Moved a canBeStored assert for struct members to TypeChecker

This is to avoid a assert from failing for forward declared user defined value types.

Author: hrkrshnn

libsolidity/analysis/DeclarationTypeChecker.cpp

@@ -100,7 +100,6 @@ bool DeclarationTypeChecker::visit(StructDefinition const& _struct)
 		m_recursiveStructSeen = false;
 		member->accept(*this);
 		solAssert(member->annotation().type, "");
-		solAssert(member->annotation().type->canBeStored(), "Type cannot be used in struct.");
 		if (m_recursiveStructSeen)
 			hasRecursiveChild = true;
 	}

libsolidity/analysis/TypeChecker.cpp

@@ -621,6 +621,16 @@ bool TypeChecker::visit(VariableDeclaration const& _variable)
 	return false;
 }
 
+void TypeChecker::endVisit(StructDefinition const& _struct)
+{
+	for (auto const& member: _struct.members())
+		solAssert(
+			member->annotation().type &&
+			member->annotation().type->canBeStored(),
+			"Type cannot be used in struct."
+		);
+}
+
 void TypeChecker::visitManually(
 	ModifierInvocation const& _modifier,
 	vector<ContractDefinition const*> const& _bases

libsolidity/analysis/TypeChecker.h

@@ -121,6 +121,7 @@ class TypeChecker: private ASTConstVisitor
 	bool visit(FunctionDefinition const& _function) override;
 	void endVisit(ArrayTypeName const& _typeName) override;
 	bool visit(VariableDeclaration const& _variable) override;
+	void endVisit(StructDefinition const& _struct) override;
 	/// We need to do this manually because we want to pass the bases of the current contract in
 	/// case this is a base constructor call.
 	void visitManually(ModifierInvocation const& _modifier, std::vector<ContractDefinition const*> const& _bases);

test/libsolidity/syntaxTests/userDefinedValueType/forward_reference_struct.sol

@@ -0,0 +1,3 @@
+struct S { U u; }
+contract C { S s; }
+type U is address;