C++: Add TranslatedElement::isIRConstant

Now that there exist constants with no QL-representable value, we need
to make sure they're not treated as constants in the IR.

Author: jbj

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/TranslatedElement.qll

@@ -49,6 +49,13 @@ private Element getRealParent(Expr expr) {
   )
 }
 
+/**
+ * Holds if `expr` is a constant of a type that can be replaced directly with
+ * its value in the IR. This does not include address constants as we have no
+ * means to express those as QL values.
+ */
+predicate isIRConstant(Expr expr) { exists(expr.getValue()) }
+
 /**
  * Holds if `expr` and all of its descendants should be ignored for the purposes
  * of IR generation due to some property of `expr` itself. Unlike
@@ -63,7 +70,7 @@ private predicate ignoreExprAndDescendants(Expr expr) {
   getRealParent(expr) instanceof SwitchCase or
   // Ignore descendants of constant expressions, since we'll just substitute the
   // constant value.
-  getRealParent(expr).(Expr).isConstant() or
+  isIRConstant(getRealParent(expr)) or
   // The `DestructorCall` node for a `DestructorFieldDestruction` has a `FieldAccess`
   // node as its qualifier, but that `FieldAccess` does not have a child of its own.
   // We'll ignore that `FieldAccess`, and supply the receiver as part of the calling
@@ -146,7 +153,7 @@ private predicate translateStmt(Stmt stmt) {
  */
 private predicate isNativeCondition(Expr expr) {
   expr instanceof BinaryLogicalOperation and
-  not expr.isConstant()
+  not isIRConstant(expr)
 }
 
 /**
@@ -159,7 +166,7 @@ private predicate isFlexibleCondition(Expr expr) {
     expr instanceof NotExpr
   ) and
   usedAsCondition(expr) and
-  not expr.isConstant()
+  not isIRConstant(expr)
 }
 
 /**

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/TranslatedExpr.qll

@@ -962,7 +962,7 @@ class TranslatedFunctionAccess extends TranslatedNonConstantExpr {
 abstract class TranslatedNonConstantExpr extends TranslatedCoreExpr {
   TranslatedNonConstantExpr() {
     this = TTranslatedValueExpr(expr) and
-    not expr.isConstant()
+    not isIRConstant(expr)
   }
 }
 
@@ -974,7 +974,7 @@ abstract class TranslatedNonConstantExpr extends TranslatedCoreExpr {
 abstract class TranslatedConstantExpr extends TranslatedCoreExpr {
   TranslatedConstantExpr() {
     this = TTranslatedValueExpr(expr) and
-    expr.isConstant()
+    isIRConstant(expr)
   }
 
   override final Instruction getFirstInstruction() {