a/include/deferred/deferred.hpp b/include/deferred/deferred.hpp
index 34c7b99..9b377f9 100644
--- a/include/deferred/deferred.hpp
+++ b/include/deferred/deferred.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_DEFERRED_HPP
 #define DEFERRED_DEFERRED_HPP
diff --git a/include/deferred/evaluate.hpp b/include/deferred/evaluate.hpp
index a21ae5b..9a4dd75 100644
--- a/include/deferred/evaluate.hpp
+++ b/include/deferred/evaluate.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_EVALUATE_HPP
 #define DEFERRED_EVALUATE_HPP
@@ -24,7 +18,7 @@ namespace deferred {
  * @return The result of evaluating the expression.
  */
 template<typename T>
-constexpr decltype(auto) evaluate(T&& t)
+constexpr auto evaluate(T&& t)
 {
   if constexpr (Deferred<T>)
   {
@@ -34,8 +28,7 @@ constexpr decltype(auto) evaluate(T&& t)
     }
     else
     {
-      auto v = evaluate(std::forward<T>(t)());
-      return v;
+      return evaluate(std::forward<T>(t)());
     }
   }
   else
@@ -46,17 +39,14 @@ constexpr decltype(auto) evaluate(T&& t)
 
 /**
  * @brief Recursively evaluates @p t until a non-callable is returned.
- * @tparam T The type of the expression to recursively evaluate.
- * @param t The expression to recursively evaluate.
- * @return The result of the recursive evaluation.
+ * @copydoc evaluate(T&&)
  */
 template<typename T>
-constexpr decltype(auto) recursive_evaluate(T&& t)
+constexpr auto recursive_evaluate(T&& t)
 {
   if constexpr (std::is_invocable_v<T>)
   {
-    auto v = recursive_evaluate(std::forward<T>(t)());
-    return v;
+    return recursive_evaluate(std::forward<T>(t)());
   }
   else
   {
diff --git a/include/deferred/expression.hpp b/include/deferred/expression.hpp
index 6fe7df6..8aa5628 100644
--- a/include/deferred/expression.hpp
+++ b/include/deferred/expression.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_EXPRESSION_HPP
 #define DEFERRED_EXPRESSION_HPP
@@ -31,14 +25,16 @@ namespace deferred {
 template<typename Operator, typename... Expressions>
 class expression_
 {
-  [[no_unique_address]] Operator m_op;
-  [[no_unique_address]] std::tuple<Expressions...> m_expressions;
-
 public:
   using operator_type       = Operator;
   using expression_types    = std::tuple<Expressions...>;
   using subexpression_types = std::tuple<Operator, Expressions...>;
 
+private:
+  [[no_unique_address]] operator_type m_op;
+  [[no_unique_address]] expression_types m_expressions;
+
+public:
   /**
    * @brief Constructs an expression.
    * @tparam Op The type of the operator.
@@ -60,22 +56,23 @@ class expression_
   expression_& operator=(expression_ const&) = delete;
   expression_& operator=(expression_&&)      = delete;
 
-  constexpr decltype(auto) operator()() const
+  [[nodiscard]] constexpr decltype(auto) operator()() const
   {
     return deferred::apply(m_op, m_expressions);
   }
 
-  constexpr decltype(auto) operator()()
+  /// @copydoc operator()() const
+  [[nodiscard]] constexpr decltype(auto) operator()()
   {
     return deferred::apply(m_op, m_expressions);
   }
 
-  constexpr operator_type const& operator_() const noexcept
+  [[nodiscard]] constexpr operator_type const& operator_() const noexcept
   {
     return m_op;
   }
 
-  constexpr expression_types const& subexpressions() const noexcept
+  [[nodiscard]] constexpr expression_types const& subexpressions() const noexcept
   {
     return m_expressions;
   }
diff --git a/include/deferred/invoke.hpp b/include/deferred/invoke.hpp
index 0df8969..6af6317 100644
--- a/include/deferred/invoke.hpp
+++ b/include/deferred/invoke.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_INVOKE_HPP
 #define DEFERRED_INVOKE_HPP
@@ -31,7 +25,7 @@ namespace deferred {
  * @return An expression representing the invocation.
  */
 template<typename F, typename... Args>
-constexpr auto invoke(F&& f, Args&&... args)
+[[nodiscard]] constexpr auto invoke(F&& f, Args&&... args)
 {
   if constexpr (is_deferred_v<std::remove_reference_t<F>>)
   {
diff --git a/include/deferred/make_function_object.hpp b/include/deferred/make_function_object.hpp
index 3b8e9c6..dcc1ef2 100644
--- a/include/deferred/make_function_object.hpp
+++ b/include/deferred/make_function_object.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_MAKE_FUNCTION_OBJECT_HPP
 #define DEFERRED_MAKE_FUNCTION_OBJECT_HPP
@@ -34,7 +28,8 @@ struct fun_ptr_wrapper
   { }
 
