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

[jketema]

This is supported out-of-the-box by the extractor.

See also https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2128r6.pdf

DCA is uneventful.

[Copilot]

Pull request overview

This PR adds support for C++23 multidimensional subscript operators, which allow operator[] to accept multiple indices (e.g., array[i, j] instead of array[i][j]). The PR updates the CodeQL library to handle this new language feature.

• Deprecates the OverloadedArrayExpr::getArrayOffset/0 predicate in favor of indexed access
• Adds getArrayOffset/1 to retrieve a specific index by position, and getAnArrayOffset to retrieve any index
• Updates existing code to use the new predicates

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cpp/ql/lib/semmle/code/cpp/exprs/Call.qll	Adds new predicates getArrayOffset(int n) and getAnArrayOffset(), deprecates getArrayOffset()
cpp/ql/lib/semmle/code/cpp/PrintAST.qll	Updates AST printing to use indexed array offset predicates
cpp/ql/src/jsf/lib/section_4_21_Operators/AV_Rule_166.qll	Updates to check all array offsets are pure using getAnArrayOffset()
cpp/ql/test/library-tests/subscript_operator/test.cpp	Test case demonstrating C++23 multidimensional subscript operator syntax
cpp/ql/test/library-tests/subscript_operator/PrintAST.qlref	Test reference to PrintAST query
cpp/ql/test/library-tests/subscript_operator/PrintAST.expected	Expected AST output showing indexed array offsets
cpp/ql/test/library-tests/ir/ir/PrintAST.expected	Updated expected output to use getArrayOffset(0)
cpp/ql/lib/change-notes/2026-01-08-multidimensional-subscript-operator-1.md	Feature change note documenting new predicates
cpp/ql/lib/change-notes/2026-01-08-multidimensional-subscript-operator-2.md	Deprecation change note for old predicate

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[Copilot]

The documentation comment "Gets the expression giving the nth index" could be clearer. Consider specifying that n is 0-indexed (i.e., n=0 gives the first index, n=1 gives the second index, etc.) to make the usage more explicit for users.

Suggested change

	* Gets the expression giving the nth index.
	* Gets the expression giving the nth index, where `n` is 0-based
	* (that is, `n = 0` gives the first index, `n = 1` gives the second, etc.).