C#: Data flow for positional patterns.

[michaelnebel]

In this PR we add data flow support via positional patterns. That is,

var x = (1, (2, 3), 4);
switch (x)
{
    case (var a1, (var b1, var c1), _):
        Sink(a1);        // has value flow 1
        Sink(c1);        // has value flow 3
        Sink(b1);       // has value flow 2.
        break;
}

Note, that positional patterns are similar to tuple patterns (var (x, y)), but they haven't been handled in the data flow library before now.

[Copilot]

Copilot review overview

🟡 Changes recommended

Nested positional patterns that capture the whole inner tuple still lose flow to the capture variable.

Review effort: Balanced
Findings: 1

Open (1)

• Nested positional patterns miss whole-tuple capture flow · New

What changed in this PR

Adds tuple-backed positional-pattern support to C# data-flow analysis, including nested patterns.

Changes:

• Models flow through positional patterns in is expressions and switch cases.
• Adds nested-pattern tests and updates generated expectations.
• Documents the analysis improvement.

File	Description
csharp/​ql/​test/​library-tests/​dataflow/​tuples/​Tuples.expected	Updates expected flow results.
csharp/​ql/​test/​library-tests/​dataflow/​tuples/​Tuples.cs	Adds nested positional-pattern tests.
csharp/​ql/​test/​library-tests/​dataflow/​tuples/​PrintAst.expected	Updates expected test syntax trees.
csharp/​ql/​test/​library-tests/​dataflow/​tuples/​DataFlowStep.expected	Updates expected flow steps.
csharp/​ql/​lib/​semmle/​code/​csharp/​dataflow/​internal/​DataFlowPrivate.qll	Adds positional-pattern flow modeling.
csharp/​ql/​lib/​change-notes/​2026-10-02-positional-pattern-flow.md	Documents the new support.

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