Add regression tests for Interlocked.Exchange with null checks

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -161655,5 +161655,94 @@ public static void M1(Func<Task> f) { }
                 //                 c1.
                 Diagnostic(ErrorCode.ERR_SemicolonExpected, "").WithLocation(15, 20));
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/51191")]
+        public void InterlockedExchange_WithNullCheckAndNullArgument()
+        {
+            var source = """
+                #nullable enable
+
+                using System.Threading;
+
+                public class C {
+                    private C? _obj;
+
+                    public void Rent()
+                    {
+                        if (_obj is not null)
+                        {
+                            C? o = Interlocked.Exchange(ref _obj, null);
+                            if (o is not null)
+                            {
+                                Use(o);
+                            }
+                        }
+                    }
+
+                    private void Use(C o) {}
+                }
+                """;
+            CreateCompilation(source).VerifyDiagnostics();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/51191")]
+        public void InterlockedCompareExchange_WithNullCheckAndNullArgument()
+        {
+            var source = """
+                #nullable enable
+
+                using System.Threading;
+
+                public class C {
+                    private C? _obj;
+
+                    public void Rent()
+                    {
+                        C? current = _obj;
+                        if (current is not null)
+                        {
+                            C? o = Interlocked.CompareExchange(ref _obj, null, current);
+                            if (o is not null)
+                            {
+                                Use(o);
+                            }
+                        }
+                    }
+
+                    private void Use(C o) {}
+                }
+                """;
+            CreateCompilation(source).VerifyDiagnostics();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/51191")]
+        public void InterlockedExchange_WithExplicitTypeArgument()
+        {
+            var source = """
+                #nullable enable
+
+                using System.Threading;
+
+                public class C {
+                    private C? _obj;
+
+                    public void Rent()
+                    {
+                        if (_obj is not null)
+                        {
+                            // Explicitly specifying the type argument should also work
+                            C? o = Interlocked.Exchange<C?>(ref _obj, null);
+                            if (o is not null)
+                            {
+                                Use(o);
+                            }
+                        }
+                    }
+
+                    private void Use(C o) {}
+                }
+                """;
+            CreateCompilation(source).VerifyDiagnostics();
+        }
     }
 }