   template<typename... T>
-  constexpr decltype(auto) operator()(T&&... t) const noexcept(noexcept(m_f(std::forward<T>(t)...)))
+  [[nodiscard]] constexpr decltype(auto)
+  operator()(T&&... t) const noexcept(noexcept(m_f(std::forward<T>(t)...)))
   {
     return m_f(std::forward<T>(t)...);
   }
@@ -49,7 +44,8 @@ struct fun_ptr_wrapper
  * @return A callable object representing @p f.
  */
 template<typename F>
-constexpr decltype(auto) make_function_object(F&& f) noexcept(noexcept(std::forward<F>(f)))
+[[nodiscard]] constexpr decltype(auto)
+make_function_object(F&& f) noexcept(noexcept(std::forward<F>(f)))
 {
   if constexpr (std::is_function_v<std::remove_reference_t<F>>)
   {
diff --git a/include/deferred/operators.hpp b/include/deferred/operators.hpp
index c44bb7b..2828748 100644
--- a/include/deferred/operators.hpp
+++ b/include/deferred/operators.hpp
@@ -1,12 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
-
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_OPERATORS_HPP
 #define DEFERRED_OPERATORS_HPP
@@ -29,7 +22,7 @@ namespace deferred {
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator+(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator+(T&& t, U&& u)
 {
   return invoke(std::plus<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -44,7 +37,7 @@ constexpr auto operator+(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator-(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator-(T&& t, U&& u)
 {
   return invoke(std::minus<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -56,7 +49,7 @@ constexpr auto operator-(T&& t, U&& u)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator+(T&& t)
+[[nodiscard]] constexpr auto operator+(T&& t)
 {
   return invoke([](auto&& x) { return +std::forward<decltype(x)>(x); }, std::forward<T>(t));
 }
@@ -68,7 +61,7 @@ constexpr auto operator+(T&& t)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator-(T&& t)
+[[nodiscard]] constexpr auto operator-(T&& t)
 {
   return invoke(std::negate<>{}, std::forward<T>(t));
 }
@@ -83,7 +76,7 @@ constexpr auto operator-(T&& t)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator*(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator*(T&& t, U&& u)
 {
   return invoke(std::multiplies<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -98,7 +91,7 @@ constexpr auto operator*(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator/(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator/(T&& t, U&& u)
 {
   return invoke(std::divides<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -113,7 +106,7 @@ constexpr auto operator/(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator%(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator%(T&& t, U&& u)
 {
   return invoke(std::modulus<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -125,7 +118,7 @@ constexpr auto operator%(T&& t, U&& u)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator++(T&& t)
+[[nodiscard]] constexpr auto operator++(T&& t)
 {
   return invoke([](auto&& x) { return ++std::forward<decltype(x)>(x); }, std::forward<T>(t));
 }
@@ -138,7 +131,7 @@ constexpr auto operator++(T&& t)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator++(T&& t, int)
+[[nodiscard]] constexpr auto operator++(T&& t, int)
 {
   return invoke([](auto&& x) { return std::forward<decltype(x)>(x)++; }, std::forward<T>(t));
 }
@@ -150,7 +143,7 @@ constexpr auto operator++(T&& t, int)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator--(T&& t)
+[[nodiscard]] constexpr auto operator--(T&& t)
 {
   return invoke([](auto&& x) { return --std::forward<decltype(x)>(x); }, std::forward<T>(t));
 }
@@ -163,7 +156,7 @@ constexpr auto operator--(T&& t)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator--(T&& t, int)
+[[nodiscard]] constexpr auto operator--(T&& t, int)
 {
   return invoke([](auto&& x) { return std::forward<decltype(x)>(x)--; }, std::forward<T>(t));
 }
@@ -178,7 +171,7 @@ constexpr auto operator--(T&& t, int)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator==(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator==(T&& t, U&& u)
 {
