C++: Add instruction for CheckedConvertOrNull and handle it in alias analysis and data flow

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -268,6 +268,7 @@ private predicate simpleInstructionLocalFlowStep(Instruction iFrom, Instruction
   iTo.(PhiInstruction).getAnOperand().getDef() = iFrom or
   // Treat all conversions as flow, even conversions between different numeric types.
   iTo.(ConvertInstruction).getUnary() = iFrom or
+  iTo.(CheckedConvertOrNullInstruction).getUnary() = iFrom or
   iTo.(InheritanceConversionInstruction).getUnary() = iFrom or
   // A chi instruction represents a point where a new value (the _partial_
   // operand) may overwrite an old value (the _total_ operand), but the alias

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/Instruction.qll

@@ -947,6 +947,10 @@ class ConvertInstruction extends UnaryInstruction {
   ConvertInstruction() { getOpcode() instanceof Opcode::Convert }
 }
 
+class CheckedConvertOrNullInstruction extends UnaryInstruction {
+  CheckedConvertOrNullInstruction() { getOpcode() instanceof Opcode::CheckedConvertOrNull }
+}
+
 /**
  * Represents an instruction that converts between two addresses
  * related by inheritance.

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/AliasAnalysis.qll

@@ -96,6 +96,10 @@ private predicate operandIsPropagated(Operand operand, IntValue bitOffset) {
         bitOffset = Ints::mul(convert.getDerivation().getByteOffset(), 8)
       )
       or
+      // Conversion using dynamic_cast results in an unknown offset
+      instr instanceof CheckedConvertOrNullInstruction and
+      bitOffset = Ints::unknown()
+      or
       // Converting to a derived class subtracts the offset of the base class.
       exists(ConvertToDerivedInstruction convert |
         convert = instr and

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/Instruction.qll

@@ -947,6 +947,10 @@ class ConvertInstruction extends UnaryInstruction {
   ConvertInstruction() { getOpcode() instanceof Opcode::Convert }
 }
 
+class CheckedConvertOrNullInstruction extends UnaryInstruction {
+  CheckedConvertOrNullInstruction() { getOpcode() instanceof Opcode::CheckedConvertOrNull }
+}
+
 /**
  * Represents an instruction that converts between two addresses
  * related by inheritance.

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/Instruction.qll

@@ -947,6 +947,10 @@ class ConvertInstruction extends UnaryInstruction {
   ConvertInstruction() { getOpcode() instanceof Opcode::Convert }
 }
 
+class CheckedConvertOrNullInstruction extends UnaryInstruction {
+  CheckedConvertOrNullInstruction() { getOpcode() instanceof Opcode::CheckedConvertOrNull }
+}
+
 /**
  * Represents an instruction that converts between two addresses
  * related by inheritance.

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/internal/AliasAnalysis.qll

@@ -96,6 +96,10 @@ private predicate operandIsPropagated(Operand operand, IntValue bitOffset) {
         bitOffset = Ints::mul(convert.getDerivation().getByteOffset(), 8)
       )
       or
+      // Conversion using dynamic_cast results in an unknown offset
+      instr instanceof CheckedConvertOrNullInstruction and
+      bitOffset = Ints::unknown()
+      or
       // Converting to a derived class subtracts the offset of the base class.
       exists(ConvertToDerivedInstruction convert |
         convert = instr and