diff --git a/libyul/CMakeLists.txt b/libyul/CMakeLists.txt
index df7f087e1..a02b3d592 100644
--- a/libyul/CMakeLists.txt
+++ b/libyul/CMakeLists.txt
@@ -160,6 +160,8 @@ add_library(yul
 	optimiser/RedundantAssignEliminator.h
 	optimiser/Rematerialiser.cpp
 	optimiser/Rematerialiser.h
+	optimiser/Scoper.h
+	optimiser/Scoper.cpp
 	optimiser/SSAReverser.cpp
 	optimiser/SSAReverser.h
 	optimiser/SSATransform.cpp
diff --git a/libyul/optimiser/Scoper.cpp b/libyul/optimiser/Scoper.cpp
new file mode 100644
index 000000000..48647ce32
--- /dev/null
+++ b/libyul/optimiser/Scoper.cpp
@@ -0,0 +1,88 @@
+/*
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+// SPDX-License-Identifier: GPL-3.0
+
+#include <libyul/optimiser/Scoper.h>
+
+#include <libyul/AST.h>
+#include <libyul/Exceptions.h>
+
+#include <libsolutil/CommonData.h>
+
+#include <boost/range/adaptor/reversed.hpp>
+
+#include <set>
+
+using namespace solidity::yul;
+using namespace solidity::util;
+using namespace std;
+
+void Scoper::operator()(VariableDeclaration& _varDecl)
+{
+	set<YulString> names;
+	for (auto const& var: _varDecl.variables)
+		names.emplace(var.name);
+	m_variableScopes.back().variables += names;
+
+	ASTModifier::operator()(_varDecl);
+}
+
+void Scoper::operator()(FunctionDefinition& _fun)
+{
+	pushScope(true);
+	for (auto const& parameter: _fun.parameters)
+		m_variableScopes.back().variables.emplace(parameter.name);
+	for (auto const& var: _fun.returnVariables)
+	{
+		m_variableScopes.back().variables.emplace(var.name);
+	}
+
+	ASTModifier::operator()(_fun);
+
+	popScope();
+}
+
+void Scoper::operator()(Block& _block)
+{
+	size_t numScopes = m_variableScopes.size();
+	pushScope(false);
+	ASTModifier::operator()(_block);
+	popScope();
+	yulAssert(numScopes == m_variableScopes.size(), "");
+}
+
+void Scoper::pushScope(bool _functionScope)
+{
+	m_variableScopes.emplace_back(_functionScope);
+}
+
+void Scoper::popScope()
+{
+	m_variableScopes.pop_back();
+}
+
+bool Scoper::inScope(YulString _variableName) const
+{
+	for (auto const& scope: m_variableScopes | boost::adaptors::reversed)
+	{
+		if (scope.variables.count(_variableName))
+			return true;
+		if (scope.isFunction)
+			return false;
+	}
+	return false;
+}
diff --git a/libyul/optimiser/Scoper.h b/libyul/optimiser/Scoper.h
new file mode 100644
index 000000000..a5dd679a1
--- /dev/null
+++ b/libyul/optimiser/Scoper.h
@@ -0,0 +1,56 @@
+/*
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+// SPDX-License-Identifier: GPL-3.0
+/**
+ * Base class that assigns scope to Yul AST
+ */
+#include <libyul/optimiser/ASTWalker.h>
+
+#include <libyul/AST.h>
+
+namespace solidity::yul
+{
+
+class Scoper: public ASTModifier
+{
+public:
+	using ASTModifier::operator();
+
+	void operator()(VariableDeclaration& _varDecl) override;
+	void operator()(FunctionDefinition& _fun) override;
+	void operator()(Block& _block) override;
+
+protected:
+	/// Returns true iff the variable is in scope.
+	bool inScope(YulString _variableName) const;
+
+	/// Creates a new inner scope.
+	void pushScope(bool _functionScope);
+	/// Removes the innermost scope and clears all variables in it.
+	virtual void popScope();
+
+	struct Scope
+	{
+		explicit Scope(bool _isFunction): isFunction(_isFunction) {}
+		std::set<YulString> variables;
+		bool isFunction;
+	};
+
+	std::vector<Scope> m_variableScopes;
+};
+
+}