   return invoke(std::equal_to<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -193,7 +186,7 @@ constexpr auto operator==(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator!=(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator!=(T&& t, U&& u)
 {
   return invoke(std::not_equal_to<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -208,7 +201,7 @@ constexpr auto operator!=(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator>(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator>(T&& t, U&& u)
 {
   return invoke(std::greater<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -223,7 +216,7 @@ constexpr auto operator>(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator<(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator<(T&& t, U&& u)
 {
   return invoke(std::less<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -238,7 +231,7 @@ constexpr auto operator<(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator>=(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator>=(T&& t, U&& u)
 {
   return invoke(std::greater_equal<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -253,7 +246,7 @@ constexpr auto operator>=(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator<=(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator<=(T&& t, U&& u)
 {
   return invoke(std::less_equal<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -268,7 +261,7 @@ constexpr auto operator<=(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator&&(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator&&(T&& t, U&& u)
 {
   return invoke(std::logical_and<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -283,7 +276,7 @@ constexpr auto operator&&(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator||(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator||(T&& t, U&& u)
 {
   return invoke(std::logical_or<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -295,7 +288,7 @@ constexpr auto operator||(T&& t, U&& u)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator!(T&& t)
+[[nodiscard]] constexpr auto operator!(T&& t)
 {
   return invoke(std::logical_not<>{}, std::forward<T>(t));
 }
@@ -310,7 +303,7 @@ constexpr auto operator!(T&& t)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator&(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator&(T&& t, U&& u)
 {
   return invoke(std::bit_and<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -325,7 +318,7 @@ constexpr auto operator&(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator|(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator|(T&& t, U&& u)
 {
   return invoke(std::bit_or<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -340,7 +333,7 @@ constexpr auto operator|(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator^(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator^(T&& t, U&& u)
 {
   return invoke(std::bit_xor<>{}, std::forward<T>(t), std::forward<U>(u));
 }
@@ -352,7 +345,7 @@ constexpr auto operator^(T&& t, U&& u)
  * @return A deferred expression representing the operation.
  */
 template<Deferred T>
-constexpr auto operator~(T&& t)
+[[nodiscard]] constexpr auto operator~(T&& t)
 {
   return invoke(std::bit_not<>{}, std::forward<T>(t));
 }
@@ -367,7 +360,7 @@ constexpr auto operator~(T&& t)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator<<(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator<<(T&& t, U&& u)
 {
   return invoke(
     [](auto&& x, auto&& y) { return std::forward<decltype(x)>(x) << std::forward<decltype(y)>(y); },
@@ -385,7 +378,7 @@ constexpr auto operator<<(T&& t, U&& u)
  */
 template<typename T, typename U>
   requires AnyDeferred<T, U>
-constexpr auto operator>>(T&& t, U&& u)
+[[nodiscard]] constexpr auto operator>>(T&& t, U&& u)
 {
   return invoke(
     [](auto&& x, auto&& y) { return std::forward<decltype(x)>(x) >> std::forward<decltype(y)>(y); },
diff --git a/include/deferred/switch.hpp b/include/deferred/switch.hpp
index 125368b..b4b58ff 100644
--- a/include/deferred/switch.hpp
+++ b/include/deferred/switch.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_SWITCH_HPP
 #define DEFERRED_SWITCH_HPP
@@ -40,7 +34,7 @@ class default_expression
   constexpr explicit default_expression(T&&... t) : m_expression(std::forward<T>(t)...)
   { }
 
