C++: Add predicates to support C++23 multidimensional subscript operators

cpp/ql/lib/change-notes/2026-01-08-multidimensional-subscript-operator-1.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* Predicates `getArrayOffset/1` and `getAnArrayOffset` have been added to the `OverloadedArrayExpr` class to support C++23 multidimensional subscript operators.

cpp/ql/lib/change-notes/2026-01-08-multidimensional-subscript-operator-2.md

@@ -0,0 +1,4 @@
+---
+category: deprecated
+---
+* The `OverloadedArrayExpr::getArrayOffset/0` predicate has been deprecated. Use `OverloadedArrayExpr::getArrayOffset/1` and `OverloadedArrayExpr::getAnArrayOffset` instead.

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

@@ -1050,10 +1050,10 @@ private predicate namedExprChildPredicates(Expr expr, Element ele, string pred)
     expr.(Call).getQualifier() = ele and
     pred = "getQualifier()"
     or
-    // OverloadedArrayExpr::getArrayBase/0 and OverloadedArrayExpr::getArrayOffset/0 also consider arguments, and are already handled below.
+    // OverloadedArrayExpr::getArrayBase/0 and OverloadedArrayExpr::getArrayOffset/1 also consider arguments, and are already handled below.
     exists(int n, Expr arg | expr.(Call).getArgument(n) = arg |
       not expr.(OverloadedArrayExpr).getArrayBase() = arg and
-      not expr.(OverloadedArrayExpr).getArrayOffset() = arg and
+      not expr.(OverloadedArrayExpr).getAnArrayOffset() = arg and
       arg = ele and
       pred = "getArgument(" + n.toString() + ")"
     )
@@ -1062,7 +1062,10 @@ private predicate namedExprChildPredicates(Expr expr, Element ele, string pred)
     or
     expr.(OverloadedArrayExpr).getArrayBase() = ele and pred = "getArrayBase()"
     or
-    expr.(OverloadedArrayExpr).getArrayOffset() = ele and pred = "getArrayOffset()"
+    exists(int n |
+      expr.(OverloadedArrayExpr).getArrayOffset(n) = ele and
+      pred = "getArrayOffset(" + n.toString() + ")"
+    )
     or
     // OverloadedPointerDereferenceExpr::getExpr/0 also considers qualifiers, and is already handled above for all Call classes.
     not expr.(OverloadedPointerDereferenceExpr).getQualifier() =

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

@@ -387,10 +387,23 @@ class OverloadedArrayExpr extends FunctionCall {
 
   /**
    * Gets the expression giving the index.
+   *
+   * DEPRECATED: Use getArrayOffset/1 instead.
+   */
+  deprecated Expr getArrayOffset() { result = this.getArrayOffset(0) }
+
+  /**
+   * Gets the expression giving the nth index.
    */
-  Expr getArrayOffset() {
-    if exists(this.getQualifier()) then result = this.getChild(0) else result = this.getChild(1)
+  Expr getArrayOffset(int n) {
+    n >= 0 and
+    if exists(this.getQualifier()) then result = this.getChild(n) else result = this.getChild(n + 1)
   }
+
+  /**
+   * Gets an expression giving an index.
+   */
+  Expr getAnArrayOffset() { result = this.getArrayOffset(_) }
 }
 
 /**

cpp/ql/src/jsf/lib/section_4_21_Operators/AV_Rule_166.qll

@@ -13,7 +13,7 @@ class SizeofImpureExprOperator extends SizeofExprOperator {
       not e.(OverloadedPointerDereferenceExpr).getExpr().isPure() and
       not exists(OverloadedArrayExpr op | op = e |
         op.getArrayBase().isPure() and
-        op.getArrayOffset().isPure()
+        forall(Expr offset | offset = op.getAnArrayOffset() | offset.isPure())
       )
     )
   }

cpp/ql/test/library-tests/ir/ir/PrintAST.expected

@@ -24309,7 +24309,7 @@ ir.cpp:
 # 2727|         getArrayBase(): [VariableAccess] x
 # 2727|             Type = [SpecifiedType] const WithBracketOperator
 # 2727|             ValueCategory = lvalue
-# 2727|         getArrayOffset(): [VariableAccess] i
+# 2727|         getArrayOffset(0): [VariableAccess] i
 # 2727|             Type = [IntType] int
 # 2727|             ValueCategory = prvalue(load)
 # 2727|       getExpr().getFullyConverted(): [ReferenceDereferenceExpr] (reference dereference)

