C++: Introduce SizeofPackOperator subclasses for expressions and types

Note that template template parameters are considered types in this context.

Author: jketema

cpp/ql/lib/change-notes/2024-12-20-sizeof-pack.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* A new classes `SizeofPackExprOperator` and `SizeofPackTypeOperator` were introduced, which represent the C++ `sizeof...` operator taking an expression and a type argument, respectively.

cpp/ql/lib/semmle/code/cpp/PrintAST.qll

@@ -1056,6 +1056,8 @@ private predicate namedExprChildPredicates(Expr expr, Element ele, string pred)
     or
     expr.(SizeofExprOperator).getExprOperand() = ele and pred = "getExprOperand()"
     or
+    expr.(SizeofPackExprOperator).getExprOperand() = ele and pred = "getExprOperand()"
+    or
     expr.(StmtExpr).getStmt() = ele and pred = "getStmt()"
     or
     expr.(ThrowExpr).getExpr() = ele and pred = "getExpr()"

cpp/ql/lib/semmle/code/cpp/exprs/Cast.qll

@@ -684,7 +684,8 @@ class TypeidOperator extends Expr, @type_id {
 }
 
 /**
- * A C++11 `sizeof...` expression which determines the size of a template parameter pack.
+ * A C++11 `sizeof...` expression which determines the size of a template
+ * parameter pack.
  *
  * This expression only appears in templates themselves - in any actual
  * instantiations, "sizeof...(x)" will be replaced by its integer value.
@@ -694,15 +695,56 @@ class TypeidOperator extends Expr, @type_id {
  * ```
  */
 class SizeofPackOperator extends Expr, @sizeof_pack {
-  override string toString() { result = "sizeof...(...)" }
-
-  override string getAPrimaryQlClass() { result = "SizeofPackOperator" }
-
   override predicate mayBeImpure() { none() }
 
   override predicate mayBeGloballyImpure() { none() }
 }
 
+/**
+ * A C++11 `sizeof...` expression which determines the size of a template
+ * parameter pack and whose operand is an expression.
+ *
+ * This expression only appears in templates themselves - in any actual
+ * instantiations, "sizeof...(x)" will be replaced by its integer value.
+ * ```
+ * template < typename... T >
+ * int count ( T &&... t ) { return sizeof... ( t ); }
+ * ```
+ */
+class SizeofPackExprOperator extends SizeofPackOperator {
+  SizeofPackExprOperator() { exists(this.getChild(0)) }
+
+  override string getAPrimaryQlClass() { result = "SizeofPackExprOperator" }
+
+  /** Gets the contained expression. */
+  Expr getExprOperand() { result = this.getChild(0) }
+
+  override string toString() { result = "sizeof...(<expr>)" }
+}
+
+/**
+ * A C++11 `sizeof...` expression which determines the size of a template
+ * parameter pack and whose operand is an type name or a template template
+ * parameter.
+ *
+ * This expression only appears in templates themselves - in any actual
+ * instantiations, "sizeof...(x)" will be replaced by its integer value.
+ * ```
+ * template < typename... T >
+ * int count ( T &&... t ) { return sizeof... ( T ); }
+ * ```
+ */
+class SizeofPackTypeOperator extends SizeofPackOperator {
+  SizeofPackTypeOperator() { sizeof_bind(underlyingElement(this), _) }
+
+  override string getAPrimaryQlClass() { result = "SizeofPackTypeOperator" }
+
+  /** Gets the contained type. */
+  Type getTypeOperand() { sizeof_bind(underlyingElement(this), unresolveElement(result)) }
+
+  override string toString() { result = "sizeof...(" + this.getTypeOperand().getName() + ")" }
+}
+
 /**
  * A C/C++ sizeof expression.
  */