C++: Make node.asExpr() instanceof ClassAggregateLiteral satisfiable

[MathiasVP]

The C++ IR represents an aggregate literal x = {.a = 1, .b = 2} as a sequence of field writes:

x.a = 1;
x.b = 2;

and as such there is no instruction for which getUnconvertedResultExpression() gives the aggregate literal. And since Node.asExpr() piggybacks on getUnconvertedResultExpression() for most nodes node.asExpr() instanceof AggregateLiteral also never holds. This gotcha has bitten users several times.

This PR solves the problem for class aggregate literals. Since class aggregate literals are represented as a sequence of field writes (see above), we can pick the last field write's post-update node as the node to represent the aggregate literal expression. Semantically this makes sense since the last post-update node represents the state of the object after the last field has been initialized.

Sadly, since we don't model array writes using post-update nodes we can't use the same trick for ArrayAggregateLiterals. However, I think we should do that eventually. Once we do that we can apply a similar trick to that case. For now I think this solution at least solves half of the problems that's been reported.

[Copilot]

Pull Request Overview

This PR enables node.asExpr() instanceof ClassAggregateLiteral by mapping the final post-update node of a class aggregate initialization back to its aggregate literal, and updates tests and change notes accordingly.

• Introduces ClassAggregateInitializerPostUpdateNode and related predicates in ExprNodes.qll to link post-update nodes to ClassAggregateLiteral.
• Updates existing IR-based dataflow tests to expect {...} nodes for class aggregate literals.
• Adds new test cases in asExpr/test.cpp for class aggregate literal expressions and documents missing support for array aggregates.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cpp/ql/test/library-tests/dataflow/fields/ir-path-flow.expected	Swapped out post-update labels for {...} to reflect the class aggregate literal.
cpp/ql/test/library-tests/dataflow/dataflow-tests/localFlow-ir.expected	Inserted {...} expectation for class aggregate literal in local flow tests.
cpp/ql/test/library-tests/dataflow/asExpr/test.cpp	Added asExpr={...} checks for struct aggregate literals; marked missing for arrays.
cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/ExprNodes.qll	Implemented post-update mapping for ClassAggregateLiteral and extended expr nodes.
cpp/ql/lib/change-notes/2025-05-15-class-aggregate-literals.md	Documented the fix for asExpr() on class aggregate literals.