Fix S3900 FP: Parameter re-assignment through coalescing assignment and tuple deconstruction

[zsolt-kolbay-sonarsource]

Description

Currently, the S3900 rule does not raise an issue if the parameter of the method is assigned a new value before it's dereferenced:

public void Method(object o)
{
    o = Unknown();
    o.ToString(); // Compliant - o no longer has the value which was passed to the method
}

private object Unkown() => null;

This currently works for simple assignments, but not for coalescing assignments, tuple deconstructions, and when the parameter is passed as ref parameter to another method.

• [ ] Coalescing assignment:

  public void CoalescingAssignment(object o)
  {
  o ??= Unknown();
  o.ToString(); // Noncompliant - FP
  }

• [ ] Null-coalescing:

  public void NullCoalescingAssignment_Unknown(object o)
  {
  arg = arg ?? Unknown<string>();
  o.ToString(); // Noncompliant - FP
  }

• [ ] Ternary operator:

  public void NullConditional_ThrowOnNull_Unknown(object o)
  {
  arg = arg == null
      ? throw new ArgumentNullException()
      : Unknown<string>();
  arg.ToString(); // Noncompliant - FP
  }

• [ ] String reassignment via compound concatenation (see also #7111)

  public void NullConditional_ThrowOnNull_Unknown(string s)
  {
  s += null;
  s.ToString(); // FP: s is not null here (should be fixed by #7111)
  }

• [ ] Tuple deconstruction:

  
  public void TupleDeconstruction(object o)
  {
  (o, _) = (Unknown(), Unknown());
  o.ToString(); // Noncompliant - FP
  }

private object Unkown() => null;

- [ ] Passed as ref parameter:
```cs
public void RefParameter(object o)
{
    MethodWithRefParam(ref o);
    o.ToString(); // Noncompliant - FP
}

private void MethodWithRefParam(ref object o) { }

• [x] Throw expression (should be working, needs UT): #7039

  public void ThrowExpression(object o)
  {
  _ = o ?? throw new NullReferenceException(nameof(o));
  o.ToString();
  }

Some of the tests needed are also in #7039

Related information

• C#/VB.NET Plugins version: 8.56
• Visual Studio version: Visual Studio 2022 17.5
• Operating System: Windows 10

[martin-strecker-sonarsource]

see also #7039