Warn about unused return values.

libsolidity/analysis/TypeChecker.cpp

@@ -830,6 +830,13 @@ void TypeChecker::endVisit(ExpressionStatement const& _statement)
 	if (type(_statement.expression())->category() == Type::Category::RationalNumber)
 		if (!dynamic_cast<RationalNumberType const&>(*type(_statement.expression())).mobileType())
 			typeError(_statement.expression().location(), "Invalid rational number.");
+
+	bool unusedReturnValue = true;
+	if (auto t = dynamic_cast<TupleType const*>(type(_statement.expression()).get()))
+		if (t->components().empty())
+			unusedReturnValue = false;
+	if (unusedReturnValue)
+		warning(_statement.location(), "Unused return value.");
 }
 
 bool TypeChecker::visit(Conditional const& _conditional)
@@ -1570,6 +1577,16 @@ void TypeChecker::typeError(SourceLocation const& _location, string const& _desc
 	m_errors.push_back(err);
 }
 
+void TypeChecker::warning(SourceLocation const& _location, string const& _description)
+{
+	auto err = make_shared<Error>(Error::Type::Warning);
+	*err <<
+		errinfo_sourceLocation(_location) <<
+		errinfo_comment(_description);
+
+	m_errors.push_back(err);
+}
+
 void TypeChecker::fatalTypeError(SourceLocation const& _location, string const& _description)
 {
 	typeError(_location, _description);

libsolidity/analysis/TypeChecker.h

@@ -59,6 +59,9 @@ private:
 	/// Adds a new error to the list of errors.
 	void typeError(SourceLocation const& _location, std::string const& _description);
 
+	/// Adds a new warning to the list of errors.
+	void warning(SourceLocation const& _location, std::string const& _description);
+
 	/// Adds a new error to the list of errors and throws to abort type checking.
 	void fatalTypeError(SourceLocation const& _location, std::string const& _description);