cpp/ql/test/library-tests/subscript_operator/PrintAST.expected

@@ -0,0 +1,69 @@
+#-----| [CopyAssignmentOperator] __va_list_tag& __va_list_tag::operator=(__va_list_tag const&)
+#-----|   <params>: 
+#-----|     getParameter(0): [Parameter] (unnamed parameter 0)
+#-----|         Type = [LValueReferenceType] const __va_list_tag &
+#-----| [MoveAssignmentOperator] __va_list_tag& __va_list_tag::operator=(__va_list_tag&&)
+#-----|   <params>: 
+#-----|     getParameter(0): [Parameter] (unnamed parameter 0)
+#-----|         Type = [RValueReferenceType] __va_list_tag &&
+test.cpp:
+#    3| [CopyAssignmentOperator] S& S::operator=(S const&)
+#    3|   <params>: 
+#-----|     getParameter(0): [Parameter] (unnamed parameter 0)
+#-----|         Type = [LValueReferenceType] const S &
+#    3| [MoveAssignmentOperator] S& S::operator=(S&&)
+#    3|   <params>: 
+#-----|     getParameter(0): [Parameter] (unnamed parameter 0)
+#-----|         Type = [RValueReferenceType] S &&
+#    5| [MemberFunction] int S::operator[](int, int)
+#    5|   <params>: 
+#    5|     getParameter(0): [Parameter] i
+#    5|         Type = [IntType] int
+#    5|     getParameter(1): [Parameter] j
+#    5|         Type = [IntType] int
+#    5|   getEntryPoint(): [BlockStmt] { ... }
+#    6|     getStmt(0): [ReturnStmt] return ...
+#    6|       getExpr(): [ArrayExpr] access to array
+#    6|           Type = [IntType] int
+#    6|           ValueCategory = prvalue(load)
+#    6|         getArrayBase(): [ArrayExpr] access to array
+#    6|             Type = [ArrayType] int[2]
+#    6|             ValueCategory = lvalue
+#    6|           getArrayBase(): [ImplicitThisFieldAccess,PointerFieldAccess] xs
+#    6|               Type = [ArrayType] int[2][2]
+#    6|               ValueCategory = lvalue
+#    6|             getQualifier(): [ThisExpr] this
+#    6|                 Type = [PointerType] S *
+#    6|                 ValueCategory = prvalue(load)
+#    6|           getArrayOffset(): [VariableAccess] i
+#    6|               Type = [IntType] int
+#    6|               ValueCategory = prvalue(load)
+#-----|           getArrayBase().getFullyConverted(): [ArrayToPointerConversion] array to pointer conversion
+#-----|               Type = [PointerType] int(*)[2]
+#-----|               ValueCategory = prvalue
+#    6|         getArrayOffset(): [VariableAccess] j
+#    6|             Type = [IntType] int
+#    6|             ValueCategory = prvalue(load)
+#    6|         getArrayBase().getFullyConverted(): [ArrayToPointerConversion] array to pointer conversion
+#    6|             Type = [IntPointerType] int *
+#    6|             ValueCategory = prvalue
+#   10| [TopLevelFunction] int foo(S)
+#   10|   <params>: 
+#   10|     getParameter(0): [Parameter] s
+#   10|         Type = [Struct] S
+#   10|   getEntryPoint(): [BlockStmt] { ... }
+#   11|     getStmt(0): [ReturnStmt] return ...
+#   11|       getExpr(): [OverloadedArrayExpr] call to operator[]
+#   11|           Type = [IntType] int
+#   11|           ValueCategory = prvalue
+#   11|         getArrayBase(): [VariableAccess] s
+#   11|             Type = [Struct] S
+#   11|             ValueCategory = lvalue
+#   11|         getArrayOffset(0): [Literal] 1
+#   11|             Type = [IntType] int
+#   11|             Value = [Literal] 1
+#   11|             ValueCategory = prvalue
+#   11|         getArrayOffset(1): [Literal] 2
+#   11|             Type = [IntType] int
+#   11|             Value = [Literal] 2
+#   11|             ValueCategory = prvalue

cpp/ql/test/library-tests/subscript_operator/PrintAST.qlref

@@ -0,0 +1 @@
+semmle/code/cpp/PrintAST.ql

cpp/ql/test/library-tests/subscript_operator/test.cpp

@@ -0,0 +1,12 @@
+// semmle-extractor-options: -std=c++23
+
+struct S {
+    int xs[2][2];
+    int operator[](int i, int j) {
+        return xs[i][j];
+    }
+};
+
+int foo(S s) {
+    return s[1, 2];
+}