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diff --git a/src/catch2/internal/catch_lazy_expr.hpp b/src/catch2/internal/catch_lazy_expr.hpp
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+++ b/src/catch2/internal/catch_lazy_expr.hpp
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+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+#ifndef CATCH_LAZY_EXPR_HPP_INCLUDED
+#define CATCH_LAZY_EXPR_HPP_INCLUDED
+
+#include <iosfwd>
+
+namespace Catch {
+
+    class ITransientExpression;
+
+    class LazyExpression {
+        friend class AssertionHandler;
+        friend struct AssertionStats;
+        friend class RunContext;
+
+        ITransientExpression const* m_transientExpression = nullptr;
+        bool m_isNegated;
+    public:
+        constexpr LazyExpression( bool isNegated ):
+            m_isNegated(isNegated)
+        {}
+        constexpr LazyExpression(LazyExpression const& other) = default;
+        LazyExpression& operator = ( LazyExpression const& ) = delete;
+
+        constexpr explicit operator bool() const {
+            return m_transientExpression != nullptr;
+        }
+
+        friend auto operator << ( std::ostream& os, LazyExpression const& lazyExpr ) -> std::ostream&;
+    };
+
+} // namespace Catch
+
+#endif // CATCH_LAZY_EXPR_HPP_INCLUDED