-  constexpr decltype(auto) operator()() const
+  [[nodiscard]] constexpr decltype(auto) operator()() const
   {
     return m_expression();
   }
@@ -67,13 +61,15 @@ class default_expression
 template<typename LabelExpression, typename BodyExpression>
 class case_expression
 {
-  [[no_unique_address]] std::tuple<LabelExpression, BodyExpression> m_expressions;
-
 public:
   using label_expression_type = LabelExpression;
   using body_expression_type  = BodyExpression;
   using subexpression_types   = std::tuple<LabelExpression, BodyExpression>;
 
+private:
+  [[no_unique_address]] subexpression_types m_expressions;
+
+public:
   /**
    * @brief Constructs a case_expression.
    * @tparam LabelEx The type of the label expression.
@@ -88,17 +84,23 @@ class case_expression
 
   /// Compares @p T with the label expression.
   template<typename T>
-  constexpr decltype(auto) compare(T&& t) const
+  [[nodiscard]] constexpr decltype(auto) compare(T&& t) const
   {
     return std::forward<T>(t) == evaluate(std::get<0>(m_expressions));
   }
 
   /// Returns the result of the body expression.
-  constexpr decltype(auto) operator()() const
+  [[nodiscard]] constexpr decltype(auto) operator()() const
   {
     return evaluate(std::get<1>(m_expressions));
   }
 
+  /**
+   * @brief Visits the case expression with a visitor.
+   * @tparam Visitor The type of the visitor.
+   * @param v The visitor.
+   * @param nesting The nesting level.
+   */
   template<typename Visitor>
   constexpr void visit(Visitor&& v, std::size_t nesting = 0) const
   {
@@ -174,7 +176,7 @@ class switch_expression
    * If none does, the default (@c std::tuple_element<1>) is returned.
    */
   template<std::size_t I, typename T>
-  constexpr result_type choose_case(T&& t) const
+  [[nodiscard]] constexpr result_type choose_case(T&& t) const
   {
     if constexpr (I < std::tuple_size<subexpression_types>::value)
     {
@@ -191,9 +193,9 @@ class switch_expression
     }
   }
 
-  /// @copydoc choose_case(T&&) const
+  /// @copydoc switch_expression::choose_case(T&&) const
   template<std::size_t I, typename T>
-  constexpr result_type choose_case(T&& t)
+  [[nodiscard]] constexpr result_type choose_case(T&& t)
   {
     if constexpr (I < std::tuple_size<subexpression_types>::value)
     {
@@ -211,22 +213,36 @@ class switch_expression
   }
 
 public:
-  constexpr result_type operator()() const
+  /**
+   * @brief Evaluates the switch expression.
+   * @return The result of the switch expression.
+   */
+  [[nodiscard]] constexpr result_type operator()() const
   {
     // start from second case, as first is the default
     return choose_case<2>(evaluate(std::get<ConditionExpression>(m_expressions)));
   }
 
-  constexpr result_type operator()()
+  /// @copydoc switch_expression::operator()() const
+  [[nodiscard]] constexpr result_type operator()()
   {
     // start from second case, as first is the default
     return choose_case<2>(evaluate(std::get<ConditionExpression>(m_expressions)));
   }
 
+  /**
+   * @brief Visits the switch expression with a visitor.
+   * @tparam Visitor The type of the visitor.
+   * @param v The visitor.
+   * @param nesting The nesting level.
+   */
   template<typename Visitor>
-  constexpr decltype(auto) visit(Visitor&& v) const
+  constexpr void visit(Visitor&& v, std::size_t nesting = 0) const
   {
-    return std::forward<Visitor>(v)(*this);
+    std::forward<Visitor>(v)(*this, nesting);
+    std::apply([&v, nesting](
+                 auto const&... args) { (args.visit(std::forward<Visitor>(v), nesting + 1), ...); },
+               m_expressions);
   }
 };
 
@@ -237,7 +253,7 @@ class switch_expression
  * @return A \ref default_expression wrapping the given expression.
  */
 template<typename Expression>
-auto default_(Expression&& ex)
+[[nodiscard]] constexpr auto default_(Expression&& ex)
 {
   using expression = make_deferred_t<Expression>;
   return default_expression<expression>(std::forward<Expression>(ex));
@@ -252,7 +268,7 @@ auto default_(Expression&& ex)
  * @return A \ref case_expression wrapping the label and body.
  */
 template<typename LabelExpression, typename BodyExpression>
-auto case_(LabelExpression&& label, BodyExpression&& body)
+[[nodiscard]] constexpr auto case_(LabelExpression&& label, BodyExpression&& body)
 {
   using label_expression = make_deferred_t<LabelExpression>;
   using body_expression  = make_deferred_t<BodyExpression>;
@@ -286,7 +302,7 @@ auto case_(LabelExpression&& label, BodyExpression&& body)
  * @return A tuple-like expression representing the switch construct.
  */
 template<typename ConditionExpression, typename DefaultExpression, typename... CaseExpressions>
-constexpr auto
+[[nodiscard]] constexpr auto
 switch_(ConditionExpression&& condition, DefaultExpression&& default_, CaseExpressions&&... case_)
 {
   static_assert(std::conjunction_v<detail::is_valid_default<std::decay_t<DefaultExpression>>>,
diff --git a/include/deferred/type_name.hpp b/include/deferred/type_name.hpp
index c10031b..411572e 100644
--- a/include/deferred/type_name.hpp
+++ b/include/deferred/type_name.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_TYPE_NAME_HPP
 #define DEFERRED_TYPE_NAME_HPP
@@ -30,7 +24,7 @@ namespace deferred {
  * https://stackoverflow.com/questions/18369128/how-can-i-see-the-type-deduced-for-a-template-type-parameter/18369732#18369732
  */
 template<typename T>
-std::string type_name()
+[[nodiscard]] std::string type_name()
 {
   using TR = std::remove_reference_t<T>;
 
