Add regression test for Interlocked.Exchange with nullable types

[Copilot]

This PR adds a regression test to verify that Interlocked.Exchange correctly handles nullable reference types without requiring explicit type parameters.

Background

Previously, the compiler would incorrectly report CS8625 ("Cannot convert null literal to non-nullable reference type") when using Interlocked.Exchange with nullable custom types:

#nullable enable
using System.Threading;

class2? c = new class2();
var c1 = Interlocked.Exchange(ref c, null); // Previously: CS8625 error

However, the same code worked fine with object?:

object? o = new object();
var o1 = Interlocked.Exchange(ref o, null); // Always worked correctly

The issue has since been fixed (likely as part of improvements to null literal type inference for generic methods mentioned in #43536), but there was no test coverage to prevent regression.

Changes

This PR adds a comprehensive test InterlockedExchangeWithNullableTypes in MethodTypeInferenceTests.cs that:

• Verifies Interlocked.Exchange works correctly with both object? and custom nullable types
• Confirms the inferred types have the correct nullable annotations
• Ensures no compilation errors occur
• Uses raw string literals as per coding standards
• Includes a WorkItem attribute linking to the original issue

The test validates that all three scenarios work correctly:

1. Interlocked.Exchange(ref object?, null)
2. Interlocked.Exchange(ref class2?, null) (the previously broken case)
3. Interlocked.Exchange<class2?>(ref class2?, null) (explicit type parameter workaround)

This test will prevent any future regressions and can be used to close the original issue.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Interlocked.Exchange: compiler choses wrong Exchange with nullable types</issue_title>
<issue_description>This issue has been moved from a ticket on Developer Community.

---

[severity:It's more difficult to complete my work] [regression] [worked-in:.NET core 3.1]

#nullable enable
using System.Threading;

namespace InterlockedExchangeNullProblem {
  public class Class1 {
    public Class1() {
      object? o = new object();
      var o1 = Interlocked.Exchange(ref o, null); //1) ok
      class2? c = new class2();
      var c1 = Interlocked.Exchange(ref c, null); //2) error CS8625 Cannot convert null literal to non-nullable reference type.
      var c2 = Interlocked.Exchange<class2?>(ref c, null); //3) ok
    }
  }
  public class class2{}
}

The compiler works as expected in 1):

But in 2), the compiler creates wrongly the error message

CS8625 Cannot convert null literal to non-nullable reference type.

Furthermore, c1 gets the wrong type class2, instead class2? like the type of variable c.

Once <class2?> gets added to Interlocked.Exchange(), the compilation works properly again:

var c2 = Interlocked.Exchange<class2?>(ref c, null);

It really doesn't make sense when Interlocked.Exchange() works for <object?> but not for <class2?> !

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Original Comments

Feedback Bot on 11/18/2020, 07:08 PM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

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Original Solutions

(no solutions)</issue_description>

<agent_instructions>@copilot When i try the users code example out, i get no error. Indicating that this was an issue that has since been fixed. Can you create an appropriate compiler test here that demonstrates that there is no error. We can use that test to close out this issue. Please use raw string literals in the test, and have an appropriate WorkItem("url") attribute on it. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@RikkiGibson This may be an instance of #43536 (`null` fails to contribute its nullability to method type argument inference). Will need investigation. /cc @jcouv

Fixes #49525

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[CyrusNajmabadi]

@dotnet/roslyn-compiler ptal