Fix and test for not really recursive structs.

Fixes #2223.

libsolidity/AST.cpp

@@ -21,6 +21,7 @@
  */
 
 #include <algorithm>
+#include <functional>
 #include <boost/range/adaptor/reversed.hpp>
 #include <libsolidity/Utils.h>
 #include <libsolidity/AST.h>
@@ -434,23 +435,29 @@ void StructDefinition::checkMemberTypes() const
 
 void StructDefinition::checkRecursion() const
 {
-	set<StructDefinition const*> definitionsSeen;
-	vector<StructDefinition const*> queue = {this};
-	while (!queue.empty())
+	using StructPointer = StructDefinition const*;
+	using StructPointersSet = set<StructPointer>;
+	function<void(StructPointer,StructPointersSet const&)> check = [&](StructPointer _struct, StructPointersSet const& _parents)
 	{
-		StructDefinition const* def = queue.back();
-		queue.pop_back();
-		if (definitionsSeen.count(def))
-			BOOST_THROW_EXCEPTION(ParserError() << errinfo_sourceLocation(def->getLocation())
-												<< errinfo_comment("Recursive struct definition."));
-		definitionsSeen.insert(def);
-		for (ASTPointer<VariableDeclaration> const& member: def->getMembers())
+		if (_parents.count(_struct))
+			BOOST_THROW_EXCEPTION(
+				ParserError() <<
+				errinfo_sourceLocation(_struct->getLocation()) <<
+				errinfo_comment("Recursive struct definition.")
+			);
+		set<StructDefinition const*> parents = _parents;
+		parents.insert(_struct);
+		for (ASTPointer<VariableDeclaration> const& member: _struct->getMembers())
 			if (member->getType()->getCategory() == Type::Category::Struct)
 			{
-				UserDefinedTypeName const& typeName = dynamic_cast<UserDefinedTypeName const&>(*member->getTypeName());
-				queue.push_back(&dynamic_cast<StructDefinition const&>(*typeName.getReferencedDeclaration()));
+				auto const& typeName = dynamic_cast<UserDefinedTypeName const&>(*member->getTypeName());
+				check(
+					&dynamic_cast<StructDefinition const&>(*typeName.getReferencedDeclaration()),
+					parents
+				);
 			}
-	}
+	};
+	check(this, {});
 }
 
 TypePointer EnumDefinition::getType(ContractDefinition const*) const

test/libsolidity/SolidityNameAndTypeResolution.cpp

@@ -190,6 +190,17 @@ BOOST_AUTO_TEST_CASE(struct_definition_indirectly_recursive)
 	BOOST_CHECK_THROW(parseTextAndResolveNames(text), ParserError);
 }
 
+BOOST_AUTO_TEST_CASE(struct_definition_not_really_recursive)
+{
+	char const* text = R"(
+		contract test {
+			struct s1 { uint a; }
+			struct s2 { s1 x; s1 y; }
+		}
+	)";
+	BOOST_CHECK_NO_THROW(parseTextAndResolveNames(text));
+}
+
 BOOST_AUTO_TEST_CASE(struct_definition_recursion_via_mapping)
 {
 	char const* text = "contract test {\n"