@@ -64,7 +58,7 @@ std::string type_name()
 
 /// @copydoc type_name()
 template<typename T>
-auto type_name(T&& t)
+[[nodiscard]] auto type_name(T&& t)
 {
   return type_name<decltype(t)>();
 }
diff --git a/include/deferred/type_traits/is_constant_expression.hpp b/include/deferred/type_traits/is_constant_expression.hpp
index ae6af32..a3bffa0 100644
--- a/include/deferred/type_traits/is_constant_expression.hpp
+++ b/include/deferred/type_traits/is_constant_expression.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_TYPE_TRAITS_IS_CONSTANT_EXPRESSION_HPP
 #define DEFERRED_TYPE_TRAITS_IS_CONSTANT_EXPRESSION_HPP
diff --git a/include/deferred/type_traits/is_deferred.hpp b/include/deferred/type_traits/is_deferred.hpp
index 78fe231..d37b3df 100644
--- a/include/deferred/type_traits/is_deferred.hpp
+++ b/include/deferred/type_traits/is_deferred.hpp
@@ -1,11 +1,5 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_TYPE_TRAITS_IS_DEFERRED_HPP
 #define DEFERRED_TYPE_TRAITS_IS_DEFERRED_HPP
@@ -14,22 +8,20 @@
 
 namespace deferred {
 
-namespace detail {
-
-template<typename T, typename = std::void_t<>>
-struct has_subexpression_types : std::false_type
-{ };
-
+/// Checks if @p T has a nested type @c subexpression_types.
+/// @tparam T The type to check against the concept.
 template<typename T>
-struct has_subexpression_types<T, std::void_t<typename T::subexpression_types>> : std::true_type
-{ };
+concept HasSubexpressionTypes = requires { typename T::subexpression_types; };
 
-} // namespace detail
+/// Concept for types that are deferred expressions.
+/// @tparam T The type to check against the concept.
+template<typename T>
+concept Deferred = HasSubexpressionTypes<std::remove_cvref_t<T>>;
 
 /// Checks if @p T is a type in @c deferred namespace.
 /// @tparam T The type to check.
 template<typename T>
-struct is_deferred : public detail::has_subexpression_types<std::remove_cv_t<T>>
+struct is_deferred : public std::bool_constant<Deferred<T>>
 { };
 
 /// Alias for @c is_deferred_datatype::type.
@@ -42,21 +34,6 @@ using is_deferred_t = typename is_deferred<T>::type;
 template<typename T>
 inline constexpr bool is_deferred_v = is_deferred<T>::value;
 
-/// Concept for types that are deferred expressions.
-/// @tparam T The type to check against the concept.
-template<typename T>
-concept Deferred = is_deferred_v<std::remove_cvref_t<T>>;
-
-/// Checks if any of @p T... satisfies @ref is_deferred.
-/// @tparam T A parameter pack of types to check.
-template<typename... T>
-using any_deferred_t = std::disjunction<is_deferred_t<T>...>;
-
-/// Alias for @c any_deferred_t::value.
-/// @tparam T A parameter pack of types to check.
-template<typename... T>
-inline constexpr bool any_deferred_v = any_deferred_t<T...>::value;
-
 /// Concept for when at least one type in a set is a deferred expression.
 /// @tparam T A parameter pack of types to check.
 template<typename... T>
diff --git a/include/deferred/variable.hpp b/include/deferred/variable.hpp
index 1fd0ea5..780a5a9 100644
--- a/include/deferred/variable.hpp
+++ b/include/deferred/variable.hpp
@@ -1,14 +1,11 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
+
 #ifndef DEFERRED_VARIABLE_HPP
 #define DEFERRED_VARIABLE_HPP
 
