C#: Data flow for property patterns.

[michaelnebel]

In this PR we added data flow support for property patterns. An example of a pattern pattern can be seen in the conditional of the if-statement below.

var i = 1;
var r = new C() { Prop = i };
if (r is C { Prop: int p })
{
    Sink(p); // $ has value flow 1
}

[Copilot]

Copilot review overview

🟡 Changes recommended

Pattern reads omit fields and can conflate hidden members with identical names.

Review effort: Balanced
Findings: 1

Open (1)

• Support field-backed property patterns · New

What changed in this PR

Adds C# data-flow support for values extracted through property patterns.

Changes:

• Adds property-pattern read steps.
• Adds tests for nested, extended, generic, and tuple patterns.
• Adds a change note.

File	Description
Patterns.cs	Adds property-pattern flow cases.
PatternFlow.expected	Updates generated test expectations.
DataFlowPrivate.qll	Implements pattern-member reads.
2026-10-06-property-pattern-flow.md	Documents the analysis improvement.

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