Index: src/AST/DeclReplacer.hpp
===================================================================
--- src/AST/DeclReplacer.hpp	(revision 148f836e345bb97af81bad4b63dcd8dfd9e1a3b0)
+++ src/AST/DeclReplacer.hpp	(revision 065867274d8bd8d2dc7817a83c4af363d233de03)
@@ -18,21 +18,26 @@
 #include <unordered_map>
 
-#include "Node.hpp"
+namespace ast {
+	class DeclWithType;
+	class Expr;
+	class Node;
+	class TypeDecl;
+}
 
 namespace ast {
-	class DeclWithType;
-	class TypeDecl;
-	class Expr;
 
-	namespace DeclReplacer {
-		using DeclMap = std::unordered_map< const DeclWithType *, const DeclWithType * >;
-		using TypeMap = std::unordered_map< const TypeDecl *, const TypeDecl * >;
-		using ExprMap = std::unordered_map< const DeclWithType *, const Expr * >;
+namespace DeclReplacer {
 
-		const Node * replace( const Node * node, const DeclMap & declMap, bool debug = false );
-		const Node * replace( const Node * node, const TypeMap & typeMap, bool debug = false );
-		const Node * replace( const Node * node, const DeclMap & declMap, const TypeMap & typeMap, bool debug = false );
-		const Node * replace( const Node * node, const ExprMap & exprMap);
-	}
+using DeclMap = std::unordered_map< const DeclWithType *, const DeclWithType * >;
+using TypeMap = std::unordered_map< const TypeDecl *, const TypeDecl * >;
+using ExprMap = std::unordered_map< const DeclWithType *, const Expr * >;
+
+const Node * replace( const Node * node, const DeclMap & declMap, bool debug = false );
+const Node * replace( const Node * node, const TypeMap & typeMap, bool debug = false );
+const Node * replace( const Node * node, const DeclMap & declMap, const TypeMap & typeMap, bool debug = false );
+const Node * replace( const Node * node, const ExprMap & exprMap);
+
+}
+
 }
 
Index: src/AST/Util.cpp
===================================================================
--- src/AST/Util.cpp	(revision 148f836e345bb97af81bad4b63dcd8dfd9e1a3b0)
+++ src/AST/Util.cpp	(revision 065867274d8bd8d2dc7817a83c4af363d233de03)
@@ -83,4 +83,23 @@
 }
 
+/// Check that the MemberExpr has an aggregate type and matching member.
+void memberMatchesAggregate( const MemberExpr * expr ) {
+	const Type * aggrType = expr->aggregate->result->stripReferences();
+	const AggregateDecl * decl = nullptr;
+	if ( auto inst = dynamic_cast<const StructInstType *>( aggrType ) ) {
+		decl = inst->base;
+	} else if ( auto inst = dynamic_cast<const UnionInstType *>( aggrType ) ) {
+		decl = inst->base;
+	}
+	assertf( decl, "Aggregate of member not correct type." );
+
+	for ( auto aggrMember : decl->members ) {
+		if ( expr->member == aggrMember ) {
+			return;
+		}
+	}
+	assertf( false, "Member not found." );
+}
+
 struct InvariantCore {
 	// To save on the number of visits: this is a kind of composed core.
@@ -108,4 +127,9 @@
 	}
 
+	void previsit( const MemberExpr * node ) {
+		previsit( (const ParseNode *)node );
+		memberMatchesAggregate( node );
+	}
+
 	void postvisit( const Node * node ) {
 		no_strong_cycles.postvisit( node );