+#include <concepts>
+#include <tuple>
 #include <type_traits>
 #include <utility>
 
@@ -28,7 +25,7 @@ class [[nodiscard]] variable_
   using subexpression_types = void;
 
 private:
-  T m_t{};
+  [[no_unique_address]] T m_t{};
 
 public:
   variable_() = default;
@@ -53,28 +50,42 @@ class [[nodiscard]] variable_
   variable_& operator=(variable_ const&) = delete;
   variable_& operator=(variable_&&)      = delete;
 
+  /**
+   * @brief Assigns a value to the variable.
+   */
   constexpr variable_& operator=(T const& t)
   {
     m_t = t;
     return *this;
   }
 
+  /// @copydoc variable_::operator=(T const&)
   constexpr variable_& operator=(T&& t) noexcept
   {
     m_t = std::move(t);
     return *this;
   }
 
-  constexpr T const& operator()() const& noexcept
+  /**
+   * @brief Returns the stored value.
+   */
+  [[nodiscard]] constexpr T const& operator()() const& noexcept
   {
     return m_t;
   }
 
-  constexpr T& operator()() & noexcept
+  /// @copydoc variable_::operator()() const& noexcept
+  [[nodiscard]] constexpr T& operator()() & noexcept
   {
     return m_t;
   }
 
+  /// @copydoc variable_::operator()() const& noexcept
+  [[nodiscard]] constexpr T operator()() && noexcept
+  {
+    return std::move(m_t);
+  }
+
   /**
    * @brief Visits the variable with a visitor.
    * @tparam Visitor The type of the visitor.
@@ -94,9 +105,9 @@ class [[nodiscard]] variable_
  * @return A newly constructed variable.
  */
 template<typename T>
-constexpr variable_<T> variable() noexcept
+[[nodiscard]] constexpr auto variable() noexcept
 {
-  return {};
+  return variable_<T>{};
 }
 
 /**
@@ -110,7 +121,7 @@ constexpr variable_<T> variable() noexcept
  * @return A variable representing the evaluated type of @p t.
  */
 template<typename T>
-constexpr auto variable(T&& t)
+[[nodiscard]] constexpr auto variable(T&& t)
 {
   using result_type = std::decay_t<decltype(recursive_evaluate(std::forward<T>(t)))>;
   return variable_<result_type>(recursive_evaluate(std::forward<T>(t)));
diff --git a/include/deferred/while.hpp b/include/deferred/while.hpp
index 3e7f1aa..6f9459b 100644
--- a/include/deferred/while.hpp
+++ b/include/deferred/while.hpp
@@ -1,15 +1,10 @@
-/** @file */
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #ifndef DEFERRED_WHILE_HPP
 #define DEFERRED_WHILE_HPP
 
+#include <tuple>
 #include <utility>
 
 #include "evaluate.hpp"
@@ -46,7 +41,19 @@ class while_expression
     m_expressions(std::forward<Condition>(condition), std::forward<Body>(body))
   { }
 
-  constexpr void operator()() const
+  /**
+   * @brief Evaluates the while loop.
+   */
+  constexpr void operator()() const&
+  {
+    while (evaluate(std::get<0>(m_expressions)))
+    {
+      evaluate(std::get<1>(m_expressions));
+    }
+  }
+
+  /// @copydoc while_expression::operator()() const&
+  constexpr void operator()() &
   {
     while (evaluate(std::get<0>(m_expressions)))
     {
@@ -54,7 +61,8 @@ class while_expression
     }
   }
 
