Defaults to external visibility for interfaces.

libsolidity/analysis/StaticAnalyzer.cpp

@@ -50,6 +50,14 @@ void StaticAnalyzer::endVisit(ContractDefinition const&)
 
 bool StaticAnalyzer::visit(FunctionDefinition const& _function)
 {
+	const bool isInterface = m_currentContract->contractKind() == ContractDefinition::ContractKind::Interface;
+	if (_function.noVisibilitySpecified())
+		m_errorReporter.warning(
+			_function.location(),
+			"No visibility specified. Defaulting to \"" +
+			(isInterface ? "external" : Declaration::visibilityToString(_function.visibility())) +
+			"\"."
+		);
 	if (_function.isImplemented())
 		m_currentFunction = &_function;
 	else