C++: Support fully qualified field names in MaD

[MathiasVP]

There's a small discrepancy between how Java and C# picks out a field in a MaD summary and how we do it in C++: In C++ we simply use the name of the field whereas Java and C# (and probably other languages as well. I didn't check this) use the fully qualified name of the field.

Using the fully qualified name is definitely the right approach as it precisely allows us to pick out the field that is being written to / read from, but we apparently missed that when we added MaD for C++.

Using the fully qualified name turns out to be crucial for some MaD summaries I'm working on for associative STL containers. So this PR adds the ability for C++ to specify fully qualified names while keeping the old "just use the field name" around for compatibility reasons. This makes some of the rendered flow summary nodes less pretty, but that's the tradeoff we have to make I think, unfortunately.

I tucked on a small one-line change in which nodes we hide in path explanations that we've been missing ever since we introduced MaD. We weren't hiding nodes that were supposed to be hidden, and caaed72 fixes that. It shouldn't change any results.

DCA is clean (the keepassxc is a well known source of wobble on DCA for C++).

[Copilot]

Pull request overview

This PR updates the C++ models-as-data (MaD) flow-summary handling to support selecting fields by fully qualified field name (aligning with how other languages model fields), while keeping the legacy unqualified field-name syntax for backward compatibility. It also tweaks path-explanation hiding so MaD nodes that are marked hidden are actually suppressed in paths.

Changes:

• Extend C++ MaD field selection/encoding to recognize and emit fully qualified field names (while still accepting unqualified names).
• Fix path explanation hiding for MaD flow-summary nodes that are marked as hidden.
• Update/extend MaD-related library tests and expected outputs; add a C++ library-pack change note.

Show a summary per file

File	Description
cpp/ql/lib/semmle/code/cpp/dataflow/internal/FlowSummaryImpl.qll	Updates MaD field encoding logic to support qualified field names while preserving legacy behavior.
cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll	Ensures flow-summary nodes that are marked hidden are treated as hidden in path explanations.
cpp/ql/test/library-tests/dataflow/external-models/flow.ext.yml	Adds external-model summaries exercising fully qualified field-name syntax.
cpp/ql/test/library-tests/dataflow/external-models/test.cpp	Adds test code that writes/reads fields from a namespaced struct and a global struct to validate qualified field handling.
cpp/ql/test/library-tests/dataflow/external-models/flow.expected	Updates expected flow graph output to reflect the new MaD behavior and added tests.
cpp/ql/test/library-tests/dataflow/external-models/sources.expected	Updates expected sources for new test code locations.
cpp/ql/test/library-tests/dataflow/external-models/sinks.expected	Updates expected sinks for new test code locations.
cpp/ql/test/library-tests/dataflow/models-as-data/testModels.expected	Updates expected rendered MaD flow summary nodes to reflect qualified field naming.
cpp/ql/test/query-tests/Security/CWE/CWE-497/semmle/tests/ExposedSystemData.expected	Updates expected path explanations/edges impacted by the MaD/path-hiding adjustments.
cpp/ql/test/query-tests/Likely Bugs/Format/NonConstantFormat/NonConstantFormat.expected	Updates expected path explanations/edges impacted by the MaD/path-hiding adjustments.
cpp/ql/lib/change-notes/2026-06-30-mad-qualified-field-names.md	Adds a change note documenting the MaD field-name qualification change and deprecation plan.

Review details

• Files reviewed: 11/11 changed files
• Comments generated: 3
• Review effort level: Low

[hvitved]

Isn't this simply the same as

Suggested change

	if arg.matches("%::%")
	then
	arg = repeatStars(c.getIndirectionIndex() - 1) + formatQualifiedName(namespace, type, base)
	else arg = repeatStars(c.getIndirectionIndex() - 1) + base
	arg = repeatStars(c.getIndirectionIndex() - 1) + formatQualifiedName(namespace, type, base)
	or
	arg = repeatStars(c.getIndirectionIndex() - 1) + base

(assuming that base cannot match %::%)?

[hvitved]

Yes, you definitely want this.