-  constexpr void operator()()
+  /// @copydoc while_expression::operator()() const&
+  constexpr void operator()() &&
   {
     while (evaluate(std::get<0>(m_expressions)))
     {
@@ -85,10 +93,10 @@ class while_expression
  * @tparam BodyExpression The type of the body expression.
  * @param condition The condition expression.
  * @param body The body expression.
- * @return A \ref while_expression capturing the condition and body.
+ * @return A @ref while_expression capturing the condition and body.
  */
 template<typename ConditionExpression, typename BodyExpression>
-constexpr auto while_(ConditionExpression&& condition, BodyExpression&& body)
+[[nodiscard]] constexpr auto while_(ConditionExpression&& condition, BodyExpression&& body)
 {
   using condition_expression = make_deferred_t<ConditionExpression>;
   using body_expression      = make_deferred_t<BodyExpression>;
diff --git a/scripts/format_changed.sh b/scripts/format_changed.sh
new file mode 100755
index 0000000..bad7a2e
--- /dev/null
+++ b/scripts/format_changed.sh
@@ -0,0 +1,17 @@
+#!/bin/bash
+
+# Find changed files (Added, Copied, Modified, Renamed) in the working tree and staged area
+FILES=$(git diff --name-only --diff-filter=ACMR HEAD | grep -E "\.(cpp|hpp)$")
+
+if [ -z "$FILES" ]; then
+  echo "No changed source files to format."
+  exit 0
+fi
+
+echo "Formatting changed files:"
+for file in $FILES; do
+  if [ -f "$file" ]; then
+    echo "  $file"
+    clang-format -i "$file"
+  fi
+done
diff --git a/test/integration/apply.cpp b/test/integration/apply.cpp
index bc228c6..bd7261e 100644
--- a/test/integration/apply.cpp
+++ b/test/integration/apply.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/integration/conditional.cpp b/test/integration/conditional.cpp
index 8e87676..4d36903 100644
--- a/test/integration/conditional.cpp
+++ b/test/integration/conditional.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/integration/constant.cpp b/test/integration/constant.cpp
index f2515df..ee82195 100644
--- a/test/integration/constant.cpp
+++ b/test/integration/constant.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/integration/main.cpp b/test/integration/main.cpp
index 6a71e87..cb2e4cf 100644
--- a/test/integration/main.cpp
+++ b/test/integration/main.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_session.hpp>
 
diff --git a/test/integration/operators.cpp b/test/integration/operators.cpp
index 975291b..8f430a8 100644
--- a/test/integration/operators.cpp
+++ b/test/integration/operators.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/integration/switch.cpp b/test/integration/switch.cpp
index ec9b6da..ae3e705 100644
--- a/test/integration/switch.cpp
+++ b/test/integration/switch.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <cstring>
 
diff --git a/test/integration/variable.cpp b/test/integration/variable.cpp
index a7e263b..2691c05 100644
--- a/test/integration/variable.cpp
+++ b/test/integration/variable.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/integration/while.cpp b/test/integration/while.cpp
index 2a4c4d5..1067b24 100644
--- a/test/integration/while.cpp
+++ b/test/integration/while.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/conditional.cpp b/test/unit/conditional.cpp
index 71db696..377a544 100644
--- a/test/unit/conditional.cpp
+++ b/test/unit/conditional.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/constant.cpp b/test/unit/constant.cpp
index 2b3256e..1053bb5 100644
--- a/test/unit/constant.cpp
+++ b/test/unit/constant.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/invoke.cpp b/test/unit/invoke.cpp
index eb2513e..9128bdb 100644
--- a/test/unit/invoke.cpp
+++ b/test/unit/invoke.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/main.cpp b/test/unit/main.cpp
index 6a71e87..cb2e4cf 100644
--- a/test/unit/main.cpp
+++ b/test/unit/main.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_session.hpp>
 
diff --git a/test/unit/make_function_object.cpp b/test/unit/make_function_object.cpp
index e79dc3a..03b60d0 100644
--- a/test/unit/make_function_object.cpp
+++ b/test/unit/make_function_object.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/switch.cpp b/test/unit/switch.cpp
index cfd297a..45521ef 100644
--- a/test/unit/switch.cpp
+++ b/test/unit/switch.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/variable.cpp b/test/unit/variable.cpp
index 46f4180..4094339 100644
--- a/test/unit/variable.cpp
+++ b/test/unit/variable.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>
 
diff --git a/test/unit/while.cpp b/test/unit/while.cpp
index 62770b4..914583e 100644
--- a/test/unit/while.cpp
+++ b/test/unit/while.cpp
@@ -1,10 +1,5 @@
-/*
- * Copyright (c) 2019-2020 Yiannis Papadopoulos
- *
- * Distributed under the terms of the MIT License.
- *
- * (See accompanying file LICENSE or copy at http://opensource.org/licenses/MIT)
- */
+// SPDX-FileCopyrightText: 2019-2026 Yiannis Papadopoulos <giannis.papadopoulos@gmail.com>
+// SPDX-License-Identifier: MIT
 
 #include <catch2/catch_test_macros.hpp>