Treat returns with expressions as return param use

There are many cases of code where the return parameters exist mostly
as a form of documentation.  This change ensures that they do not have
to be used in the function body so long as there is a return supplying
values

libsolidity/analysis/StaticAnalyzer.cpp

@@ -50,6 +50,7 @@ bool StaticAnalyzer::visit(FunctionDefinition const& _function)
 {
 	if (_function.isImplemented())
 		m_inFunction = true;
+	m_currentFunction = &_function;
 	m_localVarUseCount.clear();
 	m_nonPayablePublic = _function.isPublic() && !_function.isPayable();
 	return true;
@@ -96,6 +97,17 @@ bool StaticAnalyzer::visit(VariableDeclaration const& _variable)
 	return true;
 }
 
+bool StaticAnalyzer::visit(Return const& _return)
+{
+	// If the return has an expression, it counts as
+	// a "use" of the return parameters.
+	if (m_inFunction && _return.expression() != NULL)
+		for (auto const& var: m_currentFunction->returnParameterList()->parameters())
+			if (var->name() != "")
+				m_localVarUseCount[var.get()] += 1;
+	return true;
+}
+
 bool StaticAnalyzer::visit(ExpressionStatement const& _statement)
 {
 	if (_statement.expression().annotation().isPure)

libsolidity/analysis/StaticAnalyzer.h

@@ -63,7 +63,7 @@ private:
 	virtual bool visit(ExpressionStatement const& _statement) override;
 	virtual bool visit(VariableDeclaration const& _variable) override;
 	virtual bool visit(Identifier const& _identifier) override;
-
+	virtual bool visit(Return const& _return) override;
 	virtual bool visit(MemberAccess const& _memberAccess) override;
 
 	ErrorList& m_errors;
@@ -76,6 +76,7 @@ private:
 
 	std::map<VariableDeclaration const*, int> m_localVarUseCount;
 
+	const FunctionDefinition *m_currentFunction;
 	bool m_inFunction = false;
 };
 

test/libsolidity/SolidityNameAndTypeResolution.cpp

@@ -5663,6 +5663,22 @@ BOOST_AUTO_TEST_CASE(warn_unused_return_param)
 		}
 	)";
 	success(text);
+	text = R"(
+		contract C {
+			function f() returns (uint a) {
+				a = 1;
+			}
+		}
+	)";
+	success(text);
+	text = R"(
+		contract C {
+			function f() returns (uint a) {
+				return 1;
+			}
+		}
+	)";
+	success(text);
 }
 
 BOOST_AUTO_TEST_CASE(no_unused_warnings)