Disallow comparison for reference types.

Fixes #2690

libsolidity/Types.h

@@ -409,6 +409,10 @@ public:
 	DataLocation location() const { return m_location; }
 
 	virtual TypePointer unaryOperatorResult(Token::Value _operator) const override;
+	virtual TypePointer binaryOperatorResult(Token::Value, TypePointer const&) const override
+	{
+		return TypePointer();
+	}
 	virtual unsigned memoryHeadSize() const override { return 32; }
 
 	/// @returns a copy of this type with location (recursively) changed to @a _location,

test/libsolidity/SolidityNameAndTypeResolution.cpp

@@ -1899,6 +1899,18 @@ BOOST_AUTO_TEST_CASE(integer_boolean_operators)
 	BOOST_CHECK_THROW(parseTextAndResolveNames(sourceCode3), TypeError);
 }
 
+BOOST_AUTO_TEST_CASE(reference_compare_operators)
+{
+	char const* sourceCode1 = R"(
+		contract test { bytes a; bytes b; function() { a == b; } }
+	)";
+	BOOST_CHECK_THROW(parseTextAndResolveNames(sourceCode1), TypeError);
+	char const* sourceCode2 = R"(
+		contract test { struct s {uint a;}; s x; s y; function() { x == y; } }
+	)";
+	BOOST_CHECK_THROW(parseTextAndResolveNames(sourceCode2), TypeError);
+}
+
 BOOST_AUTO_TEST_CASE(overwrite_memory_location_external)
 {
 	char const* sourceCode = R"(