diff --git a/cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/IRBlock.qll b/cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/IRBlock.qll
index 58bd5b0a8cb..ba5e562a71e 100644
--- a/cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/IRBlock.qll
+++ b/cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/IRBlock.qll
@@ -269,7 +269,9 @@ module IRCfg implements BB::CfgSig<Language::Location> {
 
   class SuccessorType = EdgeKind;
 
-  class BasicBlock extends IRBlock {
+  final private class FinalIRBlock = IRBlock;
+
+  class BasicBlock extends FinalIRBlock {
     ControlFlowNode getNode(int i) { result = this.getInstruction(i) }
 
     ControlFlowNode getLastNode() { result = super.getLastInstruction() }
diff --git a/cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/IRBlock.qll b/cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/IRBlock.qll
index 58bd5b0a8cb..ba5e562a71e 100644
--- a/cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/IRBlock.qll
+++ b/cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/IRBlock.qll
@@ -269,7 +269,9 @@ module IRCfg implements BB::CfgSig<Language::Location> {
 
   class SuccessorType = EdgeKind;
 
-  class BasicBlock extends IRBlock {
+  final private class FinalIRBlock = IRBlock;
+
+  class BasicBlock extends FinalIRBlock {
     ControlFlowNode getNode(int i) { result = this.getInstruction(i) }
 
     ControlFlowNode getLastNode() { result = super.getLastInstruction() }
diff --git a/cpp/ql/lib/semmle/code/cpp/ir/implementation/unaliased_ssa/IRBlock.qll b/cpp/ql/lib/semmle/code/cpp/ir/implementation/unaliased_ssa/IRBlock.qll
index 58bd5b0a8cb..ba5e562a71e 100644
--- a/cpp/ql/lib/semmle/code/cpp/ir/implementation/unaliased_ssa/IRBlock.qll
+++ b/cpp/ql/lib/semmle/code/cpp/ir/implementation/unaliased_ssa/IRBlock.qll
@@ -269,7 +269,9 @@ module IRCfg implements BB::CfgSig<Language::Location> {
 
   class SuccessorType = EdgeKind;
 
-  class BasicBlock extends IRBlock {
+  final private class FinalIRBlock = IRBlock;
+
+  class BasicBlock extends FinalIRBlock {
     ControlFlowNode getNode(int i) { result = this.getInstruction(i) }
 
     ControlFlowNode getLastNode() { result = super.getLastInstruction() }