Constructors are defined using the `constructor` keyword.

libsolidity/analysis/SyntaxChecker.cpp

@@ -216,7 +216,22 @@ bool SyntaxChecker::visit(FunctionDefinition const& _function)
 
 	if (v050 && _function.noVisibilitySpecified())
 		m_errorReporter.syntaxError(_function.location(), "No visibility specified.");
-			
+
+	if (_function.isOldStyleConstructor())
+	{
+		if (v050)
+			m_errorReporter.syntaxError(
+				_function.location(),
+				"Functions are not allowed to have the same name as the contract. "
+				"If you intend this to be a constructor, use \"constructor(...) { ... }\" to define it."
+			);
+		else
+			m_errorReporter.warning(
+				_function.location(),
+				"Defining constructors as functions with the same name as the contract is deprecated. "
+				"Use \"constructor(...) { ... }\" instead."
+			);
+	}
 	return true;
 }
 

libsolidity/ast/AST.h

@@ -607,7 +607,8 @@ public:
 
 	StateMutability stateMutability() const { return m_stateMutability; }
 	bool isConstructor() const { return m_isConstructor; }
-	bool isFallback() const { return name().empty(); }
+	bool isOldStyleConstructor() const { return m_isConstructor && !name().empty(); }
+	bool isFallback() const { return !m_isConstructor && name().empty(); }
 	bool isPayable() const { return m_stateMutability == StateMutability::Payable; }
 	std::vector<ASTPointer<ModifierInvocation>> const& modifiers() const { return m_functionModifiers; }
 	std::vector<ASTPointer<VariableDeclaration>> const& returnParameters() const { return m_returnParameters->parameters(); }

libsolidity/parsing/Parser.cpp

@@ -238,7 +238,10 @@ ASTPointer<ContractDefinition> Parser::parseContractDefinition(Token::Value _exp
 		Token::Value currentTokenValue = m_scanner->currentToken();
 		if (currentTokenValue == Token::RBrace)
 			break;
-		else if (currentTokenValue == Token::Function)
+		else if (
+			currentTokenValue == Token::Function ||
+			(currentTokenValue == Token::Identifier && m_scanner->currentLiteral() == "constructor")
+		)
 			// This can be a function or a state variable of function type (especially
 			// complicated to distinguish fallback function from function type state variable)
 			subNodes.push_back(parseFunctionDefinitionOrFunctionTypeStateVariable(name.get()));
@@ -333,9 +336,19 @@ Parser::FunctionHeaderParserResult Parser::parseFunctionHeader(bool _forceEmptyN
 {
 	RecursionGuard recursionGuard(*this);
 	FunctionHeaderParserResult result;
-	expectToken(Token::Function);
-	if (_forceEmptyName || m_scanner->currentToken() == Token::LParen)
-		result.name = make_shared<ASTString>(); // anonymous function
+
+	if (m_scanner->currentToken() == Token::Function)
+		// In case of old style constructors, i.e. functions with the same name as the contract,
+		// this is set to true later in parseFunctionDefinitionOrFunctionTypeStateVariable.
+		result.isConstructor = false;
+	else if (m_scanner->currentToken() == Token::Identifier && m_scanner->currentLiteral() == "constructor")
+		result.isConstructor = true;
+	else
+		solAssert(false, "Function or constructor expected.");
+	m_scanner->next();
+
+	if (result.isConstructor || _forceEmptyName || m_scanner->currentToken() == Token::LParen)
+		result.name = make_shared<ASTString>();
 	else
 		result.name = expectIdentifierToken();
 	VarDeclParserOptions options;
@@ -426,12 +439,13 @@ ASTPointer<ASTNode> Parser::parseFunctionDefinitionOrFunctionTypeStateVariable(A
 		}
 		else
 			m_scanner->next(); // just consume the ';'
-		bool const c_isConstructor = (_contractName && *header.name == *_contractName);
+		if (_contractName && *header.name == *_contractName)
+			header.isConstructor = true;
 		return nodeFactory.createNode<FunctionDefinition>(
 			header.name,
 			header.visibility,
 			header.stateMutability,
-			c_isConstructor,
+			header.isConstructor,
 			docstring,
 			header.parameters,
 			header.modifiers,

libsolidity/parsing/Parser.h

@@ -56,6 +56,7 @@ private:
 	/// This struct is shared for parsing a function header and a function type.
 	struct FunctionHeaderParserResult
 	{
+		bool isConstructor;
 		ASTPointer<ASTString> name;
 		ASTPointer<ParameterList> parameters;
 		ASTPointer<ParameterList> returnParameters;